A discrete Function is called closed form (or sometimes ``hypergeometric'') in two variables if the
ratios
and
are both Rational Functions. A pair of
closed form functions is said to be a Wilf-Zeilberger Pair if
See also Rational Function, Wilf-Zeilberger Pair
References
Petkovsek, M.; Wilf, H. S.; and Zeilberger, D. A=B. Wellesley, MA: A. K. Peters, p. 141, 1996.
Zeilberger, D. ``Closed Form (Pun Intended!).'' Contemporary Math. 143, 579-607, 1993.