A Variational Problem Relating to Cooling Fins R. Duffin Indiana University Mathematics Journal 1959 text pdf 10.1512/iumj.1959.8.58003 10.1512/iumj.1959.8.58003 en Indiana Univ. Math. J. 8 (1959) 47 - 56 state-of-the-art mathematics http://iumj.org/access/