In number theory, the sum of two squares theorem says when an integer n > 1 can be written as a sum of two squares, that is, when 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 prime congruent to 3 (mod 4) raised to an odd power.
This theorem supplements Fermat's two-square theorem which says when a prime number can be written as a sum of two squares, in that it also covers the case for composite numbers.
Sum of two squares theorem Wikipedia
The prime decomposition of the number 2450 is given by 2450 = 2 • 52 • 72. Of the primes occurring in this decomposition, 2, 5, and 7, only 7 is congruent to 3 modulo 4. Its exponent in the decomposition, 2, is even. Therefore, the theorem states, it is expressible as the sum of two squares. Indeed, 2450 = 72 + 492.
The prime decomposition of the number 3430 is 2 • 5 • 73. This time, the exponent of 7 in the decomposition is 3, an odd number. So 3430 cannot be written as the sum of two squares.