analytic construction of points on modular elliptic curves
TRANSCRIPT
Analytic construction of pointson modular elliptic curves
Number Theory Study GroupUniversity of York
Xavier Guitart 1 Marc Masdeu 2 Mehmet Haluk Sengun 3
1Universitat de Barcelona
2University of Warwick
3University of Sheffield
May 26, 2015
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 0 / 23
Elliptic Curves
Let E be an elliptic curve defined over Q:
Y 2 + a1Y + a3XY = X3 + a2X2 + a4X + a6, ai ∈ Z
Let K/Q be a number field, and consider the abelian group
E(K) = (x, y) ∈ K2 : y2 +a1y+a3xy = x3 +a2x2 +a4x+a6∪O.
Theorem (Mordell–Weil)E(K) is finitely generated: E(K) ∼= (Torsion)⊕ Zr.
The integer r = rkZE(K) is called the algebraic rank of E(K).I Open problem: Given E and K, find r.
Theorem (Weierstrass Uniformization)There exists a computable complex-analytic group isomorphism
η : C/ΛE → E(C), ΛE = lattice of rank 2.Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 1 / 23
The Hasse-Weil L-function
Suppose that K = Q(√D) is quadratic.
Can reduce the coefficients a1, . . . , a6 modulo primes p.I For almost all primes, the reduction is an elliptic curve (nonsingular).I Obviously E(Fp) is finite.
The conductor of E is an integer N encoding the shape of E whenthis reduction is singular.
I Assume that N is square-free, coprime to disc(K/Q).
The L-function of E/K (Re(s) > 3/2)
L(E/K, s) =∏p|N
(1− ap|p|−s
)−1 ×∏p-N
(1
ap(E) = 1 + |p| −#E(Fp).
− ap|p|−s + |p|1−2s)−1.
Modularity (Wiles, Taylor–Wiles, Breuil–Conrad–Diamond–Taylor)=⇒
I Analytic continuation of L(E/K, s) to C.I Functional equation relating s↔ 2− s.
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 2 / 23
The BSD conjecture
Bryan Birch Sir Peter Swinnerton-Dyer
BSD conjecture (coarse version)
ords=1 L(E/K, s) = rkZE(K).
So L(E/K, 1) = 0BSD=⇒ ∃PK ∈ E(K) of infinite order.
Open problem: construct such PK .Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 3 / 23
Modular forms
Let N > 0 be an integer and consider
Γ0(N) = (a bc d
)∈ SL2(Z) : N | c.
Recall the upper-half plane H = z ∈ C : Im(z) > 0.I Γ0(N) acts on H via
(a bc d
)· z = az+b
cz+d .A cusp form of level N is a holomorphic map f : H→ C such that:
1 f(γz) = (cz + d)2f(z) for all γ =(a bc d
)∈ Γ0(N).
2 Cuspidal: limz→i∞ f(z) = 0.Since ( 1 1
0 1 ) ∈ Γ0(N), have Fourier expansions
f(z) =∞∑n=1
an(f)e2πinz.
The (finite) vector space of all cusp forms is denoted by S2(Γ0(N)).There is a family of commuting linear operators (Hecke algebra)acting on S2(Γ0(N)), indexed by integers coprime to N .
I A newform is a simultaneous eigenvector for the Hecke algebra.
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 4 / 23
Modularity
Theorem (Modularity, automorphic version)Given an elliptic curve E, there exists a newform fE ∈ S2(Γ0(N)) s.t.
ap(fE) = 1 + p−#E(Fp), for all p - N.
The complex manifold Y0(N)(C) = Γ0(N)\H can be compactified byadding a finite set of points (cusps), yielding X0(N)(C).Shimura proved that X0(N)(C) is the set of C-points of an algebraic(projective) curve X0(N) defined over Q.
Theorem (Modularity, geometric version)Given an elliptic curve E, there exists a surjective morphism
φE : X0(N)/Q→ E.
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 5 / 23
The main tool for BSD: Heegner points
Kurt Heegner
Only available when K = Q(√D) is imaginary: D < 0.
I will define Heegner points under the additional condition:I Heegner hypothesis: p | N =⇒ p split in K.
This ensures that ords=1 L(E/K, s) is odd (so ≥ 1).Modularity is crucial in the construction.
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 6 / 23
Heegner Points (K/Q imaginary quadratic)
Modularity =⇒ modular form fE attached to E.
ωE = 2πifE(z)dz = 2πi∑n≥1
an(f)e2πinzdz ∈ Ω1Γ0(N)\H.
I Can be obtained simply by counting points of E(Fp) (very efficient).
Given τ ∈ K ∩H, set Jτ =
∫ τ
i∞ωE ∈ C.
