Let r(n,k) = -k^2(3n+3-2k)/(2(n+1-k)^2(2n+1)) f(n,k)=factorial(n)^4/(factorial(k)^2factorial(n-k)^2factorial(2n)) g(n,k)=r(n,k)f(n,k) from Petkovsek, Wilf & Zeilberger's A=B.
The authors show how to use Mathematica or Maple to prove that f(n+1,k)-f(n,k)-g(n,k+1)+g(n,k) is zero for any integer values of k and n.
I am not able to prove this with Sage (10.3). Any idea?