# How to prove that the number $1!+2!+3!+…+n!$ is never square?

How to prove that the number $1!+2!+3!+...+n! \ \forall n \geq 4$ is never square?

I was told to count permutations but I cannot figure out what we are permuting.... Thanks!

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My apologies, for n>3 – Adam L. Apr 19 '11 at 18:16

The first few cases are easily dealt with: $1!=1$ and $1!+2!+3!=9$ are squares, while $1!+2!=3$ is not.

For $n \ge 4$, $1!+2!+3!+ \ldots + n!$ is congruent to 3 mod 5. But all squares are congruent to 0, 1, or 4 mod 5.

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Or in simpler terms, a perfect square never ends in 3 (i.e talking mod 10 instead of mod 5). – Aryabhata Apr 19 '11 at 18:11

Firstly, it's only true for n > 3. Secondly, I don't see how to do it with permutations.

But the key is to consider residue classes mod 10. $1! + 2! + ... + n!$ is congruent to either 3 or 8 mod 10, but no square ends in either 3 or 8.

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Oh, it was already answered! – mixedmath Apr 19 '11 at 18:25
It is always $3$, never $8$, when $n>3$, isn't it? – Jonas Meyer May 31 '12 at 19:05