1. M.S. Ashbaugh and E.M. Harrell II. Maximal and minimal eigen values and their associated nonlinear equations. Ans Preprint 1985.
2. C. Bandle, Isoperimetric inequalities and applications. Pitman, London 1980.
3. G. Birkhoff and G.-C. Rota, Ordinary differential equations. 2nd ed. Xerox College Publ., Lexington, Mass. 1969.
4. H. Egnell, Extremal properties of the first eigenvalue of a class of elliptic eigenvalue problems. To appear, Ann. della Scuola Norm. Sup. di Pisa.
5. M. Essén, Optimization and rearrangements of the coefficient in the operator d2/dt2 — p(t)2 on a finite interval. J. of Math. Anal. Appl. 115 (1986), 278–304.