Further results on Geršgorin Discs

This paper extends the work on Geršgorin Discs in the two articles “Geometric Multiplicities and Geršgorin Discs”, The American Mathematical Monthly, to appear, by R. Marsli and F. Hall, and “Geršgorin Discs Revisited”, Linear Algebra Appl. 438 (2013) 598–603, by M. Fiedler, F. Hall, and R. Marsli. The new results involve the algebraic multiplicity, the index of an eigenvalue, powers of matrices, and matrices in a given similarity class.