For topological spaces $X$ and $Y$, is it possible that $X \times X$ and $Y \times Y$ are homeomorphic, but $X$ and $Y$ are not homeomorphic?
(I poked around with finite spaces, and manifolds, and the Cantor set, without seeing any examples.)
This was inspired by Existence of topological space which has no “square-root” but whose “cube” has a “square-root”. Cartesian product makes the proper class of topological spaces (up to homeomorphism) into a large abelian monoid, so here's a bonus question: what is known about the structure of this monoid? For example, the dogbone space shows that it is not cancellative. Does it have torsion in the sense that sometimes $X^n \not\cong X$ but $X^{n + 1} \cong X$?
Best Answer
Copying part of my answer to this Math Overflow question: