David G. Feingold and Richard S. Varga, Block diagonally dominant matrices and generalizations of the Gerschgorin Circle Theorem, Paci c J. Math. 12 (1962), 1241-1250. MR 27, 1458. Zbl. 109, p. 248.
1. Introduction. The main purpose of this paper is to give generalizations of the well known theorem of Gerschgorin on inclusion or exclusion regions for the eigenbalues of an arbitrary square matrix A. Basically, such exlusion regions arise naturally from results which establish the nonsingularity of A. For example, if A=D+C where D is a nonsingular diagonal matrix, then Householder  shows that ||D-1C||<1 in some matrix norm is sufficient to conclude that A is nonsingular. Hence the set of all complex numbers z for which
evidently contains no eigenvalues of A. In a like manner, Fiedler  obtains exclusion regions for the eigenvalues of A by establishing the nonsingularity of A through comparisons with M-matrices. Our approach, though not fundamentally different, establishes the nonsingularity of the matrix A by the generalization of the simple concept of a diagonally dominant matrix. But one of our major results (3) is that these new exclusion regions can give significant improvements over the usual Gerschgorin circles in providing bounds for the eigenvalues of A.