the ergodic theory of continued fraction maps
TRANSCRIPT
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The ergodic theory of continuedfraction maps
Speaker: Radhakrishnan Nair
University of Liverpool
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Contents of the talk
• Contined fractions the basics
• The continued fraction map
• The ergodic theory of the continued fraction maps
• The natural extention
• Means of convergents almost everywhere
• Hurwitz’s constants
• Good universality
• Means of subsequences
• Markov Maps of the unit interval interval
• Jarnik’s theorem on badly approximable numbers
• The field of formal power series
• Continued fractions on the field of formal power series
• The continued fraction map on the field of formal power series
• p-adic numbers
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• p-adic continued fraction map
• The continued fraction expansion on the p-adic numbers
• Ergodic Properties of the p-adic continued fraction map
• Means of p-adic continued fraction maps
• Entropy of the p adic continued fraction map
• Isomorphism of dynamical systems
• Ornstein’s theorem
• Non- archemedean fields
• Examples
• Continued fraction maps on the field of formal power series
• Classifing continued fraction maps
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Euclidean Algorithm and Gauss Map
By Euclidean algorithm, any rational number a/b > 1 can beexpressed as
x =a
b= a0 +
1
c1 + 1c2+ 1
.
.
.
cn−1 + 1cn
,
where c0, . . . , cn are natural numbers with cn > 1, except for n = 0.
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Euclidean Algorithm and Gauss Map
By Euclidean algorithm, any rational number a/b > 1 can beexpressed as
x =a
b= a0 +
1
c1 + 1c2+ 1
.
.
.
cn−1 + 1cn
,
where c0, . . . , cn are natural numbers with cn > 1, except forn = 0. Note cn(x) = cn−1(Tx) for n ≥ 1, where
Tx =
1x
if x 6= 0;
0 if x = 0,
is the famous Gauss map circa 1800.
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Regular Continued fraction Expansions
For arbitrary real x we have the regular continued fractionexpansion of a real number
x = [c0; c1, c2, . . . ] = c0 +1
c1 +1
c2 +1
c3 +1
c4. . .
.
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Regular Continued fraction Expansions
For arbitrary real x we have the regular continued fractionexpansion of a real number
x = [c0; c1, c2, . . . ] = c0 +1
c1 +1
c2 +1
c3 +1
c4. . .
.
Again cn(x) = cn−1(Tx) for n ≥ 1. The terms c0, c1, · · · are calledthe partial quotients of the continued fraction expansion and thesequence of rational truncates
[c0; c1, · · · , cn] =pn
qn, (n = 1, 2, · · · )
are called the convergents of the continued fraction expansion.
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Continued fraction map on [1, 0)
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Dynamical System
By a dynamical system (X , β, µ,T ) we mean a set X , togetherwith a σ-algebra β of subsets of X , a probability measure µ on themeasurable space (X , β) and a measurable self map T of X that isalso measure preserving. i.e. if given an element A of β if we setT−1A = x ∈ X : Tx ∈ A then µ(A) = µ(T−1A).
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Dynamical System
By a dynamical system (X , β, µ,T ) we mean a set X , togetherwith a σ-algebra β of subsets of X , a probability measure µ on themeasurable space (X , β) and a measurable self map T of X that isalso measure preserving. i.e. if given an element A of β if we setT−1A = x ∈ X : Tx ∈ A then µ(A) = µ(T−1A).
We say a dynamical system is ergodic if T−1A = A for some A inβ means that µ(A) is either zero or one in value.
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Dynamical System
By a dynamical system (X , β, µ,T ) we mean a set X , togetherwith a σ-algebra β of subsets of X , a probability measure µ on themeasurable space (X , β) and a measurable self map T of X that isalso measure preserving. i.e. if given an element A of β if we setT−1A = x ∈ X : Tx ∈ A then µ(A) = µ(T−1A).
We say a dynamical system is ergodic if T−1A = A for some A inβ means that µ(A) is either zero or one in value.
We say T is weak-mixing, if (X × X , β × β, µ× µ,T × T ) isergodic
Note weak mixing is strictly stronger than ergodicity.
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Dynamical System
By a dynamical system (X , β, µ,T ) we mean a set X , togetherwith a σ-algebra β of subsets of X , a probability measure µ on themeasurable space (X , β) and a measurable self map T of X that isalso measure preserving. i.e. if given an element A of β if we setT−1A = x ∈ X : Tx ∈ A then µ(A) = µ(T−1A). We saya dynamical system is ergodic if T−1A = A for some A in βmeans that µ(A) is either zero or one in value. We say T isweak-mixing, if (X ×X , β × β, µ× µ,T ×T ) is ergodic Note weakmixing is strictly stronger than ergodicity.
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Birkhoff’s theorem
If (X , β, µ,T ) is measure preserving and ergodic and f isintegrable we have Birkhoff’s pointwise ergodic theorem
f (x) := limN→∞
1
N
N∑n=1
f (T nx) =
∫X
f (x)dµ a.e..
If (X , β, µ,T ) is not ergodic, we just know this limit is T invariantalmost everywhere i.e. f (Tx) = f (x)
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Gauss dynamical system and its natural extention
(i) If X = [0, 1], β is the σ-algebra of Borel sets on X ,µ(A) = 1
log 2
∫A
dxx+1 , for A ∈ β and T is the Gauss map then
(X , β, µ,T ) is weak mixing.
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Gauss dynamical system and its natural extention
(i) If X = [0, 1], β is the σ-algebra of Borel sets on X ,µ(A) = 1
log 2
∫A
dxx+1 , for A ∈ β and T is the Gauss map then
(X , β, µ,T ) is weak mixing.
(ii) If X = Ω = ([0, 1) \Q)× [0, 1], γ is the σ-algebra of Borelsubsets of Ω, ω is the probability measure on the measurable space(Ω, β) defined by ω(A) = 1
(log 2)
∫A
dxy(1+xy)2 , and
T (x , y) = (Tx , 1[ 1x
]+y). Then the map T preserves the measure ω
and the dynamical system (Ω, γ, ω,T ) called the natural extentionof (X , β, µ,T ) is weak mixing.
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Means of convergents
Suppose the function F with domain the non-negative realnumbers and range the real numbers is continuous and increasing.For each natural number n and arbitrary non-negative realnumbers a1, · · · , an we define
MF ,n(a1, · · · , an) = F−1[1
n
n∑j=1
F (aj)].
Then C. Ryll-Nardzewski observed that
limn→∞
MF ,n(c1(x), · · · , cn(x)) = F−1[1
log 2
∫ 1
−0F (c1(t))d
dt
1 + t],
almost every where with respect to Lebesgue measure.
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Means of convergents
Suppose the function F with domain the non-negative realnumbers and range the real numbers is continuous and increasing.For each natural number n and arbitrary non-negative real numbersa1, · · · , an we define MF ,n(a1, · · · , an) = F−1[ 1
n
∑nj=1 F (aj)].
Then C. Ryll-Nardzewski ob servedlimn→∞MF ,n(c1(x), · · · , cn(x)) = F−1[ 1
log 2
∫ 1−0 F (c1(t))d dt
1+t ],almost every where with respect to Lebesgue measure.
Special cases due to A. Khinchin
(i) limN→∞1N
∑Nn=1 cn(x) =∞a.e.;
(ii) limN→∞(c1(x) . . . cN(x))N−1
= Πk≥1(1 + 1k(k+2) )
log klog 2 a.e.
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Hurwitz’s constants
Recall the inequality |x − pnqn| ≤ 1
q2n, which is classical and well
known. One can check
θn(x) =1
(T nx)−1 + qn−1q−1n
= q2n|x −
pn
qn| ∈ [0, 1)
for each natural number n. Set
F (x) =
z
log 2 x ∈ [0, 12 );
1log 2 (1 − z + log 2z) if x ∈ [ 1
2 , 1]
Then
limn→∞
1
n|1 ≤ j ≤ n : θj(x) ≤ z| = F (z),
almost everywhere with respect to Lebesgue measure.W. Bosma, H. Jager andF. Wiedijk 1983. Conjectured H.W.Lenstra Jr.
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Good Universality
A sequence of integers (an)∞n=1 is called Lp-good universal if foreach dynamical system (X ,B, µ,T ) and f ∈ Lp(X ,B, µ) we have
f (x) = limN→∞
1
N
N∑n=1
f (T anx)
existing µ almost everywhere.
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Uniform distribution modulo 1
A sequence of real numbers (xn)∞n=1 is uniformly distributed moduloone if for each interval I ⊆ [0, 1), if |I | denotes its length, we have
limN→∞
1
N#n ≤ N : xn ∈ I = |I |.
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Subsequence ergodic theory
Lemma (Nair)
If (anγ)∞n=1 is uniformly distributed modulo one for eachirrational number γ, the dynamical system (X ,B, µ,T ) isweak-mixing and (an)n≥1 is L2-good universal then f (x) exists and
f (x) =
∫X
fdµ
µ almost everywhere.
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Polynomial like sequences
1. The natural numbers: The sequence (n)∞n=1 is L1-gooduniversal. This is Birkhoff’s pointwise ergodic theorem.
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Polynomial like sequences
1. The natural numbers: The sequence (n)∞n=1 is L1-gooduniversal. This is Birkhoff’s pointwise ergodic theorem.2. Polynomial like sequences: Note if φ(x) is a polynomial suchthat φ(N) ⊆ N (Bourgain, Nair) and p > 1 then (φ(n))∞n=1 and(φ(pn))∞n=1 (Nair) where pn is nth prime are Lp good universalsequences.
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Hartman uniformly distributed sequences
A sequence of integers (an)n≥1 is Hartman uniformly distributed if
limN→∞
1
N
N∑n=1
e(anx) = 0
for all non-integer x . Equivalenty a sequence is Hartmannuniformly distributed if (anγ)n≥1 is uniform distributed modulo 1for each irrational number γ, and the sequence (an)n≥1 isuniformly distributed in each residue class modm for each naturalnumber m > 1.
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Hartman uniformly distributed sequences
A sequence of integers (an)n≥1 is Hartman uniformly distributed if
limN→∞
1
N
N∑n=1
e(anx) = 0
for all non-integer x . Equivalenty a sequence is Hartmannuniformly distributed if (anγ)n≥1 is uniform distributed modulo 1for each irrational number γ, and the sequence (an)n≥1 isuniformly distributed in each residue class modm for each naturalnumber m > 1.
