1. Hobbs, J.R.: Granularity. In: Proc. of IJCAI, Los Angeles, USA, pp. 432–435 (1985)
2. Zadeh, L.A.: Fuzzy sets. Inf. Control 8, 338–353 (1965)
3. Pawlak, Z.: Rough Sets-Theoretical aspects of reasoning about data. Kluwer Academic Publishers, Dordrecht (1991)
4. Zhang, B., Zhang, L.: Theory and Applications of Problem Solving. Elsevier Science Publishers BV, Amsterdam (1992)
5. Zhang, L., Zhang, B.: The quotient space theory of problem solving. Fundamenta Informaticae 2, 3, 287–298 (2004)