Well-defined up to ΛE =∫
γ ωE | γ ∈ H1 (X0(N),Z)
.
Set Pτ = η(Jτ ) ∈ E(C).Fact: Pτ ∈ E(Hτ ), where Hτ/K is a ring class field attached to τ .
Theorem (Gross–Zagier, Kolyvagin)1 PK = TrHτ/K(Pτ ) is nontorsion ⇐⇒ L′(E/K, 1) 6= 0.2 If ords=1 L(E/Q, s) ≤ 1 then BSD holds for E(Q).
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 7 / 23
Computing in practice: an example of Mark WatkinsLet E be the elliptic curve of conductor NE = 66157667:
E : Y 2 + Y = X3 − 5115523309X − 140826120488927.
Watkins worked with 460 digits of precision and 600M terms of theL-series. Took less than a day (in 2006). The x-coordinate of the pointhas numerator:3677705371866775066140056423418271700879322694922855847262187700616535463492710158053651343703267430611413064645000528867046519983997664788407919153078617415072739338026281573250924797082687602171017553858718167805487654785022844156276828471927526818990949626599378706300367603592935770218062374839710749312284163465078523816968832276500720399644815972159959932997449341171062898503893640065524978358777402575345331137752028822100483561636459193457948120745710296608971732243703377010561657350085906402970902987091215062666972664619932018253973699995508681422943127563221774107305328280647596049753692423509935680307269370499116072641097827468479512837941192989412144907943309029865829912295694015235199387427463761071907702040105138183490127866378892547110594555551738109049119276198990318551492923253385898319797370264027110497425941160003806014808399829755575060358517280356452410442291650296493470492891191885968694011593251313633459625795031323398472754224400945538247051892256536774595128631179117218385529343091245081344933664374080939243620397499119074169735041423221117570585842007250226321161647201649986417295226774605259994990779421258204288795260637356926859910185168629387960475973239865371541712483169437963732171919939969937146546295368843960579247909386476566632815961781457221160982165009303338243218067269370181901361905565732088070483553355670787931266569286578590367793505932745987173797308807240343018677394437498418094567158841937203289014615526598826284058422097567571678166621399450818646421085335959899757162592592401528340509406544796171476859225008569444498220453860921224090969785448172188478976405134778065983291776042463808123777390491844755507773416209859765703930378802827649670195524084007307548226764414817153853440019798322326524148883358655673772143604560032969616681774819448090662574425967723478296641269729319041016852811289447800746467967609424309596170222574798740894035649650388853798178669200489298145202684936775070590737659026716380873664884967028363262685745931232451074203488781017631238933476570202755912488242478005942708620520821859733932900091898676772594580806760650987034535395255769756395437005076256407298723407894063143944684005844559206833619762001218344307512339014732284974905619980784862510749935288713187974033480873704269009975564425770812549105721851078566051398773310150428421211060806907435781732684894004990568983126219539479670123584145477528970810970917957442036976840460662556632012422927601267598712660045163774329619172720402171470835633999876124205952757920338556769918233682548621595584500438080514815332972700352873822470382792932239463850701180823069589872686033969240544031038574440588486055874154005176700326311212061277324813403910882777964885444157381565530147684062461546660051396904280851450982725007914162147746734845018267225005270911649442625371695958489316807540967747128604905727462240940311870432045261072392010796034682975228951065985674370150833487978753641627976939688198041395488857512826871522370782603587052302844262030644936842506142828799181077337962070672500038239594129356776240932360470386373655773263995890088045077860119731559277310730347065365574614438066227076224110878093718721572104568368924936138367920267618203822171654819989241236047827879232297391719205754470070995016783807950770131133259898013857299939208183016544242513395646068768201219283722462133998592132827925111680439534438397939011399741944793002975660976645391993846519084361887324288183733023830463888594279378938418880142666851776166056447837041357949318307502656863359340665652409440494482130055919971289855607602603992142786359126343515867623548693540215307461899928995825545976321083096385692969648000469830727362384831490147146008960565520296427479914190634547491420595642742982546549258938664049551469033300244757461635437149962496524201711710542317263364935415869714317789440514810596337383994114185743238117709497297268436126729250006313556598341642005544413154510034334524662047071238116636236628372968629480617587599286317636619851856158018862057707210320063041448677873470583163922956715800916558720872094859132869301288586404425891254542685803974845719210123188723116248983176156076281764600974413363235490318282359656362779508273280875479395111123742164365842033792484501226474060940351711307406637235476759398859593638811358930351020183894442127461462503283482426106735240223789949783920200988147219745020626928157366892297590658220939427953187053452755989894263352359355056053114113015603211922694308617337435444029085864973053536009094312149332025225287171092144929593300160658102876231441792884666648885406227023467042137524563725744495639792157824065669378853529458719945417708388719305422203077716714984665181087226221094216767415449456954035098669531672776282802324648392150034740488969680375446600297557400655812701390832499032125722304179422497954671007003939443103250096771791821099709433468073350144468396122825088243240736795841228512083604591663154848919522994493400258965092989359393577217235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Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 8 / 23
Heegner Points: revealing the trick
Why did this work?
