1. Extremal problems of approximation of fuzzy sets;Asmuss;Acta Univ. Latviensis (Ser. Math.),1997
2. A fuzzy approach to extremal problems of approximation theory;Asmuss,1997
3. Fuzzy topological spaces;Chang;J. Math. Anal. Appl.,1968
4. Izbrannyje Trudy, Moscow;Chebyshev;AN SSSR,1955
5. Basic notions of fuzzy topology;Gähler;Fuzzy Sets and Systems,1988