Solution
= Solution
Monomials are indexed by $\mathbb N^n$, a <countable set>. A <polynomial> is a finite list of monomials with rational coefficients, so $P_n$ is a countable union of countable sets.
= Solution
Monomials are indexed by $\mathbb N^n$, a <countable set>. A <polynomial> is a finite list of monomials with rational coefficients, so $P_n$ is a countable union of countable sets.