Covers in Partitioned Intersecting Hypergraphs
Given an integer $r$ and a vector $\vec{a}=(a_1, \ldots ,a_p)$ of positive numbers with $\sum_{i \le p} a_i=r$, an $r$-uniform hypergraph $H$ is said to be $\vec{a}$-partitioned if $V(H)=\bigcup_{i \le p}V_i$, where the sets $V_i$ are disjoint, and $|e \cap V_i|=a_i$ for all $e \in H,~~i \le p$. A $...
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Zusammenfassung: | Given an integer $r$ and a vector $\vec{a}=(a_1, \ldots ,a_p)$ of positive
numbers with $\sum_{i \le p} a_i=r$, an $r$-uniform hypergraph $H$ is said to
be $\vec{a}$-partitioned if $V(H)=\bigcup_{i \le p}V_i$, where the sets $V_i$
are disjoint, and $|e \cap V_i|=a_i$ for all $e \in H,~~i \le p$. A
$\vec{1}$-partitioned hypergraph is said to be $r$-partite. Let $t(\vec{a})$ be
the maximum, over all intersecting $\vec{a}$-partitioned hypergraphs $H$, of
the minimal size of a cover of $H$. A famous conjecture of Ryser is that
$t(\vec{1})\le r-1$. Tuza conjectured that if $r>2$ then $t(\vec{a})=r$ for
every two components vector $\vec{a}=(a,b)$. We prove this conjecture whenever
$a\neq b$, and also for $\vec{a}=(2,2)$ and $\vec{a}=(4,4)$. |
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DOI: | 10.48550/arxiv.1412.3067 |