TheGer? sgorin CircleTheorem, averywell-known resultin linear algebra today, stems from the paper of S. Ger? sgorin in 1931 (which is reproduced in AppendixD)where, givenanarbitrarynxncomplexmatrix, easyarithmetic operationsontheentriesofthematrixproducendisks, inthecomplexplane, whose union contain[...]