[1] A. Brauer: 
Limits for the characteristic roots of a matrix I - V. Duke Math. Journal 13 (1946), 387-395; 14 (1947), 21-26; 15 (1948), 871-877; 19 (1952), 75-91; 19 (1952), 553-562. 
MR 0020540[2] K. Fan A. J. Hoffman: 
Lower bounds for the rank and location of the eigenvalues of a matrix. Nat. Bur. Stand. Appl. Math. Ser. 39, 117 - 130. 
MR 0064732[3] M. Fiedler V. Pták: 
On matrices with non-positive off-diagonal elements and positive principal minors. Czech. Math. Journal 87 (1962), 382-400. 
MR 0142565[4] M. Fiedler V. Pták: 
Some inequalities for the spectrum of a matrix. Matem.-fyz. časopis SAV 10 (1960), 148-166. 
MR 0124334[5] S. Geršgorin: Über die Abgrenzung der Eigenwerte einer Matrix. Izv. Akad. Nauk SSSR 7 (1931), 749-754.
[6] A. Ostrowski: 
Sur les conditions générales pour la régularité des matrices. Rend. di Matem. e delle sue appl. 10 (1951), 156-168. 
MR 0049151 | 
Zbl 0044.25201