Squares is greater or factorial?

How many positive integers n are there between 1 1 and 10000 10000 such that n ! + 10 n!+10 is a perfect square?


The answer is 1.

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2 solutions

For n 4 n \geq 4

x 2 = n ! + 10 2 ( m o d 4 ) x^2=n!+10 \equiv 2 \pmod{4}

But x 2 0 , 1 ( m o d 4 ) x^2 \equiv 0,1 \pmod{4}


This implies n < 4 n<4

Checking for n = 1 , 2 , 3 n=1,2,3 we get the only solution for n = 3 n=3 .

So the answer is 1 \boxed{1}

Rushi Jogdand
Jan 2, 2015

For n>3 , n!+10 is a multiple of 2 , hence for it to be a perfect square it should have an even power of 2 .

But for n>6 , powers of 2 in n! add up to 16 or so. Thus for n>6 , n!+10 has only power of 2 i.e. 1

[We demand x × 2 y + 10 x \times 2^{y}+10 to be even power of 2 , which is not satisfied if 2 y > 10 2^{y}>10 ]

In other words, for if n>6 , n!+10 is divisible by 2 and not by 4 , so not a square.

Hence, we end up searching n between 1 and 6 .

"3" satisfies the condition.

Hence, the answer is 1 \boxed{1} .

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