Note if n ∈ N then n2 6≡ 3 mod 4 so in general the sequences(φ(n))∞n=1 and (φ(pn))∞n=1 are not Hartman uniformly distributed.We do however know that if β ∈ R \Q then (φ(n)β)∞n=1 and(φ(pn)β)∞n=1 are uniformly distributed modulo one. Condition Hsequences to follow are Hartman uniformly distributed.
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Condition H sequences of integers
3. (an)∞n=1 that are Lp-good universal and Hartman uniformlydistributed are constructed as follows.
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Condition H sequences of integers
3. (an)∞n=1 that are Lp-good universal and Hartman uniformlydistributed are constructed as follows. Denote by [y ] the integerpart of real number y
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Condition H sequences of integers
3. (an)∞n=1 that are Lp-good universal and Hartman uniformlydistributed are constructed as follows. Denote by [y ] the integerpart of real number y . Set an = [g(n)] (n = 1, . . . ) whereg : [1,∞)→ [1,∞) is a differentiable function whose derivationincreases with its argument.
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Condition H sequences of integers
3. (an)∞n=1 that are Lp-good universal and Hartman uniformlydistributed are constructed as follows. Denote by [y ] the integerpart of real number y . Set an = [g(n)] (n = 1, . . . ) whereg : [1,∞)→ [1,∞) is a differentiable function whose derivationincreases with its argument. Let An denote the cardinality of theset n : an ≤ n and suppose for some function a : [1,∞)→ [1,∞)increasing to infinity as its argument does, that we set
bM = supz∈[ 1
a(M), 1
2)
∣∣∣∣∣∣∑
n:an≤Me(zan)
∣∣∣∣∣∣ .
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Condition H sequences of integers
3. (an)∞n=1 that are Lp-good universal and Hartman uniformlydistributed are constructed as follows. Denote by [y ] the integerpart of real number y . Set an = [g(n)] (n = 1, . . . ) whereg : [1,∞)→ [1,∞) is a differentiable function whose derivationincreases with its argument. Let An denote the cardinality of theset n : an ≤ n and suppose for some functiona : [1,∞)→ [1,∞) increasing to infinity as its argument does,
that we set bM = supz∈[ 1a(M)
, 12
)
∣∣∣∑n:an≤M e(zan)∣∣∣ . Suppose also
for some decreasing function c : [1,∞)→ [1,∞), with∑∞s=1 c(θs) <∞ for θ > 1 and some positive constant C > 0 that
b(M) + A[a(M)] + Ma(M)
AM≤ Cc(M).
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Condition H sequences of integers
3. (an)∞n=1 that are Lp-good universal and Hartman uniformlydistributed are constructed as follows. Denote by [y ] the integerpart of real number y . Set an = [g(n)] (n = 1, . . . ) whereg : [1,∞)→ [1,∞) is a differentiable function whose derivationincreases with its argument. Let An denote the cardinality of theset n : an ≤ n and suppose for some functiona : [1,∞)→ [1,∞) increasing to infinity as its argument does,
that we set bM = supz∈[ 1a(M)
, 12
)
∣∣∣∑n:an≤M e(zan)∣∣∣ . Suppose also
for some decreasing function c : [1,∞)→ [1,∞), with∑∞s=1 c(θs) <∞ for θ > 1 and some positive constant C > 0 that
b(M) + A[a(M)] + Ma(M)
AM≤ Cc(M).
Then we say that k = (an)∞n=1 satisfies condition H. (Nair)
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Examples of Hartman uniformly distribution sequences
Sequences satisfying condition H are both Hartman uniformlydistributed and Lp-good universal.Specific sequences of integers that satisfy conditions H includekn = [g(n)] (n = 1, 2, . . . ) whereI. g(n) = nω if ω > 1 and ω /∈ N.II. g(n) = e logγ n for γ ∈ (1, 3
2 ).III. g(n) = P(n) = bknk + . . .+ b1n + b0 for bk , . . . , b1 not allrational multiplies of the same real number.
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Bourgain’s random sequences
4. Suppose S = (nk)∞n=1 ⊆ N is a strictly increasing sequence ofnatural numbers. By identifying S with its characteristic functionIS we may view it as a point in Λ = 0, 1N the set of maps fromN to 0, 1.
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Bourgain’s random sequences
4. Suppose S = (nk)∞n=1 ⊆ N is a strictly increasing sequence ofnatural numbers. By identifying S with its characteristic functionIS we may view it as a point in Λ = 0, 1N the set of maps fromN to 0, 1.
We may endow Λ with a probability measure by viewing it as aCartesian product Λ =
∏∞n=1 Xn where for each natural number n
we have Xn = 0, 1 and specify the probability πn on Xn byπn(1) = qn with 0 ≤ qn ≤ 1 and πn(0) = 1 − qn suchthat limn→∞ qnn = ∞.
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Bourgain’s random sequences
4. Suppose S = (nk)∞n=1 ⊆ N is a strictly increasing sequence ofnatural numbers. By identifying S with its characteristic functionIS we may view it as a point in Λ = 0, 1N the set of maps fromN to 0, 1. We may endow Λ with a probability measure byviewing it as a Cartesian product Λ =
∏∞n=1 Xn where for each
natural number n we have Xn = 0, 1 and specify the probabilityπn on Xn by πn(1) = qn with 0 ≤ qn ≤ 1 andπn(0) = 1 − qn such that limn→∞ qnn = ∞.
The desired probability measure on Λ is the corresponding productmeasure π =
∏∞n=1 πn. The underlying σ-algebra β is that
generated by the “cylinders”
λ = (λn)∞n=1 ∈ Λ : λi1 = αi1 , . . . λir = αir
for all possible choices of i1, . . . , ir and αi1 , . . . , αir .
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Bourgain’s random sequences
4. Suppose S = (nk)∞n=1 ⊆ N is a strictly increasing sequence ofnatural numbers. By identifying S with its characteristic functionIS we may view it as a point in Λ = 0, 1N the set of maps fromN to 0, 1. We may endow Λ with a probability measure byviewing it as a Cartesian product Λ =
∏∞n=1 Xn where for each
natural number n we have Xn = 0, 1 and specify the probabilityπn on Xn by πn(1) = qn with 0 ≤ qn ≤ 1 andπn(0) = 1 − qn such that limn→∞ qnn = ∞. The desiredprobability measure on Λ is the corresponding product measureπ =
∏∞n=1 πn. The underlying σ-algebra β is that generated by
the “cylinders”
λ = (λn)∞n=1 ∈ Λ : λi1 = αi1 , . . . λir = αir
for all possible choices of i1, . . . , ir and αi1 , . . . , αir .Let (kn)∞n=1 be almost any point in Λ with respect to the measureπ.
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Means of convergents for subsequences
Suppose the function F with domain the non-negative realnumbers and range the real numbers is continuous and increasing.For each natural number n and arbitrary non-negative realnumbers a1, · · · , an we define
MF ,n(a1, · · · , an) = F−1[1
n
n∑j=1
F (aj)].
Then if (an)n≥1 is Lp good universal and (anγ)n≥1 is uniformlydistributed modulo one for irrational γ we have
limn→∞
MF ,n(ca1(x), · · · , can(x)) = F−1[1
log 2
∫ 1
−0F (c1(t))d
dt
1 + t],
almost every where with respect to Lebesgue measure.
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Means of convergents for subsequences
Suppose the function F with domain the non-negative realnumbers and range the real numbers is continuous and increasing.For each natural number n and arbitrary non-negative real numbersa1, · · · , an we define MF ,n(a1, · · · , an) = F−1[ 1
n
∑nj=1 F (aj)].
Then if (an)n≥1 is Lp good universal and (anγ)n≥1 is uniformlydistributed modulo one for irrational γ we have
limn→∞
MF ,n(ca1(x), · · · , can(x)) = F−1[1
log 2
∫ 1
−0F (c1(t))d
dt
1 + t],
almost every where with respect to Lebesgue measure.Special cases
(i) limN→∞1N
∑Nn=1 can(x) =∞a.e.;
(ii) limN→∞(ca1(x) . . . caN (x))N−1
= Πk≥1(1 + 1k(k+2) )
log klog 2 a.e.
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Hurwitz’s constants for subsequences
Recall the inequality
|x − pn
qn| ≤ 1
q2n
,
which is classical and well known. Clearly
θn(x) = q2n|x −
pn
qn| ∈ [0, 1).
if for each natural number n. Set
F (x) =
z
log 2 x ∈ [0, 12 );
1log 2 (1 − z + log 2z) if x ∈ [ 1
2 , 1]
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Hurwitz’s constants for subsequences
Recall the inequality |x − pnqn| ≤ 1
q2n, which is classical and well
known. Clearly θn(x) = q2n|x −
pnqn| ∈ [0, 1). if for each natural
number n. Set
F (x) =
z
log 2 x ∈ [0, 12 );
1log 2 (1 − z + log 2z) if x ∈ [ 1
2 , 1]
Then if [ (an)n≥1 is Lp good universal and (anγ)n≥1 is uniformlydistributed modulo one for irrational γ]* we have
limn→∞
1
n|1 ≤ j ≤ n : θaj (x) ≤ z| = F (z),
almost everywhere with respect to Lebesgue measure.
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Hurwitz’s constants for subsequences
Recall the inequality |x − pnqn| ≤ 1
q2n, which is classical and well
known. Clearly θn(x) = q2n|x −
pnqn| ∈ [0, 1). if for each natural
number n. Set
F (x) =
z
log 2 x ∈ [0, 12 );
1log 2 (1 − z + log 2z) if x ∈ [ 1
2 , 1]
Then if [ (an)n≥1 is Lp good universal and (anγ)n≥1 is uniformlydistributed modulo one for irrational γ]* we have
limn→∞
1
n|1 ≤ j ≤ n : θaj (x) ≤ z| = F (z),
almost everywhere with respect to Lebesgue measure.
D. Hensley dropped condition * using a different method.
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Other sequences attached to the regular continued fractionexpansion. I
Suppose z is in [0, 1] and for irrational x in (0, 1) set
Qn(x) =qn−1(x)
qn(x)
for each positive integer n. Suppose also that (an)∞n=1 satisfies *.Then
limn→∞
1
n|1 ≤ j ≤ n : Qaj (x) ≤ z| = F2(z) =
log(1 + z)
log 2
almost everywhere with respect to Lebesgue measure.
