1. D. F. Hays, Variational formulation of the heat equation, Chapter 2 inNon-equilibrium thermodynamics, variational techniques, and stability. R. J. Donnellyet al., eds. U. of Chicago Press, (1966).
2. H. W. Butler,A minimum principle for heat conduction, Proc. 5th. U.S. National Congress of Appl. Mech. A.S.M.E., New York(1966) 739.
3. R. S. Brand and F. J. Lahey, Variational principles for steady heat conduction with mixed boundary conditions.J. Engineering Math., 3 (1969) 119?121.
4. A. M. Arthurs,Complementary variational principles, Clarendon Press, Oxford, (1970).
5. A. M. Arthurs, A note on Komkov's class of boundary value problems and associated variational principles,J. Math. Anal. Appl, 33 (1971) 402?407.