Essential Singularities of Solutions of a Class of Linear Partial Differential Equations in Three Variables Stefan Bergman Indiana University Mathematics Journal 1954 text pdf 10.1512/iumj.1954.3.53027 10.1512/iumj.1954.3.53027 en Indiana Univ. Math. J. 3 (1954) 539 - 560 state-of-the-art mathematics http://iumj.org/access/