Institute of Theoretical Physics Acadcmia Siniea P.O. Box 2735 Beijing 100080 China;
Department of Mathematics Capital Norxnal University Beijing 100037 China;
Institute of Applied Mathematics Academy of Mathematics and Systern Sciences Academia Sinica P.O. Box 2734 Beijing 100080 China;
discrete variation; Euler-Lagrange cohomology; symplectic algorithm; rnultisympleetic algorithm;