Download - K Record Values
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The k-Record Values of Discrete Random
Variables
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Subsequences Xij
X1
X3
X2
X13X2
1
X34
• Where Xi j is the “ith” subsequence and “jth” term of subseqence
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K- record value- An entry i is a k-record value if there are exactly k entries ≥ i occurring before i in the
sequence- K-record values exist iff Xi
k = i
- Hence in the 2nd subsequence, 2 is a “one-record value” - In the 3rd subsequence, 3 is a “five-record value”
X13X2
1
X35
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Ignatov’s Theorem -> Rk
• Ordered set of record values are independently distributed • Not dependent on the sequence they occur in
X13
X211X3
5
X19 X1
14X21
X214
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PMFs of k-record values
Since the Xi k = i’s are all independent and the above PMFs are independent
of the k-record number then Rk values are also independent
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Expected entires till first k-record index
We have to find E[T]
Define the first k-record entry as follows:
Consider a sequence of random variables X1, X2, . . . which take on one of the values i, i + 1, . . . ,m with probabilities p1, p2,…. respectively
Since i is the smallest value in the sequence, we check Xk for i
if Xk=i : : where Ti is the first k-record in this sequence
Otherwise we choose subsequence i+1 , where probability of each entry from i+1,..,m is given as :
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In this case, if Ti+1 is the number of entries till first k–record entry , we can show that Ti is given as
Where, Ni is the number of occurrences of i in the original sequence, till we reach the Ti+1 entry.
Let Ti+1 be a value n (≥k) , then the distribution of Time taken to get n successes if geometric with the following success probability
Since the mean of the geometric random variable will be 1/(1/ λi) , we get
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Taking expectation on both sides
Thus we get the following expectation of T by conditioning on Xk= i and Xk>i