How many positive integers n are there between and such that is a perfect square?
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For n ≥ 4
x 2 = n ! + 1 0 ≡ 2 ( m o d 4 )
But x 2 ≡ 0 , 1 ( m o d 4 )
This implies n < 4
Checking for n = 1 , 2 , 3 we get the only solution for n = 3 .
So the answer is 1