Add Matroid Rainbow Base Cover notes

notes/main.md

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+----
+title: Matroid Rainbow Base Cover
+----
+
+Let $M=(E,\mathcal B)$  be a rank-$r$ matroid whose ground set decomposes into two disjoint bases. Furthermore,
+assume that $E$ is colored by $r$ colors, each color appearing exactly twice. A basis of $M$ is called rainbow if
+it does not contain two elements of the same color.
+
+::: Problem
+What is the minimum number of rainbow bases needed to cover $E$?
+:::
+
+Kristóf Bérczi conjectured the minimum number is 3.
+
+Currently known bounds:
+
+- upperbound: $\floor{\log_2 |E|}+1$ by matroid intersection;
+- lowerbound: $3$ on graphic matroid of $K_4$.
+
+::: {.Conjecture #conj-doublecover}
+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.
+Then $M_1$ and $M_2$ has four common bases that cover each element exactly twice.
+:::
+
+Let $M_1$ be the partition matroid of colors and let $M_2$ be $M$.
+If [@conj-doublecover] is true, then 3 common bases will be enough to cover $E$.