A sharp bound on the size of a connected matroid

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This paper proves that a connected matroid A/ in which a largest circuit and a largest cocircuit have c and c* elements, respectively, has at most cc* elements. It is also shown that if e is an element of M and ce and c£ are the sizes of a largest circuit containing e and a largest cocircuit containing e, then | E(A/) | (ce -l)(c* -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 ce and c* 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 |E(G)| cannot exceed the product of the length of a longest (u, u)-path and the size of a largest minimal edge-cut separating u from ν. © 2001 American Mathematical Society.

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Transactions of the American Mathematical Society

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