The Algebra of Singular Integral Operators in R^n H. Cordes Indiana University Mathematics Journal 1965 text pdf 10.1512/iumj.1965.14.14064 10.1512/iumj.1965.14.14064 en Indiana Univ. Math. J. 14 (1965) 1007 - 1032 state-of-the-art mathematics http://iumj.org/access/