On prescribed characteristic polynomials.

Abstract

Let F be a field. We show that given any nth degree monic polynomial q(x) ∈ F[x] and any matrix A ∈ Mn(F) whose trace coincides with the trace of q(x) and consisting in its main diagonal of k 0-blocks of order one, with k < n − k, and an invertible non-derogatory block of order n − k, we can construct a square-zero matrix N such that the characteristic polynomial of A + N is exactly q(x). We also show that the restriction k < n − k is necessary in the sense that, when the equality k = n − k holds, not every characteristic polynomial having the same trace as A can be obtained by adding a square-zero matrix. Finally, we apply our main result to decompose matrices into the sum of a square-zero matrix and some other matrix which is either diagonalizable, invertible, potent or torsion.