# Lindelöf's Lemma/Lemma/Lemma/Lemma

 It has been suggested that this article or section be renamed: This is such a basic result it really should not be hidden as obscurely as a lemma of a lemma of a lemma of a lemma. One may discuss this suggestion on the talk page.

## Lemma

Let $S$ be countable set.

Let $T$ be a set.

Let $T$ be in one-to-one correspondence with $S$.

Then $T$ is countable.

## Proof

$S$ is countable.

Therefore, $S$ is in one-to-one correspondence with a subset of the natural numbers by a definition of countable set.

$T$ is in one-to-one correspondence with $S$.

Therefore, $T$ is in one-to-one correspondence with a subset of the natural numbers by Composite of Bijections is Bijection.

Accordingly, $T$ is countable by a definition of countable.

$\blacksquare$