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Other sequences attached to the regular continued fractionexpansion II
For irrational x in (0, 1) set
rn(x) =|x − pn
qn|
|x − pn−1
qn−1|. (n = 1, 2, · · · )
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Other sequences attached to the regular continued fractionexpansion II
For irrational x in (0, 1) set
rn(x) =|x − pn
qn|
|x − pn−1
qn−1|. (n = 1, 2, · · · )
Further for z in [0, 1] let
F3(z) =1
log 2(log(1 + z) − z
1 + zlog z).
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Other sequences attached to the regular continued fractionexpansion II
For irrational x in (0, 1) set
rn(x) =|x − pn
qn|
|x − pn−1
qn−1|. (n = 1, 2, · · · )
Further for z in [0, 1] let
F3(z) =1
log 2(log(1 + z) − z
1 + zlog z).
Suppose also that (an)∞n=1 satisfes *. Then
limn→∞
1
n|1 ≤ j ≤ n : raj (x) ≤ z| = F3(z),
almost everywhere with respect to Lebesgue measure.
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Continued fraction map on [1, 0)
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Markov Partitions
Let T be a self map of [0, 1] and let
P0 = P(j) : j ∈ Λ
be a partition of [0, 1] into open intervals, disregarding a set ofLebesgue measure 0.
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Markov Partitions
Let T be a self map of [0, 1] and let
P0 = P(j) : j ∈ Λ
be a partition of [0, 1] into open intervals, disregarding a set ofLebesgue measure 0.
We may for instance take Λ to be 1, 2, · · · , n (n = 1, 2, · · · ) orwe may take Λ to be the natural numbers.
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Markov Partitions
Let T be a self map of [0, 1] and let P0 = P(j) : j ∈ Λ be apartition of [0, 1] into open intervals, disregarding a set ofLebesgue measure 0. We may for instance take Λ to be1, 2, · · · , n (n = 1, 2, · · · ) or we may take Λ to be the naturalnumbers. Define further partitions
Pk = P(j0, · · · , jk) : (j0, · · · , jk) ∈ Λk+1
of [0, 1] inductively for k in N by setting
P(j0, · · · , jk) = P(j0, · · · , jk−1) ∩ T−k(P(jk)),
so thatPk = Pk−1 ∨ T−1(Pk−1).
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Markov Partitions
Let T be a self map of [0, 1] and let P0 = P(j) : j ∈ Λ be apartition of [0, 1] into open intervals, disregarding a set ofLebesgue measure 0. We may for instance take Λ to be1, 2, · · · , n (n = 1, 2, · · · ) or we may take Λ to be the naturalnumbers. Define further partitions
Pk = P(j0, · · · , jk) : (j0, · · · , jk) ∈ Λk+1
of [0, 1] inductively for k in N by setting
P(j0, · · · , jk) = P(j0, · · · , jk−1) ∩ T−k(P(jk)),
so thatPk = Pk−1 ∨ T−1(Pk−1).
Of course some of these sets P(j0, · · · , jk) may be empty. We shalldisregard these.
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Markov Maps of the unit interval
We say the map T is Markov with partition P0 if the followingconditions hold :
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Markov Maps of the unit interval
We say the map T is Markov with partition P0 if the followingconditions hold :(i) for each j in Λ there exists Λj ⊂ Λ such that
T (P(j)) = int ∪i∈ΛjP(i);
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Markov Maps of the unit interval
We say the map T is Markov with partition P0 if the followingconditions hold :(i) for each j in Λ there exists Λj ⊂ Λ such that
T (P(j)) = int ∪i∈ΛjP(i);
(ii) we haveinf j∈Λλ(T (P(j)) > 0;
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Markov Maps of the unit interval
We say the map T is Markov with partition P0 if the followingconditions hold :(i) for each j in Λ there exists Λj ⊂ Λ such that
T (P(j)) = int ∪i∈ΛjP(i);
(ii) we haveinf j∈Λλ(T (P(j)) > 0;
(iii) the derivative T ′ of T is defined and 1T ′ is bounded off
endpoints;
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Markov Maps of the unit interval
We say the map T is Markov with partition P0 if the followingconditions hold :(i) for each j in Λ there exists Λj ⊂ Λ such that
T (P(j)) = int ∪i∈ΛjP(i);
(ii) we haveinf j∈Λλ(T (P(j)) > 0;
(iii) the derivative T ′ of T is defined and 1T ′ is bounded off
endpoints;
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Markov Maps of the unit interval
We say the map T is Markov with partition P0 if the followingconditions hold :(i) for each j in Λ there exists Λj ⊂ Λ such that
T (P(j)) = int ∪i∈ΛjP(i);
(ii) we haveinf j∈Λλ(T (P(j)) > 0;
(iii) the derivative T ′ of T is defined and 1T ′ is bounded on U0;
(iv) there exists β > 1 such that (T n)′ βn off endpoints;
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Markov Maps of the unit interval
We say the map T is Markov with partition P0 if the followingconditions hold :(i) for each j in Λ there exists Λj ⊂ Λ such that
T (P(j)) = int ∪i∈ΛjP(i);
(ii) we haveinf j∈Λλ(T (P(j)) > 0;
(iii) the derivative T ′ of T is defined and 1T ′ is bounded on U0;
(iv) there exists β > 1 such that (T n)′ βn on Un and(v) there exists γ in (0, 1) such that
|1 − T ′(x)
T ′(y)| |x − y |γ ,
for x and y belonging to the same element of P0.
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Examples of Markov Maps
(a) For a Pisot-Vijayaraghavan number β > 1 let
Tβ(x) = βx;
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Examples of Markov Maps
(a) For a Pisot-Vijayaraghavan number β > 1 let
Tβ(x) = βx;
and let(b)
Tx =
1x
if x 6= 0;
0 if x = 0,
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Examples of Markov Maps
(a) For a Pisot-Vijayaraghavan number β > 1 let
Tβ(x) = βx;
and let(b)
Tx =
1x
if x 6= 0;
0 if x = 0,
The example (a) is known as the β-transformation. Note that inthe special case where β is an integer Tβ(Tβ(x)) = β2x. Thisis not true for non-integer β and this gives the dynamics a quitedifferent character. The example (b) is the famous Gauss mapwhich is associated to the continued fraction expansion of a realnumber.
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Continued fraction map on [1, 0)
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Invariant Measures for Markov Measures
If the map T : [0, 1] → [0, 1] is Markov in the sense describedabove then it preserves a measure η equivalent to Lebesguemeasure.Further the dynamical system ([0, 1], β, η,T ), where β denotes theusual Borel σ-algebra on [0, 1], is exact. In particular it is ergodic.
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Invariant Measures for Markov Measures
If the map T : [0, 1] → [0, 1] is Markov in the sense describedabove then it preserves a measure η equivalent to Lebesguemeasure.Further the dynamical system ([0, 1], β, η,T ), where β denotes theusual Borel σ-algebra on [0, 1], is exact. In particular it is ergodic.As a consequence, G. Birkhoff’s pointwise ergodic theorem tells usthat
(1.1) limN→∞
1
N
N∑n=1
χB(T n(x)) = η(B),
almost everywhere with respect to Lebesgue measure.
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An exceptional set
For x in [0, 1] let Ω(x) = ΩT (x) denote the closure of the setT n(x) : n = 1, 2, · · · .
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An exceptional set
For x in [0, 1] let Ω(x) = ΩT (x) denote the closure of the setT n(x) : n = 1, 2, · · · .Henceforth we denote N ∪ 0 by N0.
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An exceptional set
For x in [0, 1] let Ω(x) = ΩT (x) denote the closure of the setT n(x) : n = 1, 2, · · · . Henceforth we denote N ∪ 0 by N0.
For a subset A of [0, 1] let d(x ,A) denote infa∈A|x − a|.
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An exceptional set
For x in [0, 1] let Ω(x) = ΩT (x) denote the closure of the setT n(x) : n = 1, 2, · · · . Henceforth we denote N ∪ 0 by N0.For a subset A of [0, 1] let d(x ,A) denote infa∈A|x − a|.
If x = (xr )∞r=0 is a sequence of real numbers such that0 ≤ xr ≤ 1 and f : N0 → R is positive, set
E (x , f ) = x ∈ [0, 1] : | log d(xr ,Ω(x))| f (r).
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An exceptional set
For x in [0, 1] let Ω(x) = ΩT (x) denote the closure of the setT n(x) : n = 1, 2, · · · . Henceforth we denote N ∪ 0 by N0.For a subset A of [0, 1] let d(x ,A) denote infa∈A|x − a|. Ifx = (xr )∞r=0 is a sequence of real numbers such that0 ≤ xr ≤ 1 and f : N0 → R is positive, set
E (x , f ) = x ∈ [0, 1] : | log d(xr ,Ω(x))| f (r).
As a consequence of Birkhoff’s theorem and the fact that η isequivalent to Lebesgue measure λ we see that λ(E (x , f )) = 0.
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Hausdorff Dimension
Suppose M is a metric space endowed with a metric d .
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Hausdorff Dimension
Suppose M is a metric space endowed with a metric d . Alsosuppose E ⊆ M.
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Hausdorff Dimension
Suppose M is a metric space endowed with a metric d . Alsosuppose E ⊆ M. Suppose δ > 0.
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Hausdorff Dimension
Suppose M is a metric space endowed with a metric d . Alsosuppose E ⊆ M. Suppose δ > 0.
We say a collection of subsets of M denoted Cδ is a δ–cover for Eif E ⊆ ∪U∈CδU, and if we set
diam(U) := supx ,y∈U
d(x , y),
then U ∈ Cδ implies diam(U) ≤ δ.
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Hausdorff Dimension
Suppose M is a metric space endowed with a metric d . Alsosuppose E ⊆ M. Suppose δ > 0. We say a collection of subsets ofM denoted Cδ is a δ–cover for E if E ⊆ ∪U∈CδU, and if we set
diam(U) := supx ,y∈U
d(x , y),
then U ∈ Cδ implies diam(U) ≤ δ.
We setHsδ(E ) = sup
Cδ
∑i
(diamUi )s ,
where the supremum is taken over all δ-covers Cδ.
