Proof. As above, we may assume that n is odd, or n = 2m, m odd. Assume
first that n is odd. Writing n = x2
0+x2
1+x2
2+x2
3, we have by parity consideration
that either exactly three odd xi’s, say x0, x1, and x2 or exactly one odd, say
x0.
Proof. As above, we may assume that n is odd, or n = 2m, m odd. Assumefirst that n is odd. Writing n = x20+x21+x22+x23, we have by parity considerationthat either exactly three odd xi’s, say x0, x1, and x2 or exactly one odd, sayx0.
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