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Simplifying Inclusion–Exclusion Formulas

Published online by Cambridge University Press:  14 October 2014

XAVIER GOAOC
Affiliation:
Université Paris–Est Marne-la-Vallée, France (e-mail: goaoc@univ-mlv.fr)
JIŘÍ MATOUŠEK
Affiliation:
Department of Applied Mathematics, Charles University, Malostranské nám. 25, 118 00 Praha 1, Czech Republic (e-mail: matousek@kam.mff.cuni.cz, zuzka@kam.mff.cuni.cz, tancer@kam.mff.cuni.cz) Institute of Theoretical Computer Science, ETH Zurich, 8092 Zurich, Switzerland
PAVEL PATÁK
Affiliation:
Department of Algebra, Charles University, Sokolovská 83, 186 75 Praha 8, Czech Republic (e-mail: patak@kam.mff.cuni.cz)
ZUZANA SAFERNOVÁ
Affiliation:
Department of Applied Mathematics, Charles University, Malostranské nám. 25, 118 00 Praha 1, Czech Republic (e-mail: matousek@kam.mff.cuni.cz, zuzka@kam.mff.cuni.cz, tancer@kam.mff.cuni.cz)
MARTIN TANCER
Affiliation:
Department of Applied Mathematics, Charles University, Malostranské nám. 25, 118 00 Praha 1, Czech Republic (e-mail: matousek@kam.mff.cuni.cz, zuzka@kam.mff.cuni.cz, tancer@kam.mff.cuni.cz)
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Abstract

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Let $\mathcal{F}$ = {F1, F2,. . ., Fn} be a family of n sets on a ground set S, such as a family of balls in ℝd. For every finite measure μ on S, such that the sets of $\mathcal{F}$ are measurable, the classical inclusion–exclusion formula asserts that

$\[\mu(F_1\cup F_2\cup\cdots\cup F_n)=\sum_{I:\emptyset\ne I\subseteq[n]} (-1)^{|I|+1}\mu\biggl(\bigcap_{i\in I} F_i\biggr),\]$
that is, the measure of the union is expressed using measures of various intersections. The number of terms in this formula is exponential in n, and a significant amount of research, originating in applied areas, has been devoted to constructing simpler formulas for particular families $\mathcal{F}$. We provide an upper bound valid for an arbitrary $\mathcal{F}$: we show that every system $\mathcal{F}$ of n sets with m non-empty fields in the Venn diagram admits an inclusion–exclusion formula with mO(log2n) terms and with ±1 coefficients, and that such a formula can be computed in mO(log2n) expected time. For every ϵ > 0 we also construct systems with Venn diagram of size m for which every valid inclusion–exclusion formula has the sum of absolute values of the coefficients at least Ω(m2−ϵ).

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Copyright © Cambridge University Press 2014