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Hausdorff Dimension
Suppose M is a metric space endowed with a metric d . Alsosuppose E ⊆ M. Suppose δ > 0. We say a collection of subsets ofM denoted Cδ is a δ–cover for E if E ⊆ ∪U∈CδU, and if we set
diam(U) := supx ,y∈U
d(x , y),
then U ∈ Cδ implies diam(U) ≤ δ. We set
Hsδ(E ) = sup
Cδ
∑i
(diamUi )s ,
where the supremum is taken over all δ-covers Cδ. We set
Hs(E ) := limδ→0Hsδ(E ),
which always exists.
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Hausdorff Dimension
Suppose M is a metric space endowed with a metric d . Alsosuppose E ⊆ M. Suppose δ > 0. We say a collection of subsets ofM denoted Cδ is a δ–cover for E if E ⊆ ∪U∈CδU, and if we set
diam(U) := supx ,y∈U
d(x , y),
then U ∈ Cδ implies diam(U) ≤ δ. We set
Hsδ(E ) = sup
Cδ
∑i
(diamUi )s ,
where the supremum is taken over all δ-covers Cδ. We set
Hs(E ) := limδ→0Hsδ(E ),
which always exists. We call the specific s0 where Hs changes from∞ to 0 the Hausdorff dimension of E .
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Some examples
(i) If M = Rn for n > 1 and E ⊆ M has positive lebesgue measurethen s0 = n i.e. dim(E ) = n.
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Some examples
(i) If M = Rn for n > 1 and E ⊆ M has positive lebesgue measurethen s0 = n i.e. dim(E ) = n.(ii) If E ⊆ M is countable then dim(E ) = 0.
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Some examples
(i) If M = Rn for n > 1 and E ⊆ M has positive lebesgue measurethen s0 = n i.e. dim(E ) = n.(ii) If E ⊆ M is countable then dim(E ) = 0.(iii) Cantor’s middle third set : Let
C = x ∈ [0, 1) : x =∞∑n=1
xn3n
s.t.xn ∈ 0, 2.
C is well known to be uncountanle.One can show dim(C ) = log 2
log 3 .
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Abercrombie, Nair
For each sequence x = (xr )∞r=0 of real numbers in [0, 1] andpositive function f : N0 → R such that f (r) r 2, theHausdorff dimension of E (x , f ) is 1.
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A special case
An immediate consequence is the following result.For x0 ∈ [0, 1] set
E (x0) = x ∈ [0, 1] : x0 ∈ [0, 1] \ ΩT (x).
Then for each x0 in [0, 1] the Hausdorff dimension of E (x0) is 1.
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A special case
An immediate consequence is the following result.For x0 ∈ [0, 1] set
E (x0) = x ∈ [0, 1] : x0 ∈ [0, 1] \ ΩT (x).
Then for each x0 in [0, 1] the Hausdorff dimension of E (x0) is 1.
Take x0 = 0 and T is the Gauss continued fraction map.
Thus the set of x ∈ [0, 1] with bound convergents had dimension 1.
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Continued fraction map on [1, 0)
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Equivalent characterisations of badly approximability
(i) We say an irrational real number α is badly approximable if
there exists a constant c(α) > 0 such that |α − pq | >
c(α)q2 , for
every rational pq .
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Equivalent characterisations of badly approximability
(i) We say an irrational real number α is badly approximable if
there exists a constant c(α) > 0 such that |α − pq | >
c(α)q2 , for
every rational pq .
(ii) Suppose α has a continued fraction expansion [a0; a1, a2, . . . ].We say α has bounded partial quotients if there exists a constantK (α) such that
|cn| ≤ K (α). (n = 1, 2, · · · )
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Equivalent characterisations of badly approximability
(i) We say an irrational real number α is badly approximable if
there exists a constant c(α) > 0 such that |α − pq | >
c(α)q2 , for
every rational pq .
(ii) Suppose α has a continued fraction expansion [a0; a1, a2, . . . ].We say α has bounded partial quotients if there exists a constantK (α) such that
|cn| ≤ K (α). (n = 1, 2, · · · )
(i) and (ii) are equivalent.
Corollary : (V. Jarnik 1929) : The set of badly approximablenumbers has Hausdorff dimension 1
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The Field of Formal Power series
Let Fq denote the finite field of q elements, where q is a power ofa prime p. If Z is an indeterminate, we denote by Fq[Z ] and Fq(Z )the ring of polynomials in Z with coefficients in Fq and thequotient field of Fq[Z ], respectively.
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The Field of Formal Power series
Let Fq denote the finite field of q elements, where q is a power ofa prime p. If Z is an indeterminate, we denote by Fq[Z ] and Fq(Z )the ring of polynomials in Z with coefficients in Fq and thequotient field of Fq[Z ], respectively.
For each P,Q ∈ Fq[Z ] with Q 6= 0, define |P/Q| = qdeg(P)−deg(Q)
and |0| = 0.
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The Field of Formal Power series
Let Fq denote the finite field of q elements, where q is a power ofa prime p. If Z is an indeterminate, we denote by Fq[Z ] and Fq(Z )the ring of polynomials in Z with coefficients in Fq and thequotient field of Fq[Z ], respectively. For each P,Q ∈ Fq[Z ] withQ 6= 0, define |P/Q| = qdeg(P)−deg(Q) and |0| = 0.
The field Fq((Z−1)) of formal Laurent series is the completion ofFq(Z ) with respect to the valuation | · |.
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The Field of Formal Power series
Let Fq denote the finite field of q elements, where q is a power ofa prime p. If Z is an indeterminate, we denote by Fq[Z ] and Fq(Z )the ring of polynomials in Z with coefficients in Fq and thequotient field of Fq[Z ], respectively. For each P,Q ∈ Fq[Z ] withQ 6= 0, define |P/Q| = qdeg(P)−deg(Q) and |0| = 0. The fieldFq((Z−1)) of formal Laurent series is the completion of Fq(Z )with respect to the valuation | · |.
That is,
Fq((Z−1)) = anZn + · · ·+ a0 + a−1Z−1 + · · · : n ∈ Z, ai ∈ Fq
and we have |anZn + an−1Zn−1 + · · · | = qn (an 6= 0) and |0| = 0,where q is the number of elements of Fq.
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Haar measure on the field of Formal Power Series
It is worth keeping in mind that | · | is a non-Archimedean norm,since |α + β| ≤ max(|α|, |β|). In fact, Fq((Z−1)) is thenon-Archimedean local field of positive characteristic p.
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Haar measure on the field of Formal Power Series
It is worth keeping in mind that | · | is a non-Archimedean norm,since |α + β| ≤ max(|α|, |β|). In fact, Fq((Z−1)) is thenon-Archimedean local field of positive characteristic p.
As a result, there exists a unique, up to a positive multiplicativeconstant, countably additive Haar measure µq on the Borel subsetsof Fq((Z−1)).
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Haar measure on the field of Formal Power Series
It is worth keeping in mind that | · | is a non-Archimedean norm,since |α + β| ≤ max(|α|, |β|). In fact, Fq((Z−1)) is thenon-Archimedean local field of positive characteristic p. As aresult, there exists a unique, up to a positive multiplicativeconstant, countably additive Haar measure µq on the Borel subsetsof Fq((Z−1)).
Sprindzuk found a characterization of Haar measure on Fq((Z−1))by its value on the ballsB(α; qn) = β ∈ Fq((Z−1)) : |α− β| < qn.
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Haar measure on the field of Formal Power Series
It is worth keeping in mind that | · | is a non-Archimedean norm,since |α + β| ≤ max(|α|, |β|). In fact, Fq((Z−1)) is thenon-Archimedean local field of positive characteristic p. As aresult, there exists a unique, up to a positive multiplicativeconstant, countably additive Haar measure µq on the Borel subsetsof Fq((Z−1)).
Sprindzuk found a characterization of Haar measure on Fq((Z−1))by its value on the ballsB(α; qn) = β ∈ Fq((Z−1)) : |α− β| < qn.
It was shown that the equation µq(B(α; qn)) = qn completelycharacterizes Haar measure here.
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Continued fractions on Fq((Z−1))
For each α ∈ Fq((Z−1)), we can uniquely write
α = A0 +1
A1 +1
A2 +.. .
= [A0; A1,A2, . . . ],
where (An)∞n=0 is a sequence of polynomials in Fq[Z ] with |An| > 1for all n ≥ 1.
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Continued fractions on Fq((Z−1))
For each α ∈ Fq((Z−1)), we can uniquely write
α = A0 +1
A1 +1
A2 +.. .
= [A0; A1,A2, . . . ],
where (An)∞n=0 is a sequence of polynomials in Fq[Z ] with |An| > 1for all n ≥ 1.
We define recursively the two sequences of polynomials (Pn)∞n=0
and (Qn)∞n=0 by
Pn = AnPn−1 + Pn−2 and Qn = AnQn−1 + Qn−2,
with the initial conditions P0 = A0, Q0 = 1, P1 = A1A0 + 1 andQ1 = A1.
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Continued fractions on Fq((Z−1))
For each α ∈ Fq((Z−1)), we can uniquely write
α = A0 +1
A1 +1
A2 +.. .
= [A0; A1,A2, . . . ],
where (An)∞n=0 is a sequence of polynomials in Fq[Z ] with |An| > 1for all n ≥ 1. We define recursively the two sequences ofpolynomials (Pn)∞n=0 and (Qn)∞n=0 by
Pn = AnPn−1 + Pn−2 and Qn = AnQn−1 + Qn−2,
with the initial conditions P0 = A0, Q0 = 1, P1 = A1A0 + 1 andQ1 = A1.Then we have QnPn−1 − PnQn−1 = (−1)n, and whence Pn and Qn
are coprime. In addition, we have Pn/Qn = [A0; A1, . . . ,An].
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Continued fractions map on Fq((Z−1))
Define Tq on the unit ballB(0; 1) = a−1Z−1 + a−2Z−2 + · · · : ai ∈ Fq by
Tqα =
1
α
and T 0 = 0.
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Continued fractions map on Fq((Z−1))
Define Tq on the unit ballB(0; 1) = a−1Z−1 + a−2Z−2 + · · · : ai ∈ Fq by
Tqα =
1
α
and T 0 = 0.
Here anZn + · · ·+ a0 + a−1Z−1 + · · · = a−1Z−1 + a−2Z−2 + · · ·denotes its fractional part.
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Continued fractions map on Fq((Z−1))
Define Tq on the unit ballB(0; 1) = a−1Z−1 + a−2Z−2 + · · · : ai ∈ Fq by
Tqα =
1
α
and T 0 = 0.
