1. Afrouzi, G.A., Brown, K.J.: On principal eigenvalues for boundary value problems with indefinite weight and Robin boundary conditions. Proc. Am. Math. Soc. 127(1), 125–130 (1999)
2. Kolmogorov, A.N., Petrovsky, I.G., Piskunov, N.S.: Etude de l’équation de la diffusion avec croissance de la quantité de matière et son application à un problème biologique. Moscow Univ. Math. Bull. 1, 1–26 (1937)
3. Bandle, C.: Isoperimetric inequality for some eigenvalues of an inhomogeneous, free membrane. SIAM J. Appl. Math. 22, 142–147 (1972)
4. Bandle, C.: Isoperimetric inequalities and applications, monographs and studies in mathematics, vol. 7. Pitman (Advanced Publishing Program), Boston (1980)
5. Berestycki, H.: Personal communication (2012)