Let be the class of expressions generated by
See also Recursion, Undecidable
References
Caviness, B. F. ``On Canonical Forms and Simplification.'' J. Assoc. Comp. Mach. 17, 385-396, 1970.
Petkovsek, M.; Wilf, H. S.; and Zeilberger, D. A=B. Wellesley, MA: A. K. Peters, 1996.
Richardson, D. ``Some Unsolvable Problems Involving Elementary Functions of a Real Variable.'' J. Symbolic Logic 33,
514-520, 1968.