ThmDex – An index of mathematical definitions, results, and conjectures.

ThmDex – An index of mathematical definitions, results, and conjectures.

Formulation F7955 on R3506: Finite union of countable sets is countable

F7955

Formulation 0

Let $X_1, \dots, X_N$ each be a D16: Countable set.

Then $\bigcup_{n = 1}^N X_n$ is a D16: Countable set.