Square product

Answer: 8

We make use of two key facts:

First, $n^2 -1 = (n-1)(n+1)$.

Second, when a square number is factorised, each prime factor appears an even number of times.

 

Now $2^2 -1 = 1\times 3$.

We next get a prime factor $3$ with $4^2-1= 3\times 5$.

We next get a prime factor $5$ with $6^2-1= 5\times 7$.

We next get a prime factor $7$ with $8^2-1= 7\times 9$.

 

As $9=3^2$, it does not require any further factors. Hence we need $n\geq 8$. Checking the product $n=8$, we get

$(2^2 -1)(3^2 -1)(4^2 -1)(5^2 -1)(6^2 -1)(7^2 -1)(8^2 -1)$

$$\begin{array}{rl}& =1\times 3\times 2\times 4\times 3\times 5\times 4\times 6\times 5\times 7\times 6\times 8\times 7\times 9\\ & =2\times 8\times 3\times 3\times 4\times 4\times 5\times 5\times 6\times 6\times 7\times 7\times 3\times 3\\ & =4\times 4\times 3\times 3\times 4\times 4\times 5\times 5\times 6\times 6\times 7\times 7\times 3\times 3\\ & =(4\times 3\times 4\times 5\times 6\times 7\times 3{)}^2\end{array}$$

So in fact $n=8$ is sufficient, and is thus the minimum.