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Mathematics Colloquium: James Fill (Johns Hopkins University)

Event Type
Department of Mathematics
245 Altgeld Hall
Feb 21, 2013   4:00 pm  
Steve Bradlow
Originating Calendar

James Fill (Johns Hopkins University) will present "Distributional Convergence for the Number of Symbol Comparisons Used by Quicksort." Abstract: We will begin by reviewing the operation of the sorting algorithm QuickSort. Most previous analyses of QuickSort have used the number of key comparisons as a measure of the cost of executing the algorithm. In contrast, we suppose that the n independent and identically distributed (iid) keys are each represented as a sequence of symbols from a probabilistic source and that QuickSort operates on individual symbols, and we measure the execution cost as the number of symbol comparisons. Assuming only a mild "tameness" condition on the source, we show that there is a limiting distribution for the number of symbol comparisons after normalization: first centering by the mean and then dividing by n. Additionally, under a condition that grows more restrictive as p increases, we have convergence of moments of orders p and smaller. In particular, we have convergence in distribution and convergence of moments of every order whenever the source is memoryless, i.e., whenever each key is generated as an infinite string of iid symbols. This is somewhat surprising: Even for the classical model that each key is an iid string of unbiased ("fair") bits, the mean exhibits periodic fluctuations of order n.

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