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On a random search tree: asymptotic enumeration of vertices by distance from leaves

Part of: Combinatorics Graph theory Combinatorial probability Extremal combinatorics Lattices

Published online by Cambridge University Press:  08 September 2017

Miklós Bóna  and
Boris Pittel
Miklós Bóna*
Affiliation:
University of Florida
Boris Pittel*
Affiliation:
The Ohio State University
*
* Postal address: Department of Mathematics, University of Florida, 358 Little Hall, Gainesville, FL 32611-8105, USA. Email address: bona@ufl.edu
** Postal address: Department of Mathematics, The Ohio State University, 231 West 18th Avenue, Columbus, OH 43210-1175, USA. Email address: bgp@math.ohio-state.edu
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Abstract

A random binary search tree grown from the uniformly random permutation of [n] is studied. We analyze the exact and asymptotic counts of vertices by rank, the distance from the set of leaves. The asymptotic fraction c k of vertices of a fixed rank k ≥ 0 is shown to decay exponentially with k. We prove that the ranks of the uniformly random, fixed size sample of vertices are asymptotically independent, each having the distribution {c k }. Notoriously hard to compute, the exact fractions c k have been determined for k ≤ 3 only. We present a shortcut enabling us to compute c 4 and c 5 as well; both are ratios of enormous integers, the denominator of c 5 being 274 digits long. Prompted by the data, we prove that, in sharp contrast, the largest prime divisor of the denominator of c k is at most 2k+1 + 1. We conjecture that, in fact, the prime divisors of every denominator for k > 1 form a single interval, from 2 to the largest prime not exceeding 2k+1 + 1.

Keywords

Search treerootleavesrankenumerationasymptoticdistributionnumerical data

MSC classification

Primary: 05A05: Permutations, words, matrices
Secondary: 05A15: Exact enumeration problems, generating functions 05A16: Asymptotic enumeration 05C05: Trees 06B05: Structure theory 05C80: Random graphs 05D40: Probabilistic methods 60C05: Combinatorial probability

Information

Type
Research Article
Information
Advances in Applied Probability , Volume 49 , Issue 3 , September 2017 , pp. 850 - 876
Copyright
Copyright © Applied Probability Trust 2017 

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