  Let $m$ be a fixed square-free positive integer, then equivalence classes of
solutions of Diophantine equation $x^2+m\cdot y^2=z^2$ form an infinitely
generated abelian group under the operation induced by the complex
multiplication. A basis of this group is constructed here using prime ideals
and the ideal class group of the field $\mathbb Q (\sqrt{-m})$.
