1. Alessandrini G.: Critical points of solutions to the p-Laplace equation in dimension two. Boll. Univ. Math. Ital. A (7) 1(2), 239–246 (1987)
2. Alessandrini G., Magnanini R.: The index of isolated critical points and solutions of elliptic equations in the plane. Ann. Scuola Norm. Super. Pisa Cl. Sci. (4) 19(4), 567–589 (1992)
3. Alessandrini G., Magnanini R.: Elliptic equations in divergence form, geometric critical points of solutions, and Stekloff eigenfunctions. SIAM J. Math. Anal. 25(5), 1259–1268 (1994)
4. Ambrosio L., Fusco N., Pallara D.: Functions of Bounded Variation and Free Discontinuity Problems. Oxford Mathematical Monographs. The Clarendon Press/Oxford University Press, New York (2000)
5. Aronsson G.: On the partial differential equation u x 2 u xx + 2u x u y u xy + u y 2 u yy = 0. Ark. Mat. 7, 395–425 (1968)