# hw4sol - Computer Science 340 Reasoning about Computation...

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Computer Science 340 Reasoning about Computation Homework 4 Due at the beginning of class on Wednesday, October 17, 2007 Problem 1 (10 points) We analyzed the process of throwing n balls into n bins independently and at random and showed that the maximum load is at most O ( log n log log n ) with high probability. Suppose instead that the balls are assigned to bins by a function f which is chosen from a universal family. Prove that, with probability at least 1 / 2, the maximum load is O ( n ). Hint: Let X i be the load of the i th bin. Give an upper bound on E [ X 2 i ]. Solution: Let indicator random variables y jk be 1 if ball j is mapped to bin k , and zero otherwise. We know that X i = j y ji . We have E [ i X 2 i ] = i E [ X 2 i ] = i a,b E [ y ai y bi ] = i a E [ y ai ] + a = b E [ y ai y bi ] = a E i y ai + a = b E i y ai y bi n + n ( n - 1) 1 n = 2 n - 1 . Besides changing the order of summations and using linearity of expectations, we use the following: the inequality uses universality property, while the third equation uses the fact that for a zero-one variable x , we have x 2 = x . Using Markov’s bound, we know that with probability at least 1 / 2, i X 2 i < 4 n . Then the maximum is also bounded by 4 n = 2 n = O ( n ). Problem 2 (15 points) In order to implement a counter capable of counting from 1 to N, at least log N bits are needed. In some cases, allowing this number of bits is too expensive. If we allow the counter

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to make some errors, we can get by with much fewer bits. In this question you’ll see how to implement a “probabilistic counter” - a counter that counts with small errors, and uses only Θ(log log N ) bits. The counter is defined as follows: Initially the counter is 0. An Increment operation is performed as follows: If the current value of the counter is i , increase the counter value by 1 with probability 1 2 i , and leave the counter in its current state ( i ) with probability 1 - 1 2 i . Let C k be a random variable denoting the value of the counter after k Increment operations have occurred.
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• Fall '07
• CharikarandChazelle
• Probability, Probability theory, Yi

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