Prove that 1^3 + 2^3 + · · · + n^3 = (1 + 2 + · · · + n)^2 for
every n ∈ N. That is, the sum of the first n perfect cubes is the
square of the sum of the first n natural numbers. (As a student, I
found it very surprising that the sum of the first n perfect cubes
was always a perfect square at all.)