1. Gould S.: Variational methods for eigenvalue problems. Oxford University Press/University of Toronto Press, Oxford/Toronto (1996)
2. Fabrisio, P., Jean-Paul, Z.: Derivees par rapport au domaine des valeurs propres du Laplacien. C.R. Acad. Sci., Par 321, Ser. I, Math. (10), 1310–1337 (1995)
3. Bugur D., Buttazzo G., Figueiredo I.: On the attainable eigenvalues of the Laplace operator. SIAM J. Math. Anal. 30, 527–536 (1999)
4. Bucur, D., Buttazo, G.: Variational methods in shape optimization problems. Progress in nonlinear differential equations and their applications, 216 p. Birkhauser, Boston (2005)
5. Vladimirov V.S.: Equations of mathematical physics, 512 p. Nauka, Moscow (1988)