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Partial Shadows of Set Systems

Part of: Extremal combinatorics Combinatorics

Published online by Cambridge University Press:  20 January 2015

BÉLA BOLLOBÁS  and
TOM ECCLES
BÉLA BOLLOBÁS
Affiliation:
Department of Pure Mathematics and Mathematical Statistics, Wilberforce Road, Cambridge CB3 0WB, UK (e-mail: eccles.tom@gmail.com) Department of Mathematical Sciences, University of Memphis, Memphis TN 38152, USA (e-mail: b.bollobas@dpmms.cam.ac.uk)
TOM ECCLES
Affiliation:
Department of Pure Mathematics and Mathematical Statistics, Wilberforce Road, Cambridge CB3 0WB, UK (e-mail: eccles.tom@gmail.com)
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Abstract

The shadow of a system of sets is all sets which can be obtained by taking a set in the original system, and removing a single element. The Kruskal-Katona theorem tells us the minimum possible size of the shadow of $\mathcal A$, if $\mathcal A$ consists of m r-element sets.

In this paper, we ask questions and make conjectures about the minimum possible size of a partial shadow for $\mathcal A$, which contains most sets in the shadow of $\mathcal A$. For example, if $\mathcal B$ is a family of sets containing all but one set in the shadow of each set of $\mathcal A$, how large must $\mathcal B$ be?

MSC classification

Primary: 05D05: Extremal set theory
Secondary: 05A99: None of the above, but in this section
Type
Paper
Information
Combinatorics, Probability and Computing , Volume 24 , Issue 5 , September 2015 , pp. 825 - 828
Copyright
Copyright © Cambridge University Press 2015 

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References

[1] Katona, G. O. H. (1968) A theorem of finite sets. In Theory of Graphs (Erdős, P. and Katona, G. O. H., eds), Conference in Tihany, Hungary, 1966, Academic Press and Akadémiai Kiadó, pp. 187–207.Google Scholar
[2] Kruskal, J. B. (1963) The number of simplices in a complex. In Mathematical Optimization Techniques, University of California Press, pp. 251–278.Google Scholar
[3] Lovász, L. (1979) Combinatorial Problems and Exercises, North-Holland.Google Scholar