On the Summability of Bivariate Rational Functions
  We present criteria for deciding whether a bivariate rational function in two variables can be written as a sum of two (q-)differences of bivariate rational functions. Using these criteria, we show how certain double sums can be evaluated, first, in terms of single sums and, finally, in terms of values of special functions. 
Shaoshi Chen and Michael F. Singer
math.CO cs.SC