Kristaly, A., Radelescu, V., & Varga, C. G. (2010). Variational principles in mathematical physics, geometry and economics: Qualitative analysis of nonlinear equations and unilateral problems. Cambridge University press.

Kristaly, Alexandru, V.D Radelescu, and Cs. Gy Varga. Variational Principles in Mathematical Physics, Geometry and Economics: Qualitative Analysis of Nonlinear Equations and Unilateral Problems. New York: Cambridge University press, 2010.

Kristaly, Alexandru, et al. Variational Principles in Mathematical Physics, Geometry and Economics: Qualitative Analysis of Nonlinear Equations and Unilateral Problems. Cambridge University press, 2010.