A General Variational Identity Patrizia PucciJames Serrin Indiana University Mathematics Journal 1986 text pdf 10.1512/iumj.1986.35.35036 10.1512/iumj.1986.35.35036 en Indiana Univ. Math. J. 35 (1986) 681 - 703 state-of-the-art mathematics http://iumj.org/access/