On Some Approximation Methods for Solutions of the Dirichlet Problem for Linear Elliptic Equations of Arbitrary Order Felix Browder Indiana University Mathematics Journal 1958 text pdf 10.1512/iumj.1958.7.57006 10.1512/iumj.1958.7.57006 en Indiana Univ. Math. J. 7 (1958) 69 - 80 state-of-the-art mathematics http://iumj.org/access/