Optimum Heat Transfer and Network Programming R. Duffin Indiana University Mathematics Journal 1968 text pdf 10.1512/iumj.1968.17.17047 10.1512/iumj.1968.17.17047 en Indiana Univ. Math. J. 17 (1968) 759 - 768 state-of-the-art mathematics http://iumj.org/access/