Generalized Variational Principles for Convective Heat Transfer and Irreversible Thermodynamics M. Biot Indiana University Mathematics Journal 1966 text pdf 10.1512/iumj.1966.15.15013 10.1512/iumj.1966.15.15013 en Indiana Univ. Math. J. 15 (1966) 177 - 186 state-of-the-art mathematics http://iumj.org/access/