Prove that there does not exist a power of 2 that is expressible as the sum of three positive perfect squares.
Any perfect square is congruent to 0 or 1 mod 4. So if the sum of three perfect squares is to be a power of 2 it must equal 1 or 2 or be a multiple of 4. The cases 1 and 2 are ruled out since we have three *positive* squares. So the sum must be congruent to 0 mod 4 and each square must be even, and thus the square of an even number. Let these even numbers be 2a, 2b and 2c. Then we have:
(2a)² + (2b)² + (2c)² = 2^k, for some integer k >= 2. Hence:
a² + b² + c² = 2^(k2).
Clearly we can continue in this way until the power of 2 is either 2^0 or 2^1. But neither of these is a solution, so by contradiction no solution is possible.
This is an example of a proof by infinite descent, pioneered by Fermat.

Posted by Nick
on 20100228 16:22:36 