- In this paper a theory of dual-complementary variational principles is developed in connection with problems characterized by the equations of the form T*ETu + f = 0. A canonical triple of equations associated with this equation is developed and are shown to be Euler equations for a certain functional. Correspondingly, a dual triple of canonical equations is developed which give rise to a system of equations of the type S E-1S + = 0. A variety of applications in mathematical physics is discussed. 1974.