1Dept. oF PURE MATH., FACULTY OF MATH. AND COMPUTER SCI., AMIRKABIR UNI- VERSITY OF TECHNOLOGY (TEHRAN POLYTECHNIC), 424, HAFEZ AVE., TEHRAN 15914, IRAN
2ScHOOL OF MATHEMATICS, INSTITUTE FOR RESEARCH IN FUNDAMENTAL SCIENCES (IPM), P.O.Box: 19395-5746, TEHRAN,IRAN