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Discrete Math

How Prolific is a Random Permutation?

Peter Winkler (Dartmouth College)

Location:  Hill 705
Date & time: Monday, 24 February 2020 at 2:00PM - 3:00PM


A "pattern" of length k in a permutation \(P\) in \(S_n\) is a permutation in \(S_k\) determined by choosing \(k\) elements from {1,2,...,n} and looking at the order of their images under \(P\). For example, if \(P_3 > P_5\) then the positions 3 and 5 produce the pattern 21.

\(P\) is "\(d\)-prolific" if every pattern of length n-d is different; equivalently, the set of patterns of \(P\) of length \(n-d\) has size \(n\) choose \(d\).

We show that the probability that a uniformly random \(P\) in \(S_n\) is \(d\)-prolific tends to \(e^{-d^2-d}\) as \(n\) grows.

Joint work with Simon Blackburn and Cheyne Homberger.

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