Factorial sum

Level 2

How many positive integers x x satisfy 2 i = 1 n i ! = x 2 2\displaystyle\sum^{n}_{i=1}{i!}=x^2 , where n n is a positive integer?


The answer is 0.

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1 solution

Jeffery Li
Jun 7, 2014

Note that i = 1 n i ! \displaystyle \sum_{i=1}^{n}i! is always odd (Since 1 ! 1! is odd and all the other factorials are even), so 2 i = 1 n i ! 2 ( m o d 4 ) \displaystyle 2\sum_{i=1}^{n}i!\equiv2\pmod{4} . Since 2 2 is not a quadratic residue ( m o d 4 ) \pmod{4} , 2 i = 1 n i ! \displaystyle 2\sum_{i=1}^{n}i! will never be the square of an integer, and 0 \boxed{0} such values of x x exist.

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