1 The Riemann surface Γ0(N)\H has an algebraic model X0(N)/Q.
2 Existence of the morphism φE defined over Q:
φE : X0(N)→ E. (geometric modularity)
3 CM theory shows that τ ∈ Γ0(N)\H are defined in X0(N)(Hτ ).An explicit description of φ shows that:
Pτ = φ(τ) ∈ E(Hτ ).
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 9 / 23
Generalization
One can replace Q with any totally real field F , m = [F : Q].I i.e. the defining polynomial of F factors completely over R.
Consider an elliptic curve E defined over F , of conductor NE .The field K/F needs then to be a CM extension.
I i.e. the defining polynomial for K over Q has no linear terms over R.
Suppose that NE is coprime to the discriminant of K/F .Let Sf (E,K) = p | NE : p inert in K.The Heegner hypothesis can be relaxed to:
(new) Heegner Hypothesis: m+ #Sf (E,K) is odd.
I This still ensures that ords=1 L(E/K, s) is odd.I Old Heegner hypothesis is equivalent to Sf (E,K) = ∅ (and m = 1).
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 10 / 23
Darmon’s Idea
Henri Darmon
What if K/F is not CM?
I Simplest case: F = Q, K real quadratic.
I Or what if F is not totally real?
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 11 / 23
However. . .
. . . this will get us in trouble!
1 Algebraic model X/F .
2 Geometric modularity: φE : X → E.
3 CM points τ ∈ X(Hτ ).
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 12 / 23
History
Henri Darmon Adam Logan Xevi Guitart Jerome Gartner
H. Darmon (2000): F totally real, Sf (E,K) = ∅.I Darmon-Logan (2003): F quadratic norm-euclidean, NE trivial.I Guitart-M. (2011): F quadratic norm-euclidean, NE trivial.I Guitart-M. (2012): F quadratic norm-euclidean, NE trivial.
J. Gartner (2010): F totally real, Sf (E,K) 6= ∅.I ?
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 13 / 23
Notation
Let E be an elliptic curve over a number field F .I If v is an infinite real place of F , then:
1 It may extend to two real places of K (splits), or2 It may extend to one complex place of K (ramifies).
I If v is complex, then it extends to two complex places of K (splits).
n = #v | ∞F : v splits in K.
K/F is CM ⇐⇒ n = 0.Can compute only n ≤ 1, although construction works in general.
S(E,K) =v | NE∞F : v not split in K
.
Sign of functional equation for L(E/K, s) should be (−1)#S(E,K).I All constructions assume that #S(E,K) is odd.I In this talk: assume that S(E,K) = ν, with ν an infinite place.
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 14 / 23
Darmon’s quadratic ATR points
Let E be defined over a real quadratic F (with h+F = 1).
Suppose that K/F is an ATR extension:I The 1st embedding v1 of F extends to one complex place of K.I The 2nd embedding v2 of F extends to two real places of K.
Suppose that p | NE =⇒ p is split in K ( =⇒ S(E,K) = v1).Let τ ∈ K \ F . One has StabΓ0(NE)(v1(τ)) = 〈γτ 〉.Given τ2 ∈ H, consider the geodesic path joining τ2 with τ ′2 = γττ2.
×H H
τ1τ2
τ ′2
γτ
Γ0(NE)
X0(NE)
Fact: τ1 × γτ in Z1(Γ0(NE)\(H×H),Z) is null-homologous.; ∆τ a 2-chain such that ∂∆τ = τ1 × γτ .
I ∆τ is well-defined up to H2(X0(NE),Z).Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 15 / 23
Darmon’s quadratic ATR points (II)
DefinitionA Hilbert modular form (HMF) of level N is a holomorphic functionf : H×H→ C, such that
f(γ1z1, γ2z2) = (c1z1 + d1)2(c2z2 + d2)2f(z1, z2), γ ∈ Γ0(N).
Have also Fourier expansions
fE(z1, z2) =∑n>>0
an(fE)e2πi(n1z1/δ1+n2z2/δ2).
Theorem (Freitas–Le-Hung–Siksek)There is a HMF fE of level NE such that ap(fE) = ap(E) for all p.
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 16 / 23
Darmon’s quadratic ATR points (III)
Need to symmetrize the HMF fE to account for units of F .I Yields fE , which is no longer holomorphic.
