If n is congruent, then multiplying n by the square of a whole number gives another congruent
number. For example, since 5 is congruent, it follows that 20 = 4 · 5 is congruent. The sides of
the triangle are doubled and the area of the triangle is quadrupled. So, the search for congruent
numbers focuses on those n which are not multiples of a square. These are called square-free.