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Let $I, J$ be two bases of a matroid. For every $x$ in $I$, there is some $y$ in $J$ such that, if we exchange $x$ with $y$, then both resulting sets ($I \setminus x \cup y$ and $J \setminus y \cup x$) are bases (this is the strong basis exchange property).

Can we extend this property as follows: there exists a bijection $f$ between $I\setminus J$ and $J\setminus I$, such that for every $x$ in $I$, if we exchange $x$ with $f(x)$, then both resulting sets are bases?

The nearest result I found was in lecture notes by Goemans. In Lemma 5, he proves that there is a perfect matching between $I\setminus J$ and $J\setminus I$ in a bipartite graph that he denotes by $D_M(I)$. This means that for every $x$ in $I$, if we exchange $x$ with $f(x)$, then $I \setminus x \cup f(x)$ is a base. But, it does not imply that $J \setminus f(x) \cup x$ is a base too.

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1 Answer 1

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No, not every matroid satisfies this property. For example, it is known to fail for the cycle matroid of $K_4$. The matroids that satisfy your property are called base orderable matroids. There are important classes of matroids that are base orderable, such as transversal matroids. Moreover, base orderability is a minor-closed property, but Ingleton proved that there are actually an infinite number of excluded minors. See these slides of Joseph Bonin for more information. For example, the slides include a proof that $M(K_4) $ is not base orderable.

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