On set-valued non-Boolean functions. Tošić, Ratko, Čomić, Lidija (2000) Novi Sad Journal of Mathematics
On the approximation of elliptic operators with discontinuous coefficients William H. Mc Connell (1976) Annali della Scuola Normale Superiore di Pisa - Classe di Scienze
On the characteristics for a system of partial differential equations of the first order in a special case Z. Kamont, W. Pawelski (1974) Annales Polonici Mathematici