On Pythagorean triplets

We discuss properties of diophantine solutions of the Pythagoras equation, $a^2+b^2=c^2$, where the three numbers have no common factor. Some of the highlights are: (1) All triplets for which $c$ (called the `peak') is non-prime can be deduced from the triplets with prime peaks; (2) If a peak has $n+1$ prime factors, there are $2^n$ independent solutions of the Pythagoras equation; (3) All Pythagorean peaks have to be of the form $12k+1$ or $12k+5$ for integer $k$; (4) A Pythagorean peak cannot have 3, or any number of the form $12k+7$ or $12k+11$, as its prime factors.