Abstract: The main purpose of this paper is to clear some foundamental concepts, to generalize legendre transformation including the method of lagrange multipher, to unifyly deal with varions complementary variational principles.
In adition, some new mixed comple variatprinciples are developed, based on the idea of duality, for the theory of disloction. The functional saddle theorems are derived also for emphasizing the methos of finite elements and boundry elements.