A rank bound for bases and circuits in binary matroids

Abstract

Let \(b(M)\), \(d(M)\), and \(r(M)\) denote the number of bases, the number of circuits, and the rank of a matroid \(M\), respectively. We prove that every nonempty simple binary matroid with no coloops satisfies \[ 2b(M)\ge(r(M)+1)d(M), \] with equality if and only if \(M\) is isomorphic to the Fano matroid. This confirms a conjecture recorded by Oxley in 1983. For the same class, we prove that deleting any element leaves at least as many bases as there are circuits in the original matroid: \(b(M\backslash e)\ge d(M)\) for every \(e\in E(M)\). We determine all equality cases and deduce a sharp linear lower bound for basis growth under successive series extensions. The main counting step is a joint estimate for the three largest possible circuit sizes, obtained from contraction-normalized fundamental-circuit counts and an exact folded-cube edge correspondence.