Perfect Squares as Divisors of $1!\cdot 2! \cdot 3! \cdots 9!$

[anemone]

Here is this week's POTW:

-----

How many perfect squares are divisors of the product $1!\cdot 2! \cdot 3! \cdots 9!$?

-----

Remember to read the https://mathhelpboards.com/showthread.php?772-Problem-of-the-Week-%28POTW%29-Procedure-and-Guidelines to find out how to https://mathhelpboards.com/forms.php?do=form&fid=2!

 

[anemone]

Congratulations to the following members for their correct solution!

1. castor28
2. lfdahl
3. kaliprasad
4. Oxide

Solution from Oxide:

Spoiler

By expanding out the product $$n=1! 2! \cdots 9!$$, we get $$2^8 3^7 4^6 5^5 6^4 7^3 8^2 9^1$$, which can be factored into primes as $$2^{30} 3^{13} 5^5 7^3$$. Since a perfect square that divides $$n$$ must be of the form $$2^a 3^b 5^c 7^d$$ where $$a,b,c,d$$ are even, we can choose $$a,b,c,d$$ from the following sets respectively:
$$\{0,2,\dots, 30\}$$
$$\{0,2,\dots,12\}$$
$$\{0,2,4\}$$
$$\{0,2\}$$
which gives us $$16 \cdot 7 \cdot 3 \cdot 2 = 672$$ perfect squares that divide $$n$$.

Alternate solution from castor28:

Spoiler

We factorize $n!=2^\alpha3^\beta5^\gamma7^\delta$ for $2\le n\le9$ as follows (note that each line is easily computed from the previous line):
$$
\begin{array}{c|r|r|r|r}
n!&\alpha&\beta&\gamma&\delta\\
\hline
2!&1&0&0&0\\
3!&1&1&0&0\\
4!&3&1&0&0\\
5!&3&1&1&0\\
6!&4&2&1&0\\
7!&4&2&1&1\\
8!&7&2&1&1\\
9!&7&4&1&1\\
\hline
\prod{n!}&30&13&5&3
\end{array}
$$
The square divisors of the product are of the form $2^x 3^y 5^z7^w$, with $x$, $y$, $z$, $w$ even and $(x,y,z,w)\le (30,13,5,3)$.
Since the number of even integers between $0$ and $k$ is $\left\lfloor\dfrac{k}{2}\right\rfloor+1$, the number of square divisors of the product is $16\times 7\times 3\times 2 = \bf 672$.