I haven't dabbled with GP's algebraic functions very much. I recently came across this problem: for which n is n plus a perfect square a triangular number? That is, given an even number 2n, does the Diophantine equation
y^2 + y - 2x^2 = 2n
have a solution?
I've only used qfbsolve and thue before, which of course don't apply here, but I'm pretty sure this is implemented in PARI. Hint, anyone?
Charles Greathouse
Analyst/Programmer
Case Western Reserve University