Hi,

I've seen this problem somewhere, but have no idea on how to solve it ^^'

And it looks interesting, so here it is :

Let a and b be two integers that are sums of 4 squared integers (a=m²+n²+p²+q²)

Show that ab is also the sum of 4 squared integers.

(Brute force can lead you to the result, but it has no interest ~)