Integration yields an element Jτ =∫∫
∆τfEdz1dz2 ∈ C.
Well-defined up to a lattice
L =∫∫
∆fEdz1dz2 : ∆ ∈ H2(X0(NE),Z).
Conjecture 1 (Oda)There is an isogeny β : C/L→ E(C).
Pτ = β(Jτ ) ∈ E(C).
Conjecture 2 (Darmon)1 The local point Pτ is global, and belongs to E(Hτ ).2 Pτ is nontorsion if and only if L′(E/K, 1) 6= 0.
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 17 / 23
Archimedean cubic Darmon points (I)
Let F be a cubic field of signature (1, 1) (and h+F = 1).
Let E/F be an elliptic curve, of conductor NE .Consider the arithmetic group
Γ0(NE) ⊂ SL2(OF ) ⊂ SL2(R)× SL2(C).
H3 = C× R>0 = hyperbolic 3-space, on which SL2(C) acts:(a bc d
)· (x, y) =
((ax+ b)(cx+ d) + acy2
|cx+ d|2 + |cy|2 ,y
|cx+ d|2 + |cy|2
).
Get an action of Γ0(NE) on the symmetric space H×H3.
R>0
C
×
H H3
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 18 / 23
Archimedean cubic Darmon points (II)
Assume that E is modular.E ; an automorphic form ωE with Fourier-Bessel expansion:
ωE(z, x, y) =∑
α∈δ−1OFα0>0
a(δα)(E)e2πi(α0z+α1x+α2x)yH (α1y) ·(−dx∧dzdy∧dzdx∧dz
)
H(t) =
(− i
2eiθK1(4πρ),K0(4πρ),
i
2e−iθK1(4πρ)
)t = ρeiθ.
I K0 and K1 are hyperbolic Bessel functions of the second kind:
K0(x) =
∫ ∞0
e−x cosh(t)dt, K1(x) =
∫ ∞0
e−x cosh(t) cosh(t)dt.
ωE descends to a harmonic 2-form on Γ0(NE)\ (H×H3).
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 19 / 23
Archimedean cubic Darmon points (III)
Let K be a totally complex quadratic extension of F .I Suppose that p | NE =⇒ p is split in K. ( =⇒ S(E,K) = v1).
Choose τ ∈ K \ F .R>0
C
×
H H3
τ1
γτ
τ2
τ ′2
Γ0(NE)
τ ; ∆τ ∈ C2(Γ0(NE),Z).
Jτ =
∫∆τ
ωE ∈ C.
Jτ ; Pτ via C/ΛE → E(C).Conjecture: Pτ is defined over a finite abelian extension of K.
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 20 / 23
Remarks
When n > 1 the cycles constructed are analogous, but of higherdimension.
I Need to develop computational homology of arithmetic groups.
When #S(E,K) > 1 the group Γ0(NE) is replaced with the(norm-one) units of a certain quaternion algebra over F .
I No computations have been done in this setting.
There is a p-adic counterpart to all these constructions, where therole of the place v1 is substituted with a (finite) prime.
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 21 / 23
Example (I)
Let F = Q(r) with r3 − r2 + 1 = 0.
F signature (1, 1) and discriminant −23.
Consider the elliptic curve E/F given by the equation:
E/F : y2 + (r − 1)xy +(r2 − r
)y = x3 +
(−r2 − 1
)x2 + r2x.
E has prime conductor NE =(r2 + 4
)of norm 89.
K = F (α), with α2 + (r + 1)α+ 2r2 − 3r + 3 = 0.
I K has class number 1, thus we expect the point to be defined over K.
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 22 / 23
Example (II)
Take τ = α. This gives
γτ =
(−4r − 3 −r2 + 2r + 3
−2r2 − 4r − 3 −r2 + 4r + 2
)Finding ∆τ with ∂∆τ = τ × γτ amounts to decomposing γτ into aproduct of elementary matrices.
I Effective version of congruence subgroup problem.
Jτ =∑i
∫ si
ri
∫ γi·O
OωE(z, x, y).
We obtain, summing over all ideals (α) of norm up to 400, 000:
Jτ = 0.1419670770183− 0.0550994633√−1 ∈ C/ΛE ; Pτ ∈ E(C).
Numerically (up to 32 decimal digits) we obtain:
Pτ?= 10×
(r − 1, α− r2 + 2r
)∈ E(K).
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 23 / 23
Thank you !
“The fun of the subject seems to me to be in the examples.
B. Gross, in a letter to B. Birch, 1982”Bibliography, code and slides at:http://www.warwick.ac.uk/mmasdeu/
Marc Masdeu Analytic construction of pointson modular elliptic curves May 26, 2015 23 / 23