Approximation of Non-Parametric Surfaces of Finite Area Casper Goffman Indiana University Mathematics Journal 1963 text pdf 10.1512/iumj.1963.12.12050 10.1512/iumj.1963.12.12050 en Indiana Univ. Math. J. 12 (1963) 737 - 745 state-of-the-art mathematics http://iumj.org/access/