1. Bauer, F. L.: An elementary proof of the Hopf inequality for positive operators. Num. Math.7, 331?337 (1965).
2. Bauer, F. L., Deutsch, E., Stoer, J.: Abschätzungen für die Eigenwerte positiver linearer Operatoren. Lin. Algebra and its Appl.2, 275?301 (1969).
3. Birkhoff, G.: Extensions of Jentzsch's theorem. Trans. Amer. Math. Soc.85, 219?227 (1957).
4. Hopf, E.: An inequality for positive linear integral operators. J. Math. Mech.12, 683?692 (1963).
5. Ostrowski, A. M.: Positive matrices and functional analysis in ?Recent advances in matrix theory?, 81?101. Madison: Univ. of Wisconsin Press 1964.