# $2AB$ is a perfect square and $A+B$ is not a perfect square

If :$A=1!2!\cdots 1002!$, and $B=1004 ! 1005!\cdots2006!$, how to prove that:

a) $2AB$ is a perfect square

b) $A+B$ is not a perfect square

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Hint: The exponent of a prime $p$ in $n!$ is $\lfloor\frac np\rfloor + \lfloor\frac n{p^2}\rfloor+ \lfloor\frac n{p^3}\rfloor+\ldots$. –  Hagen von Eitzen Sep 16 '12 at 14:18

Let $e(n,p)$ denote the exponent of a prime $p$ in the number $n$. It is well-known that $$e(n!,p):=\left\lfloor\frac np\right\rfloor+\left\lfloor\frac n{p^2}\right\rfloor+\left\lfloor\frac n{p^3}\right\rfloor+\ldots.$$

For part b), let $p$ be a prime with $2p\le 1002<3p$ (and of course $p^2>1002$). This is equivalent to $334<p\le501$ and a prime in this range is readily found, e.g. $p=337$ or $p=499$. Then $e(n!,p)=0$ for $n<p$, $e(n!,p)=1$ for $p\le n<2p$, $e(n!,p)=2$ for $2p\le n\le 1002$. Therefore the exponent of $p$ in $A$ is given by $$e(A,p):=\sum_{n=1}^{1002} e(n,p) = p\cdot1+(1003-2p)\cdot 2=2006-3p.$$ Clearly $$e(B,p):=\sum_{n=1004}^{2006} e(n,p)\ge(2006-1003)\cdot2=2006>e(A,p).$$ Therefore, we have $e(A+B,p)=\min\{e(A,p), e(B,p)\}=e(A,p)$ is odd, hence $A+B$ cannot be a square.

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Can you complete part a ,please ? –  Frank Sep 19 '12 at 10:49

Hint for a

$$x!(x+1)!=[x!]^2 (x+1)$$

It follows that

$$A= (..)^2 \cdot 2 \cdot 4 ... \cdot 1002= (...)^2 \cdot 2^{501} \cdot 501! \,.$$

do the same to $B$, which has an odd number of terms and you are done.

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Or note that $A\cdot1003!\cdot B=(\ldots)^2\cdot 2^{1003}\cdot 1003!$. –  Hagen von Eitzen Sep 16 '12 at 15:01