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Sperner's Theorem and a Problem of Erdős, Katona and Kleitman

Published online by Cambridge University Press:  01 December 2014

SHAGNIK DAS ,
WENYING GAN  and
BENNY SUDAKOV
SHAGNIK DAS
Affiliation:
Department of Mathematics, UCLA, Los Angeles, CA 90095, USA (e-mail: shagnik@ucla.edu, wgan@math.ucla.edu)
WENYING GAN
Affiliation:
Department of Mathematics, UCLA, Los Angeles, CA 90095, USA (e-mail: shagnik@ucla.edu, wgan@math.ucla.edu)
BENNY SUDAKOV
Affiliation:
Department of Mathematics, ETH, 8092 Zürich, Switzerland (e-mail: benjamin.sudakov@math.ethz.ch)
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Abstract

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A central result in extremal set theory is the celebrated theorem of Sperner from 1928, which gives the size of the largest family of subsets of [n] not containing a 2-chain, F1 ⊂ F2. Erdős extended this theorem to determine the largest family without a k-chain, F1 ⊂ F2 ⊂ . . . ⊂ Fk. Erdős and Katona, followed by Kleitman, asked how many chains must appear in families with sizes larger than the corresponding extremal bounds.

In 1966, Kleitman resolved this question for 2-chains, showing that the number of such chains is minimized by taking sets as close to the middle level as possible. Moreover, he conjectured the extremal families were the same for k-chains, for all k. In this paper, making the first progress on this problem, we verify Kleitman's conjecture for the families whose size is at most the size of the k + 1 middle levels. We also characterize all extremal configurations.

Keywords

Primary 05D05
Type
Paper
Information
Combinatorics, Probability and Computing , Volume 24 , Special Issue 4: Oberwolfach Special Issue Part 1 , July 2015 , pp. 585 - 608
Copyright
Copyright © Cambridge University Press 2014 

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