Gersgorin variations II: On themes of Fan and Gudkov

Assume F = {f1,.∈.∈.,fn} is a family of nonnegative functions of n-1 nonnegative variables such that, for every matrix A of order n, |a ii|>f i (moduli of off-diagonal entries in row i of A) for all i implies A nonsingular. We show that there is a positive vector x, depending only on F, such that for all A=(a ij), and all i, f i ≥ ∑j|a ij|x j/xi. This improves a theorem of Ky Fan, and yields a generalization of a nonsingularity criterion of Gudkov. © Springer 2006.