1. D. Dubois and H. Prade ,In Possibility Theory: An Approach to ComputerizedProcessing of Uncertainty, Plenum , New York , 1988 62 – 63
2. D. Dubois and H. Prade ,Decision making under fuzziness. In Advances in Fuzzy Set Theory, North-Holland , Amsterdam , 1979 279 – 302 .
3. M. Mizumoto and K. Tanaka ,Some properties of fuzzy numbers. In Advances in Fuzzy Set Theory, North-Holland , Amsterdam , 1979 153 – 164 .
4. The use of fuzzy variables in pert