On the Singularities of Zonal Harmonic Expansions G. Szego Indiana University Mathematics Journal 1954 text pdf 10.1512/iumj.1954.3.53028 10.1512/iumj.1954.3.53028 en Indiana Univ. Math. J. 3 (1954) 561 - 564 state-of-the-art mathematics http://iumj.org/access/