generated from congyu/work_with_codex
Yu Cong
89a458c76c
Introduce a "Known results" section discussing β(M1∩M2), state the Aharoni–Berger conjectures and the Davies–McDiarmid theorem, and add three bibliography entries (emlektabla16_2024, rainbow bases SIAM, berczi_partitioning_2024).
51 lines
2.1 KiB
Markdown
51 lines
2.1 KiB
Markdown
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title: Matroid Rainbow Base Cover
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----
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This problem is proposed in [@emlektabla16_2024].
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Let $M=(E,\mathcal B)$ be a rank-$r$ matroid whose ground set decomposes into two disjoint bases. Furthermore,
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assume that $E$ is colored by $r$ colors, each color appearing exactly twice. A basis of $M$ is called rainbow if
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it does not contain two elements of the same color.
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::: Problem
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What is the minimum number of rainbow bases needed to cover $E$?
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:::
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Kristóf Bérczi conjectured the minimum number is 3.
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Currently known bounds:
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- upperbound: $\floor{\log_2 |E|}+1$ by matroid intersection;
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- lowerbound: $3$ on graphic matroid of $K_4$.
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::: {.Conjecture #conj-doublecover}
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Let $M_1$ and $M_2$ be two matroid on the same ground set $E$. Assume that $E$ decomposes into two bases in both of them.
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Then $M_1$ and $M_2$ has four common bases that cover each element exactly twice.
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:::
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Let $M_1$ be the partition matroid of colors and let $M_2$ be $M$.
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If [@conj-doublecover] is true, then 3 common bases will be enough to cover $E$.
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# Known results
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Let $\beta(M)$ be the covering number of matroid $M$. Given two matroids $M_1,M_2$ on the same ground set, $\beta(M_1\cap M_2)$ is the minimum number of common independent sets needed to cover the ground set.
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[@conj-doublecover] is basically asking for $\beta(M_1\cap M_2)$ when the ground set $E$ is partitioned into 2 common bases of $M_1$ and $M_2$.
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## Results and conjectures on $\beta(M_1\cap M_2)$
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One can see that $\beta(M_1\cap M_2)\geq \min \set{\beta(M_1),\beta(M_2)}$. Aharoni and Berger showed that $\beta(M_1\cap M_2)\leq 2 \max \set{\beta(M_1),\beta(M_2)}$
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::: {.Conjecture title="Aharoni and Berger"}
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Let $M_1$ and $M_2$ be two matroids on the same ground set.
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1. If $\beta(M_1)\neq \beta(M_2)$, then $\beta(M_1\cap M_2)=\max \set{\beta(M_1),\beta(M_2)}$.
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2. If $\beta(M_1)= \beta(M_2)$, then $\beta(M_1\cap M_2)\leq \max \set{\beta(M_1),\beta(M_2)}+1$.
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:::
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::: {.Theorem title="Davies and McDiarmid"}
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Let $M_1$ and $M_2$ be strongly base orderable matroids on the same ground set.
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Then $\beta(M_1\cap M_2)= \max \set{\beta(M_1),\beta(M_2)}$.
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:::
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See [@berczi_partitioning_2024] for refs. |