Here
anZn + · · ·+ a0 + a−1Z−1 + · · · = a−1Z−1 + a−2Z−2 + · · ·
denotes its fractional part.We note that if α = [0; A1(α),A2(α), . . . ], then we have, for allm, n ≥ 1,
T nα = [0; An+1(α),An+2(α), . . . ] and Am(T nα) = An+m(α).
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Exactness the CF map on Fq((Z−1))
Let (X ,B, µ,T ) be a dynamical system consisting of a set X withthe σ-algebra B of its subsets, a probability measure µ, and atransformation T : X → X . We say that (X ,B, µ,T ) ismeasure-preserving if, for all E ∈ B, µ(T−1E ) = µ(E ).
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Exactness the CF map on Fq((Z−1))
Let (X ,B, µ,T ) be a dynamical system consisting of a set X withthe σ-algebra B of its subsets, a probability measure µ, and atransformation T : X → X . We say that (X ,B, µ,T ) ismeasure-preserving if, for all E ∈ B, µ(T−1E ) = µ(E ).
Let N = E ∈ B : µ(E ) = 0 or µ(E ) = 1 denote the trivialσ-algebra of subsets of B of either null or full measure.
We say that the measure-preserving dynamical system (X ,B, µ,T )is exact if
∞⋂n=0
T−nB = N ,
where T−nB = T−nE : E ∈ B.
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Exactness the CF map on Fq((Z−1))
Let (X ,B, µ,T ) be a dynamical system consisting of a set X withthe σ-algebra B of its subsets, a probability measure µ, and atransformation T : X → X . We say that (X ,B, µ,T ) ismeasure-preserving if, for all E ∈ B, µ(T−1E ) = µ(E ). LetN = E ∈ B : µ(E ) = 0 or µ(E ) = 1 denote the trivial σ-algebraof subsets of B of either null or full measure. We say that themeasure-preserving dynamical system (X ,B, µ,T ) is exact if
∞⋂n=0
T−nB = N ,
where T−nB = T−nE : E ∈ B.
TheoremThe dynamical system (B(0; 1),B, µq,Tq) is exact.(Lertchoosakul, Nair)
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Exactness implies mixing, ergodicity
If (X ,B, µ,T ) is exact, then a number of strictly weaker propertiesarise. Firstly, for any natural number n and any E0,E1, . . . ,En ∈ B,we have
limj1,...,jn→∞
µ(E0 ∩ T−j1E1 ∩ · · · ∩ T−(j1+···+jn)En) = µ(E0)µ(E1) · · ·µ(En).
This is called mixing of order n.
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Exactness implies mixing, ergodicity
If (X ,B, µ,T ) is exact, then a number of strictly weaker propertiesarise. Firstly, for any natural number n and any E0,E1, . . . ,En ∈ B,we have
limj1,...,jn→∞
µ(E0 ∩ T−j1E1 ∩ · · · ∩ T−(j1+···+jn)En) = µ(E0)µ(E1) · · ·µ(En).
This is called mixing of order n. This implies
limm→∞
1
m
m∑j=1
|µ(E0 ∩ T−jE1)− µ(E0)µ(E1)| = 0
which is called weak mixing.
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Exactness implies mixing, ergodicityIf (X ,B, µ,T ) is exact, then a number of strictly weaker propertiesarise. Firstly, for any natural number n and any E0,E1, . . . ,En ∈ B,we have
limj1,...,jn→∞
µ(E0 ∩ T−j1E1 ∩ · · · ∩ T−(j1+···+jn)En) = µ(E0)µ(E1) · · ·µ(En).
This is called mixing of order n. This implies
limm→∞
1
m
m∑j=1
|µ(E0 ∩ T−jE1)− µ(E0)µ(E1)| = 0
which is called weak mixing.Weak-mixing property implies the condition that if E ∈ B and ifT−1E = E , then either µ(E ) = 0 or µ(E ) = 1. This last propertyis referred to as ergodicity in measurable dynamics. All theseimplications are known to be strict in general.
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Good Universality
• A sequence of integers (an)∞n=1 is called Lp-good universal iffor each dynamical system (X ,B, µ,T ) and f ∈ Lp(X ,B, µ)we have
f (x) = limN→∞
1
N
N∑n=1
f (T anx)
existing µ almost everywhere.
• A sequence of real numbers (xn)∞n=1 is uniformly distributedmodulo one if for each interval I ⊆ [0, 1), if |I | denotes itslength, we have
limN→∞
1
N#n ≤ N : xn ∈ I = |I |.
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Subsequence ergodic theory
LemmaIf (anγ)∞n=1 is uniformly distributed modulo one for eachirrational number γ, the dynamical system (X ,B, µ,T ) isweak-mixing and (an)n≥1 is L2-good universal then f (x) exists and
f (x) =
∫X
fdµ
µ almost everywhere.(Nair)
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V. Berthe, H. Nakada
Specializing for instance to the case where F (x) = logq x , werecover the positive characteristic analogue of Khinchin’s famousresult that
limn→∞
|A1(α) · · ·An(α)|1n = q
qq−1
almost everywhere with respect to Haar measure.
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Let (an)∞n=1 be an Lp-good universal sequence with, for anyirrational number γ, (anγ)∞n=1 is uniformly distributed modulo 1.Suppose that F : R≥0 → R is a continuous increasing function with∫
B(0;1)|F (|A1(α)|)|p dµ <∞.
For each n ∈ N and arbitrary non-negative real numbers d1, . . . , dn,we define
MF ,n(d1, . . . , dn) = F−1
(F (d1) + · · ·+ F (dn)
n
).
Then we have
limn→∞
MF ,n(|Aa1(α)|, . . . , |Aan(α)|) = F−1
(∫B(0;1)
F (|A1(α)|) dµ
)
almost everywhere with respect to Haar measure.
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Let (an)∞n=1 be an Lp-good universal sequence with, for anyirrational number γ, (anγ)∞n=1 is uniformly distributed modulo 1.Suppose that H : Nm → R is a function with∫
B(0;1)|H(|A1(α)|, . . . , |Am(α)|)|p dµ <∞.
Then we have
limn→∞
1
n
n∑j=1
H(|Aaj (α)|, . . . , |Aaj+m−1(α)|)
=∑
(i1,...,im)∈Nm
H(qi1 , . . . , qim)
((q − 1)m
qi1+···+im
)
almost everywhere with respect to Haar measure.
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New application 1
Let (an)∞n=1 be an Lp-good universal sequence with, for anyirrational number γ, (anγ)∞n=1 is uniformly distributed modulo 1.Then
limn→∞
1
n
n∑j=1
deg(Aaj (α)) =q
q − 1
almost everywhere with respect to Haar measure. (Lertchoosakul,Nair)Apply with f (α) =
∑∞n=1 n · χqn(|A1(α)|).
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New application 2
Let (an)∞n=1 be an Lp-good universal sequence with, for anyirrational number γ, (anγ)∞n=1 is uniformly distributed modulo 1.Then, for any A ∈ Fq[Z ]∗,
limn→∞
1
n·#1 ≤ j ≤ n : Aaj (α) = A = |A|−2
almost everywhere with respect to Haar measure. (Lertchoosakul,Nair)Apply with f (α) = χA(A1(α)).
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New application 3
Let (an)∞n=1 be an Lp-good universal sequence with, for anyirrational number γ, (anγ)∞n=1 is uniformly distributed modulo 1.Then, for any natural numbers k < l ,
limn→∞
1
n·#1 ≤ j ≤ n : deg(Aaj (α)) = l =
q − 1
ql,
limn→∞
1
n·#1 ≤ j ≤ n : deg(Aaj (α)) ≥ l =
1
ql−1,
limn→∞
1
n·#1 ≤ j ≤ n : k ≤ deg(Aaj (α)) < l =
1
qk−1
(1− 1
ql−k
)almost everywhere with respect to Haar measure. (Lertchoosakul,Nair)
Apply with f1(α) = χql(|A1(α)|), f2(α) = χ[ql ,∞)(|A1(α)|), andf3(α) = χ[qk ,ql )(|A1(α)|), respectively.
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The Gal-Koksma Theorem
Let S be a measurable set. For any non-negative integers M andN, let ϕ(M,N; x) ≥ 0 be a function defined on S such that
(i) ϕ(M, 0; x) = 0 for all M ≥ 0;
(ii) ϕ(M,N; x) ≤ ϕ(M,N ′; x) + F (M + N ′,N − N ′; x) for allM,N ≥ 0 and 0 ≤ N ′ ≤ N.
Suppose that, for all M ≥ 0,∫Sϕ(M,N; x)p dx = O(φ(N)),
where φ(N)/N is a non-decreasing function. Then, given anyε > 0, we have
ϕ(0,N; x) = o(φ(N)1p (log N)1+ 1
p+ε)
almost everywhere x ∈ S .
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New application 4
Suppose that F : R≥0 → R is a function such that∫B(0;1)
|F (|A1(α)|)|2 dµ(α) <∞.
Then, given any ε > 0, we have
1
N
N∑n=1
F (|A1(T nα)|) =
∫B(0;1)
F (|A1(α)|) dµ(α) + o(N−12 (log N)
32
+ε)
almost everywhere with respect to Haar measure.
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New application 5
Suppose that H : Nm → R is a function such that∫B(0;1)
|H(|A1(α)|, |A2(α)|, . . . , |Am(α)|)|2 dµ(α) <∞.
Then, given any ε > 0, we have
1
N
N∑n=1
H(|A1(T nα)|, |A2(T nα)|, . . . , |Am(T nα)|)
=∑
(i1,...,im)∈Nm
H(qi1 , . . . , qim)
((q − 1)m
qi1+···+im
)+ o(N−
12 (log N)
32
+ε)
almost everywhere with respect to Haar measure.
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A special case
Specializing for instance to the case F (x) = logq x , we establishthe positive characteristic analogue of the quantitative version ofKhinchin’s famous result that
|A1(α) · · ·AN(α)|1N = q
qq−1 + o(N−
12 (log N)
32
+ε) (1)
almost everywhere with respect to Haar measure. Results formeans other than the geometric mean can be obtained by makingdifferent choices of F and H.
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New application 6
Given any ε > 0, we have
1
N
N∑n=1
deg(An(α)) =q
q − 1+ o(N−
12 (log N)
32
+ε)
almost everywhere with respect to Haar measure.
