Sum of two squares theorem
In number theory, the sum of two squares theorem relates the prime decomposition of any integer n > 1 to whether it can be written as a sum of two squares, such that n = a2 + b2 for some integers a, b. An integer greater than one can be written as a sum of two squares if and only if its prime decomposition contains no factor pk, where prime p ≡ 3 ( mod 4 ) {\displaystyle p\equiv 3{\pmod {4}}} and k is odd.
Source: Wikipedia — Sum of two squares theorem (CC BY-SA 4.0)