21.1.2 Use unique prime factorization to show that a positive integer $n$ is a square
if and only if each prime in the prime factorization of $n$ occurs to an even
power.
21.1.3 If $l$ and $m$ are positive integers with no common prime divisor, and $lm$ is a
square, use Exercise 21.1.2 to show that $l$ and $m$ are both squares.
21.1.4 Show similarly that if $l$ and $m$ are integers with no common prime divisor,
and if $lm$ is a cube, then $l$ and $m$ are both cubes.