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New application 7
Given any A ∈ Fq[Z ]∗ and ε > 0, we have
1
N·#1 ≤ n ≤ N : An(α) = A = |A|−2 + o(N−
12 (log N)
32
+ε)
almost everywhere with respect to Haar measure.
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New application 8
Let k < l be two natural numbers. Given any ε > 0, we have
1
N·#1 ≤ n ≤ N : deg(An(α)) = l =
q − 1
ql+ o(N−
12 (log N)
32
+ε),
1
N·#1 ≤ n ≤ N : deg(An(α)) ≥ l =
1
ql−1+ o(N−
12 (log N)
32
+ε),
1
N·#1 ≤ n ≤ N : k ≤ deg(An(α)) < l =
1
qk−1
(1− 1
ql−k
)+ o(N−
12 (log N)
32
+ε)
almost everywhere with respect to Haar measure.
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DefinitionThe p-adic absolute value of a ∈ Q is defined by
|a|p = p−α and |0|p = 0.
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p-adic numbers
Let p be a prime. Any nonzero rational number a can be written inthe form a = pα(r/s) where α ∈ Z, r , s ∈ Z and p - r , p - s.
DefinitionThe p-adic absolute value of a ∈ Q is defined by
|a|p = p−α and |0|p = 0.
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p-adic numbers
Let p be a prime. Any nonzero rational number a can be written inthe form a = pα(r/s) where α ∈ Z, r , s ∈ Z and p - r , p - s.
DefinitionThe p-adic absolute value of a ∈ Q is defined by
|a|p = p−α and |0|p = 0.
The p-adic field Qp is constructed by completing Q w.r.t. p-adicabsolute value.
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p-adic numbersLet p be a prime. Any nonzero rational number a can be written inthe form a = pα(r/s) where α ∈ Z, r , s ∈ Z and p - r , p - s.
DefinitionThe p-adic absolute value of a ∈ Q is defined by
|a|p = p−α and |0|p = 0.
The p-adic field Qp is constructed by completing Q w.r.t. p-adicabsolute value.
The p-adic absolute value |.|p satisfies the following properties:
1. |a|p = 0 if and only if a = 0,
2. |ab|p = |a|p|b|p for all a, b ∈ Qp,
3. |a + b|p ≤ |a|p + |b|p for all a, b ∈ Qp,
4. |a + b|p ≤ max|a|p, |b|p for all a, b ∈ Qp.
The p-adic absolute value is non-archimedian.
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The topology of Qp
Let a ∈ Qp and r ≥ 0 be a real number. The open ball of radius rcentered at a is the set
B(a, r) = x ∈ Qp : |x − a|p < r.
The closed ball of radius r and center a is the set
B(a, r) = x ∈ Qp : |x − a|p ≤ r.
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The topology of Qp
Let a ∈ Qp and r ≥ 0 be a real number. The open ball of radius rcentered at a is the set
B(a, r) = x ∈ Qp : |x − a|p < r.
The closed ball of radius r and center a is the set
B(a, r) = x ∈ Qp : |x − a|p ≤ r.
The ring of p-adic integers is
Zp = x ∈ Qp : |x |p ≤ 1.
Next, we will consider the setpZp = px : x ∈ Zp = x ∈ Qp : |x |p < 1.
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p-adic continued fraction expansion
Let p be a prime. We will consider the continued fractionexpansion of a p-adic integer x ∈ pZp in the form
x =pa1
b1 +pa2
b2 +pa3
b3 + .. .
(2)
where bn ∈ 1, 2, . . . , p − 1, an ∈ N for n = 1, 2, . . . .
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p-adic continued fraction map
For x ∈ pZp define the map Tp : pZp → pZp to be
Tp(x) =pv(x)
x−
(pv(x)
xmod p
)=
pa(x)
x− b(x) (3)
where v(x) is the p-adic valuation of x , a(x) ∈ N andb(x) ∈ 1, 2, . . . , p − 1.
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p-adic continued fraction map
For x ∈ pZp define the map Tp : pZp → pZp to be
Tp(x) =pv(x)
x−
(pv(x)
xmod p
)=
pa(x)
x− b(x) (4)
where v(x) is the p-adic valuation of x , a(x) ∈ N andb(x) ∈ 1, 2, . . . , p − 1.We will consider the dynamical system (pZp,B, µ,Tp) where B isσ-algebra on pZp and µ is Haar measure on pZp.For the Haar measure it it is the case that µ(pa + pmZp) = p1−m.
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Properties of the p-adic continued fraction map
The following properties are due to Hirsch and Washington (2011).
• Tp is measure-preserving with respect to µ, i.e.µ(T−1
p (A)) = µ(A) for all A ∈ B.
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Properties of the p-adic continued fraction map
The following properties are due to Hirsch and Washington (2011).
• Tp is measure-preserving with respect to µ, i.e.µ(T−1
p (A)) = µ(A) for all A ∈ B.• Tp is ergodic, i.e. µ(B) = 0 or 1 for any B ∈ B with
T−1p (B) = B.
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Properties of the p-adic continued fraction map
The following properties are due to Hirsch and Washington (2011).
• Tp is measure-preserving with respect to µ, i.e.µ(T−1
p (A)) = µ(A) for all A ∈ B.• Tp is ergodic, i.e. µ(B) = 0 or 1 for any B ∈ B with
T−1p (B) = B.
• The p-adic analogue of Khinchin’s Theorem: For almost allx ∈ pZp the p-adic continued fraction expansion satisfies
limn→∞
a1 + a2 + · · ·+ ann
=p
p − 1.
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Other properties of the p-adic continued fraction map
DefinitionLet T be a measure-preserving transformation of a probabilityspace (X ,B, µ). The transformation T is exact if
∞⋂n=0
T−nB = N .
where N = B ∈ B |B = ∅ a.e. or B = X a.e..
Theorem (Hancl, Nair, Lertchoosakul, Jassova)
The p-adic continued fraction map Tp is exact.
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Other properties of the p-adic continued fraction map
Because (pZp,B, µ,Tp) is exact, it implies other strictly weakerproperties:
• Tp is strong-mixing, i.e. for all A,B ∈ B we have
limn→∞
µ(T−np A ∩ B) = µ(A)µ(B)
which impies
• Tp is weak-mixing, i.e. for all A,B ∈ B we have
limn→∞
1
n
n−1∑j=0
|µ(T−jp A ∩ B)− µ(A)µ(B)| = 0
which implies
• Tp is ergodic, i.e. µ(B) = 0 or 1 for any B ∈ B withT−1p (B) = B.
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Good Universality
• A sequence of integers (an)∞n=1 is called Lp-good universal iffor each dynamical system (X ,B, µ,T ) and f ∈ Lp(X ,B, µ)we have
f (x) = limN→∞
1
N
N∑n=1
f (T anx)
existing µ almost everywhere.
• A sequence of real numbers (xn)∞n=1 is uniformly distributedmodulo one if for each interval I ⊆ [0, 1), if |I | denotes itslength, we have
limN→∞
1
N#n ≤ N : xn ∈ I = |I |.
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Subsequence ergodic theory
LemmaIf (anγ)∞n=1 is uniformly distributed modulo one for eachirrational number γ, the dynamical system (X ,B, µ,T ) isweak-mixing and (an)n≥1 is L2-good universal then f (x) exists and
f (x) =
∫X
fdµ
µ almost everywhere.
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Results
Theorem (Hancl, Nair, Lertchoosakul, Jassova)
For any Lp-good universal sequence (kn)n≥1 where (knγ)∞n=1 isuniformly distributed modulo one for each irrational number γ wehave
limN→∞
1
N
N∑n=1
akn =p
p − 1,
and
limN→∞
1
N
N∑n=1
bkn =p
2,
almost everywhere with respect to Haar measure on pZp.
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Results
Theorem (Hancl, Nair, Lertchoosakul, Jassova)
For any Lp-good universal sequence (kn)n≥1 where (knγ)∞n=1 isuniformly distributed modulo one for each irrational number γ wehave
limN→∞
1
N#1 ≤ n ≤ N : akn = i =
p − 1
pi;
limN→∞
1
N#1 ≤ n ≤ N : akn ≥ i =
1
pi−1;
limN→∞
1
N#1 ≤ n ≤ N : i ≤ akn < j =
1
pi−1
(1− 1
pj
);
almost everywhere with respect to Haar measure on pZp.
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Partitions
Let (X ,A,m) be a probability space where X is a set, A is aσ-algebra of its subsets and m is a probability measure. A partitionof (X ,A,m) is defined as a denumerable collection of elementsα = A1,A2, . . . of A such that
Ai ∩ Aj = ∅
for all i , j ∈ I , i 6= j and ⋃i∈I
Ai = X .
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Partitions
Let (X ,A,m) be a probability space where X is a set, A is aσ-algebra of its subsets and m is a probability measure. A partitionof (X ,A,m) is defined as a denumerable collection of elementsα = A1,A2, . . . of A such that Ai ∩ Aj = ∅ for all i , j ∈ I , i 6= jand
⋃i∈I Ai = X . For a measure-preserving transformation T we
haveT−1α = T−1Ai |Ai ∈ α
which is also a denumerable partition.
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Partitions
Let (X ,A,m) be a probability space where X is a set, A is aσ-algebra of its subsets and m is a probability measure. A partitionof (X ,A,m) is defined as a denumerable collection of elementsα = A1,A2, . . . of A such that Ai ∩ Aj = ∅ for all i , j ∈ I , i 6= jand
⋃i∈I Ai = X . For a measure-preserving transformation T we
have T−1α = T−1Ai |Ai ∈ α which is also a denumerablepartition.Given partitions α = A1,A2, . . . and β = B1,B2, . . . we definethe join of α and β to be the partition
α ∨ β = Ai ∩ Bj |Ai ∈ α,Bj ∈ β
.
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Entropy of a Partition
For a finite partition α = A1, . . . ,An we define its entropyH(α) = −
∑ni=1 m(Ai ) log m(Ai ). Let A′ ⊂ A be a sub-σ-algebra.
Then we define the conditional entropy of α given A′ to beH(α|A′) = −
∑ni=1 m(Ai |A′) log m(Ai |A′).
Here of course m(A|A′) means E(χA|A′) where E(.|A′) denotesthe projection operator L1(X ,A,m)→ L1(X ,A′,m) and χA is thecharacteristic function of the set A.
