When teaching Algebra, I try to share my fascination about two apparently unrelated questions, which turn out to involve the same theory:
- classifying the finitely generated abelian groups,
- classifying the endomorphism of a vector space, up to similarity.
These problems are solved by using the structure of finitely generated modules over a a principal idea domain, $\mathbb Z$ in the first case, $k[X]$ in the second case.
I presume that these problems were first solved independently from each other. But,
who discovered that these are two applications of the same theorem ?