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A sharp bound on the size of a connected matroid

Authors: Manoel Lemos and James Oxley
Journal: Trans. Amer. Math. Soc. 353 (2001), 4039-4056
MSC (2000): Primary 05B35; Secondary 05C35
Published electronically: June 1, 2001
MathSciNet review: 1837219
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This paper proves that a connected matroid $M$ in which a largest circuit and a largest cocircuit have $c$ and $c^*$ elements, respectively, has at most $\frac{1}{2}cc^*$ elements. It is also shown that if $e$ is an element of $M$ and $c_e$ and $c^*_e$ are the sizes of a largest circuit containing $e$ and a largest cocircuit containing $e$, then $\vert E(M)\vert \le (c_e -1)(c^*_e - 1) + 1$. Both these bounds are sharp and the first is proved using the second. The second inequality is an interesting companion to Lehman's width-length inequality which asserts that the former inequality can be reversed for regular matroids when $c_e$ and $c^*_e$ are replaced by the sizes of a smallest circuit containing $e$ and a smallest cocircuit containing $e$. Moreover, it follows from the second inequality that if $u$ and $v$ are distinct vertices in a $2$-connected loopless graph $G$, then $\vert E(G)\vert$ cannot exceed the product of the length of a longest $(u,v)$-path and the size of a largest minimal edge-cut separating $u$ from $v$.

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Additional Information

Manoel Lemos
Affiliation: Departamento de Matemática, Universidade Federal de Pernambuco, Recife, Pernambuco 50740-540, Brazil

James Oxley
Affiliation: Department of Mathematics, Louisiana State University, Baton Rouge, Louisiana 70803-4918

Received by editor(s): April 12, 1999
Received by editor(s) in revised form: January 18, 2000
Published electronically: June 1, 2001
Article copyright: © Copyright 2001 American Mathematical Society

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