Michael F. Singer, B. David Saunders, B. F. Caviness. An extension of Liouville's theorem on integration in finite terms. In Paul S. Wang, editor, SYMSAC 1981, Proceedings of the Symposium on Symbolic and Algebraic Manipulation, Snowbird, Utah, USA, August 5-7, 1981. pages 23-24, ACM, 1981. [doi]
Abstract is missing.