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Entropy of a transformation
The entropy of a measure-preserving transformation T relative to apartition α is defined to be
hm(T , α) = limn→∞
1
nH
(n−1∨i=0
T−iα
)
where the limit always exists.
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Entropy of a transformation
The entropy of a measure-preserving transformation T relative to apartition α is defined to be
hm(T , α) = limn→∞
1
nH
(n−1∨i=0
T−iα
)
where the limit always exists. The alternative formula for hm(T , α)which is used for calculating entropy is
hm(T , α) = limn→∞
H
(α|
n∨i=1
T−iα
)= H
(α|∞∨i=1
T−iα
). (5)
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Entropy of a transformation
The entropy of a measure-preserving transformation T relative to apartition α is defined to be
hm(T , α) = limn→∞
1
nH
(n−1∨i=0
T−iα
)
where the limit always exists. The alternative formula for hm(T , α)which is used for calculating entropy is
hm(T , α) = limn→∞
H
(α|
n∨i=1
T−iα
)= H
(α|∞∨i=1
T−iα
). (6)
We define the measure-theoretic entropy of T with respect to themeasure m (irrespective of α) to be hm(T ) = supα hm(T , α) wherethe supremum is taken over all finite or countable partitions αfrom A with H(α) <∞.
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Theorem (Jassova,Nair)
Let B denote the Haar σ-algebra restricted to pZp and let µdenote Haar measure on pZp. Then the measure-preservingtransformation (pZp,B, µ,Tp) has measure-theoretic entropyp
p−1 log p.
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Isomorphism of measure preserving transformations
Suppose (X1, β1,m1,T1) and (X2, β2,m2,T2) are two dynamicalsystems.
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Isomorphism of measure preserving transformations
Suppose (X1, β1,m1,T1) and (X2, β2,m2,T2) are two dynamicalsystems.
They are said to be isomorphic if there exist sets M1 ⊆ X1 andM2 ⊆ X2 with m1(M1) = 1 and m2(M2) = 1 such thatT1(M1) ⊆ M1 and T2(M2) ⊆ M2 and such that there exists a mapφ : M1 → M2 satisfying φT1(x) = T2φ(x) for all x ∈ M1 andm1(φ−1(A)) = m2(A) for all A ∈ β2.
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Isomorphism of measure preserving transformations
Suppose (X1, β1,m1,T1) and (X2, β2,m2,T2) are two dynamicalsystems.
They are said to be isomorphic if there exist sets M1 ⊆ X1 andM2 ⊆ X2 with m1(M1) = 1 and m2(M2) = 1 such thatT1(M1) ⊆ M1 and T2(M2) ⊆ M2 and such that there exists a mapφ : M1 → M2 satisfying φT1(x) = T2φ(x) for all x ∈ M1 andm1(φ−1(A)) = m2(A) for all A ∈ β2.
The importance of measure theoretic entropy, is that twodynamical systems with different entropies can not be isomorphic.
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Bernoulli Space
Suppose (Y , α, l) is a probability space, and let(X , β,m) = Π∞−∞(Y , α, l) i.e. the bi-infinite product probabilityspace.
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Bernoulli Space
Suppose (Y , α, l) is a probability space, and let(X , β,m) = Π∞−∞(Y , α, l) i.e. the bi-infinite product probabilityspace.
For shift map τ(xn) = (xn+1), the measure preservingtransformation (X , β,m, τ) is called the Bernoulli process withstate space (Y , α, l). Here xn is a bi-infinite sequence ofelements of the set Y .
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Bernoulli Space
Suppose (Y , α, l) is a probability space, and let(X , β,m) = Π∞−∞(Y , α, l) i.e. the bi-infinite product probabilityspace.
For shift map τ(xn) = (xn+1), the measure preservingtransformation (X , β,m, τ) is called the Bernoulli process withstate space (Y , α, l). Here xn is a bi-infinite sequence ofelements of the set Y .
Any measure preserving transformation isomorphic to a Bernoulliprocess will be refered to as Bernoulli.
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Ornstein’s theorem
The fundamental fact about Bernoulli processes, famously provedby D. Ornstein in 1970, is that Bernoulli processes with the sameentropy are isomorphic.
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The natural extention
To any measure-preserving transformation, (X , β,m,T0) setX∞ = Π∞n=0X and set
XT0 = x = (x0, x1, . . . ) ∈ X∞ : xn = T0(xn+1), xn ∈ X , n = 0, 1, 2, . . . .
Let T : XT0 → XT0 be defined by
T ((x0, x1, . . . , )) = (T (x0), x0, x1, . . . , ).
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The natural extention
To any measure-preserving transformation, (X , β,m,T0) setX∞ = Π∞n=0X and set
XT0 = x = (x0, x1, . . . ) ∈ X∞ : xn = T0(xn+1), xn ∈ X , n = 0, 1, 2, . . . .
Let T : XT0 → XT0 be defined by
T ((x0, x1, . . . , )) = (T (x0), x0, x1, . . . , ).
The map T is bijective on XT0 . If T0 preserves a measure m, thenwe can define a measure m on XT0 , by defining m on the cylindersets C (A0,A1, . . . ,Ak) = x : x0 ∈ A0, x1 ∈ A1, . . . , xk ∈ Ak by
m(C (A0,A1, . . . ,Ak)) = m(T−k0 (A0) ∩ T−k+10 (A1) ∩ . . . ∩ Ak),
for k ≥ 1.
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The natural extentionTo any measure-preserving transformation, (X , β,m,T0) setX∞ = Π∞n=0X and set
XT0 = x = (x0, x1, . . . ) ∈ X∞ : xn = T0(xn+1), xn ∈ X , n = 0, 1, 2, . . . .
Let T : XT0 → XT0 be defined by
T ((x0, x1, . . . , )) = (T (x0), x0, x1, . . . , ).
The map T is bijective on XT0 . If T0 preserves a measure m, thenwe can define a measure m on XT0 , by defining m on the cylindersets C (A0,A1, . . . ,Ak) = x : x0 ∈ A0, x1 ∈ A1, . . . , xk ∈ Ak by
m(C (A0,A1, . . . ,Ak)) = m(T−k0 (A0) ∩ T−k+10 (A1) ∩ . . . ∩ Ak),
for k ≥ 1. One can check that the invertable transformation(XT0 , β,m,T0) called the natural extention of (X , β,m,T0) ismeasure preserving as a consequence of the measure preservationof the transformation (X , β,m,T0).
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Fundamental Dynamical property of the Schneider Map
Theorem (Jassova,Nair)
Suppose (pZp,B, µ,Tp) is the Schneider continued fraction map.Then the dynamical system (pZp,B, µ,Tp) has a natural extentionthat is Bernoulli.
This property implies all the mixing properties of the map and viaergodic theorems all the properties of averages of convergents.Also, via Ornstein’s theorem, it is isomphorphic as a dynamicalsystem to all Bernoulli shifts with the same entropy and hence iscompletely classified.
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Absolute values on topological fields
Let K denote a topological field. By this we mean that the field Kis a locally compact group under the addition , with respect atopology (which in our case is discrete). This ensures that Kcomes with a translation invariant Haar measure µ on K , that isunique up to scalar multiplication. For an element a ∈ K , we arenow able it absolute value, as
|a| =µ(aX )
µ(X ),
for every µ masureable X ⊆ K of finite µ measure.
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Absolute values on topological fields
Let K denote a topological field. By this we mean that the field Kis a locally compact group under the addition , with respect atopology (which in our case is discrete). This ensures that Kcomes with a translation invariant Haar measure µ on K , that isunique up to scalar multiplication. For an element a ∈ K , we arenow able it absolute value, as
|a| =µ(aX )
µ(X ),
for every µ masureable X ⊆ K of finite µ measure.
An absolute value is a function |.| : K → R≥0 such that (i) |a| = 0if and only if a = 0; (ii) |ab| = |a||b| for all a, b ∈ K and (iii)|a + b| ≤ |a|+ |b| for all pairs a, b ∈ K . The absolute value justdefined gives rise to a metric defined by d(a, b) = |a− b| witha, b ∈ K , whose topology coincides with original topology on thefield K .
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Archemedian and Non-Archemedian
Topological fields come in two types:
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Archemedian and Non-Archemedian
Topological fields come in two types:
(a)The first where (iii) can be replaced by the stronger condition(iii)* |a + b| ≤ max(|a|, |b|) a, b ∈ K , called non-archimedeanspaces
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Archemedian and Non-Archemedian
Topological fields come in two types:
(a)The first where (iii) can be replaced by the stronger condition(iii)* |a + b| ≤ max(|a|, |b|) a, b ∈ K , called non-archimedeanspaces and
(b) spaces where (iii)* is not true called archimedean spaces.
From now on we shall concern ourselves solely withnon-archimedean fields.
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Valuations and absolute values
Another approach to defining a non-archimedan field is via discretevaluations
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Valuations and absolute values
Another approach to defining a non-archimedan field is via discretevaluations.
Let K ∗ = K\0. A map v : K ∗ → R is a valuation if(i) v(K ∗) 6= 0;(ii) v(xy) = v(x) + v(y) for x , y ∈ Kand(iii) v(x + y) ≥ minv(x), v(y).
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Valuations and absolute values
Another approach to defining a non-archimedan field is via discretevaluations. Let K ∗ = K\0. A map v : K ∗ → R is a valuation if(i) v(K ∗) 6= 0; (ii) v(xy) = v(x) + v(y) for x , y ∈ K and (iii)v(x + y) ≥ minv(x), v(y). Two valuations v and cv , for c > 0 areal constant, are called equivalent.
A valuation determines a non-trivial non-Archimedean absolutevalue and vice versa.
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Valuations and absolute values
Another approach to defining a non-archimedan field is via discretevaluations. Let K ∗ = K\0. A map v : K ∗ → R is a valuation if(i) v(K ∗) 6= 0; (ii) v(xy) = v(x) + v(y) for x , y ∈ K and (iii)v(x + y) ≥ minv(x), v(y). Two valuations v and cv , for c > 0 areal constant, are called equivalent. A valuation determines anon-trivial non-Archimedean absolute value and vice versa.
We extend v to K formally by letting v(0) = 1.
