How many different integer values such that and there is some integer such that ?
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Claim: All perfect squares are equivalent to 0 , 1 , or 4 (mod 8) . Proof: If x is even, then x 2 is divisible by 4 , meaning it is equivalent to 0 or 4 (mod 8) . Since 8 2 ≡ 0 (mod 8) and 2 2 ≡ 4 (mod 8) , both cases are possible.
If x is odd, then x can be written in the form 2 y + 1 for some integer y . Squaring this yields 4 y 2 + 4 y + 1 . We factor this as 4 y ( y + 1 ) + 1 . Since either y or y + 1 must be even, the first term is divisible by 8 and the expression is equivalent to 1 (mod 8) .
Thus the possible values are 0 , 1 , 4 , so there are 3