A counterexample to a basis conjecture of Brualdi, Friedland, and Pothen

Abstract

Brualdi, Friedland, and Pothen studied sparse bases of row spaces and proposed a rank-intersection criterion for elementary row-space vectors of sparse generic matrices. We give a \(4\times 8\) sparse generic counterexample. The example has four elementary vectors in its row space whose zero sets satisfy all the proposed rank-intersection inequalities, but the four vectors satisfy a nontrivial linear relation and hence do not form a basis. The construction also explains the obstruction: the proposed inequalities see only ranks of column sets indexed by intersections of zero sets, while linear independence of the corresponding elementary vectors is governed by the relative position of the hyperplanes annihilated by their coefficient vectors.