A Note on the Critical Problem for Matroids

Let M be a matroid representable over GF(q) and S be a subset of its ground set. In this note we prove that S is maximal with the property that the critical exponent c(M|S; q) does not exceed k if and only if S is maximal with the property that c(M · S) ≤ k. In addition, we show that, for regular matroids, the corresponding result holds for the chromatic number. © 1984, Academic Press Inc. (London) Limited. All rights reserved

On bipartite restrictions of binary matroids

In a 1965 paper, Erdos remarked that a graph G has a bipartite subgraph that has at least half as many edges as G. The purpose of this note is to prove a matroid analogue of Erdos\u27s original observation. It follows from this matroid result that every loopless binary matroid has a restriction that uses more than half of its elements and has no odd circuits; and, for 2≤k≤5, every bridgeless graph G has a subgraph that has a nowhere-zero k-flow and has more than k-1/k|E(G)| edges. © 2011 Elsevier Ltd