Possible Duplicate:
Alternative proof that $(a^2+b^2)/(ab+1)$ is a square when it's an integer
Prove that if $c=\frac{a^2+b^2}{ab+1}\in \mathbb{N}$ then $c$ is a square given $a, b\in \mathbb{N}$.
I know that this's a very hard question, I welcome any ideas, guesses and hints, and I accept all kinds of method to solve this . Thank you.
P.S. Please don't be restricted by the tag "number theory".