Perturbed Boundary Value Problems Whose Reduced Solutions are Nonsmooth F. Howes Indiana University Mathematics Journal 1981 text pdf 10.1512/iumj.1981.30.30021 10.1512/iumj.1981.30.30021 en Indiana Univ. Math. J. 30 (1981) 267 - 280 state-of-the-art mathematics http://iumj.org/access/