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Межкафедральный семинар МФТИ по дискретной математике
4 мая 2017 г. 18:30, г. Долгопрудный, Актовый зал Лабораторного корпуса МФТИ
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Simplifying inclusion-exclusion formulas
M. Tancer |
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Аннотация:
Let $\mathcal{F}=\{F_1,F_2, \ldots,F_n\}$ be a family of $n$ sets on a
ground set $S$, such as a family of balls in $\mathbb R^d$. For every finite
measure $\mu$ on $S$, such that the sets of $\mathcal{F}$ are measurable,
the classical inclusion-exclusion formula asserts that
$\mu(F_1\cup F_2\cup\cdots\cup F_n)=\sum_{I:\emptyset\ne I\subseteq[n]}
(-1)^{|I|+1}\mu\Bigl(\bigcap_{i\in I} F_i\Bigr)$; 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 $\mathcal F$.
During the talk, I will discuss how to get an upper bound valid for an
arbitrary $\mathcal F$: we show that every system $\mathcal F$ of $n$ sets
with $m$ nonempty fields in the Venn diagram admits an inclusion-exclusion
formula with $m^{O(\log^2n)}$ terms and with $\pm1$ coefficients, and that
such a formula can be computed in $m^{O(\log^2n)}$ expected time. For
every $\varepsilon>0$ 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 $\Omega(m^{2-\varepsilon})$.
Based on a joint work with X. Goaoc, J. Matousek, P. Patak and Z.
Safernova (Patakova).
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