1. Duvaut, G., Lions, J.L.: Les inéquations en mécanique et en physique. Paris: Dunod, 1972
2. Duvaut, G.: Solution of two phases Stefan problem by variational inequality. In: Proc. of the Symp. on moving boundary problems. oxford, 1974
3. Fagé, D.: Approximate solution of a nonlinear problem with constraint for a two-dimensional elliptic equation. Soviet Math. Dokl.22, 722?725 (1980)
4. Fagé, D.: The indicatrix method in free boundary problems. Numer. Math.38, 39?52 (1981)
5. Fagé, D.: The indicatrix method. II. Numer. Math.38, 255?261 (1981)