Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Cross-intersecting subfamilies of levels of hereditary families   A set $A$ $t$-intersects a set $B$ if $A$ and $B$ have at least $t$ common
elements. Families $\mathcal{A}_1, \mathcal{A}_2, \dots, \mathcal{A}_k$ of sets
are cross-$t$-intersecting if, for every $i$ and $j$ in $\{1, 2, \dots, k\}$
with $i \neq j$, each set in $\mathcal{A}_i$ $t$-intersects each set in
$\mathcal{A}_j$. An active problem in extremal set theory is to determine, for
a given finite family $\mathcal{F}$, the structure of $k$
cross-$t$-intersecting subfamilies whose sum or product of sizes is maximum.
For a family $\mathcal{H}$, the $r$-th level $\mathcal{H}^{(r)}$ of
$\mathcal{H}$ is the family of all sets in $\mathcal{H}$ of size $r$, and, for
$s \leq r$, $\mathcal{H}^{(s)}$ is called a $(\leq r)$-level of $\mathcal{H}$.
We solve the problem for any union $\mathcal{F}$ of $(\leq r)$-levels of any
union $\mathcal{H}$ of power sets of sets of size at least a certain integer
$n_0$, where $n_0$ is independent of $\mathcal{H}$ and $k$ but depends on $r$
and $t$ (dependence on $r$ is inevitable, but dependence on $t$ can be
avoided). Our primary result asserts that there are only two possible optimal
configurations for the sum. A special case was conjectured by Kamat in 2011. We
also prove generalizations, whereby $\mathcal{A}_1, \mathcal{A}_2, \dots,
\mathcal{A}_k$ are not necessarily contained in the same union of levels.
Various Erd\H{o}s-Ko-Rado-type results follow. The sum problem for a level of a
power set was solved for $t=1$ by Hilton in 1977, and for any $t$ by Wang and
Zhang in 2011.

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The categories of this paper are: adap-org
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The categories of this paper are: stat
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The categories of this paper are: astro-ph
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The categories of this paper are: bayes-an
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The categories of this paper are: q-alg
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The categories of this paper are: gr-qc
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The categories of this paper are: hep-th
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The categories of this paper are: cs
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The categories of this paper are: solv-int
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The categories of this paper are: patt-sol
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The categories of this paper are: acc-phys
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The categories of this paper are: quant-ph
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The categories of this paper are: funct-an
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The categories of this paper are: plasm-ph
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The categories of this paper are: cond-mat
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The categories of this paper are: q-bio