Fermat's assertion about primes of the form x² + 3y²

Fermat's assertion that "every prime number which surpasses by one a multiple of three [sic] is composed of a square and the triple of another square" raises further questions about primes and other quadratic forms. The interest over primes does not stop at x² + 3y², although this form by itself is a complex way of analyzing simple primes. We present an algorithm for solving p= x² + 3y², first discovered by Lagrange, and show that this algorithm does not work for other representations of numbers by quadratic forms.