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Valuations and absolute values
Another approach to defining a non-archimedan field is via discretevaluations. Let K ∗ = K\0. A map v : K ∗ → R is a valuation if(i) v(K ∗) 6= 0; (ii) v(xy) = v(x) + v(y) for x , y ∈ K and (iii)v(x + y) ≥ minv(x), v(y). Two valuations v and cv , for c > 0 areal constant, are called equivalent. A valuation determines anon-trivial non-Archimedean absolute value and vice versa. Weextend v to K formally by letting v(0) = 1.
The image v(K ∗) is an additive subgroup of R, called the valuegroup of v .
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Valuations and absolute values
Another approach to defining a non-archimedan field is via discretevaluations. Let K ∗ = K\0. A map v : K ∗ → R is a valuation if(i) v(K ∗) 6= 0; (ii) v(xy) = v(x) + v(y) for x , y ∈ K and (iii)v(x + y) ≥ minv(x), v(y). Two valuations v and cv , for c > 0 areal constant, are called equivalent. A valuation determines anon-trivial non-Archimedean absolute value and vice versa. Weextend v to K formally by letting v(0) = 1. The image v(K ∗) isan additive subgroup of R, called the value group of v .
If it is discrete, i.e., isomorphic to Z, we say v is a discretevaluation.
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Valuations and absolute values
Another approach to defining a non-archimedan field is via discretevaluations. Let K ∗ = K\0. A map v : K ∗ → R is a valuation if(i) v(K ∗) 6= 0; (ii) v(xy) = v(x) + v(y) for x , y ∈ K and (iii)v(x + y) ≥ minv(x), v(y). Two valuations v and cv , for c > 0 areal constant, are called equivalent. A valuation determines anon-trivial non-Archimedean absolute value and vice versa. Weextend v to K formally by letting v(0) = 1. The image v(K ∗) isan additive subgroup of R, called the value group of v . If it isdiscrete, i.e., isomorphic to Z, we say v is a discrete valuation.
If v(K ∗) = Z, we call v a normalised discrete valuation.
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Valuations and absolute values
Another approach to defining a non-archimedan field is via discretevaluations. Let K ∗ = K\0. A map v : K ∗ → R is a valuation if(i) v(K ∗) 6= 0; (ii) v(xy) = v(x) + v(y) for x , y ∈ K and (iii)v(x + y) ≥ minv(x), v(y). Two valuations v and cv , for c > 0 areal constant, are called equivalent. A valuation determines anon-trivial non-Archimedean absolute value and vice versa. Weextend v to K formally by letting v(0) = 1. The image v(K ∗) isan additive subgroup of R, called the value group of v . If it isdiscrete, i.e., isomorphic to Z, we say v is a discrete valuation. Ifv(K ∗) = Z, we call v normalised discrete valuation.
To our initial valuation we associate the valuation described asfollows. Pick 0 < α < 1 and write |a| = αv(a), i.e., letv(a) = logα |a|. Then v(a) is a valuation, an additive version of |a|.
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Rings of integers and maximal ideals
Let v : K ∗ → R be a valuation corresponding to the absolute value|.| : K → R≥0.
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Rings of integers and maximal ideals
Let v : K ∗ → R be a valuation corresponding to the absolute value|.| : K → R≥0.
Then
O = Ov := x ∈ K : v(x) ≥ 0 = OK := x ∈ K : |x | ≤ 1
is a ring, called the valuation ring of v .
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Rings of integers and maximal ideals
Let v : K ∗ → R be a valuation corresponding to the absolute value|.| : K → R≥0.
Then
O = Ov := x ∈ K : v(x) ≥ 0 = OK := x ∈ K : |x | ≤ 1
is a ring, called the valuation ring of v .
K is its field of fractions, and if x ∈ K\O then 1x ∈ O.
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Rings of integers and maximal ideals
Let v : K ∗ → R be a valuation corresponding to the absolute value|.| : K → R≥0.
Then
O = Ov := x ∈ K : v(x) ≥ 0 = OK := x ∈ K : |x | ≤ 1
is a ring, called the valuation ring of v .
K is its field of fractions, and if x ∈ K\O then 1x ∈ O.
The set of units in O areO× = x ∈ K : v(x) = 0 = x ∈ K : |x | = 1andM = x ∈ K : v(x) > 0 = x ∈ K : |x | < 1 is an ideal in O.k = O/M is a field, called the residue field of v or of K .
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The structure of maximal ideals
In the sequel throughout these lectures we assume that k is a finitefield.
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The structure of maximal ideals
In the sequel throughout these lectures we assume that k is a finitefield.
Suppose v : K ∗ → Z is normalised discrete.
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The structure of maximal ideals
In the sequel throughout these lectures we assume that k is a finitefield.
Suppose v : K ∗ → Z is normalised discrete.
Take π ∈M such that v(π) = 1. We call π a uniformiser.
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The structure of maximal ideals
In the sequel throughout these lectures we assume that k is a finitefield.
Suppose v : K ∗ → Z is normalised discrete.
Take π ∈M such that v(π) = 1. We call π a uniformiser.
Then every x ∈ K can be written uniquely as x = uπn withu ∈ O× and n ∈ Z≥0. Also every x ∈M can be written uniquelyas x = uπn for a unit u ∈ O× and n ≥ 1.
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The structure of maximal ideals
In the sequel throughout these lectures we assume that k is a finitefield.
Suppose v : K ∗ → Z is normalised discrete.
Take π ∈M such that v(π) = 1. We call π a uniformiser.
Then every x ∈ K can be written uniquely as x = uπn withu ∈ O× and n ∈ Z≥0. Also every x ∈M can be written uniquelyas x = uπn for a unit u ∈ O× and n ≥ 1.
In particular, M = (πn) is a principal ideal .
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The structure of maximal ideals
In the sequel throughout these lectures we assume that k is a finitefield.
Suppose v : K ∗ → Z is normalised discrete.
Take π ∈M such that v(π) = 1. We call π a uniformiser.
Then every x ∈ K can be written uniquely as x = uπn withu ∈ O× and n ∈ Z≥0. Also every x ∈M can be written uniquelyas x = uπn for a unit u ∈ O× and n ≥ 1.
In particular, M = (πn) is a principal ideal .
Moreover, every ideal I ⊂ O is principal, as (0) 6= I ⊂ O impliesI = (πn) where n = minv(x) : x ∈ I, so O is a principal idealdomain (PID).
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There are two examples
(i) The p-adic numbers Qp and their finite extentions. For instanceif K = Qp then O = Zp M = pZp. Here we can take π = p.
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There are two examples
(i) The p-adic numbers Qp and their finite extentions. For instanceif K = Qp then O = Zp M = pZp. Here we can take π = p.
(ii) The field of formal power series K = Fq((X−1)) for q = pn forsome prime p, with O = Fq[X ] and M = I (x)Fq[X ] for someirreducible polynomial I . Here we can take π = I .
These two are the only two possibilities. This is the structuretheorem for non-archemedian fields.
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Schneider’s Map on an arbitrary non-archemedean field
We define the map Tv :M→M defined by
Tv (x) =πv(x)
x− b(x)
where b(x) denotes the residue class to which πv(x)
x in k .This gives rise to the continued fraction expansion of x ∈M in theform
x =πa1
b1 +πa2
b2 +πa3
b3 + .. .
(7)
where bn ∈ k×, an ∈ N for n = 1, 2, . . . .
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The start of continued fractions on a non-archemedeanfield
The rational approximants to x ∈M arise in a manner similar tothat in the case of the real numbers as follows. We supposeA0 = b0,B0 = 1,A1 = b0b1 + πa1 ,B1 = b1. Then set
An = πanAn−2 + bnAn−1 and Bn = πanBn−2 + bnBn−1 (8)
for n ≥ 2. A simple inductive argument gives for n = 1, 2, . . . .
An−1Bn − AnBn−1 = (−1)nπa1+...+an . (9)
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Dynamics of the Schneider’s map on a non-archemedeanfield
The map Tv :M→M preserves Haar measure on M. We alsohave the following.
TheoremLet B denote the Haar σ-algebra restricted to M and let µ denoteHaar measure on M. Then the measure-preserving transformation(M,B, µ,Tv ) has measure-theoretic entropy |k|
|k×| log(|k|).
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Dynamics of the Schneider’s map on a non-archemedeanfield
The map Tv :M→M preserves Haar measure on M. We alsohave the following.
TheoremLet B denote the Haar σ-algebra restricted to M and let µ denoteHaar measure on M. Then the measure-preserving transformation(M,B, µ,Tv ) has measure-theoretic entropy |k|
|k×| log(|k|).
TheoremSuppose (M,B, µ,Tv ) is as in our first theorem. Then thedynamical system (M,B, µ,Tv ) has a natural extension that isBernoulli.
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Dynamics of the Schneider’s map on a non-archemedeanfield
The map Tv :M→M preserves Haar measure on M. We alsohave the following.
TheoremLet B denote the Haar σ-algebra restricted to M and let µ denoteHaar measure on M. Then the measure-preserving transformation(M,B, µ,Tv ) has measure-theoretic entropy |k|
|k×| log(|k|).
TheoremSuppose (M,B, µ,Tv ) is as in our first theorem. Then thedynamical system (M,B, µ,Tv ) has a natural extension that isBernoulli.This tells us the isomorphism class of the dynamical system(M,B, µ,Tv ) is determined by its residue class field irrespective ofthe characteristic. This means for different p each Sneider map onthe p-adics non-isomorphic.
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Results
Theorem (Nair,Jassova)
For any Lp-good universal sequence (kn)n≥1 where (knγ)∞n=1 isuniformly distributed modulo one for each irrational number γ wehave
limN→∞
1
N
N∑n=1
akn =|k ||k×|
,
and
limN→∞
1
N
N∑n=1
bkn =|k |2,
almost everywhere with respect to Haar measure on M.
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Results
Theorem (Nair, Jassova)
For any Lp-good universal sequence (kn)n≥1 where (knγ)∞n=1 isuniformly distributed modulo one for each irrational number γ wehave
limN→∞
1
N#1 ≤ n ≤ N : akn = i =
|k×||k|i
;
limN→∞
1
N#1 ≤ n ≤ N : akn ≥ i =
1
|k |i−1;
limN→∞
1
N#1 ≤ n ≤ N : i ≤ akn < j =
1
|k |i−1
(1− 1
|k |j
);
almost everywhere with respect to Haar measure on M.
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Thank you for your attention.