What do you know about Squares?

The set consists of all possible positive perfect squares.Let an integer be $a$ .Then it's square will be $a^2$ and let a second integer be $b$ .Then it's square will be $b^2$ .Both of these are integers which are present in the set $u$ .Then: $a^2\times b^2\\=a\times a\times b\times b\\=(a\times b)\times(a\times b)\\=ab\times ab=(ab)^2$ And as $a$ and $b$ are integers so $ab$ is an integer so $(ab)^2$ is an integer which is present in the set.So the set $u$ is closed under $\boxed{Multiplication}$

(x^2)*(y^2) = (xy)^2. Ed Gray