# SP20:Lecture 9 prep

We will cover countability and diagonalization. Last semester, some of this material was covered in lecture 8 and the rest was in lecture 9.

Please come to lecture with the following definitions:

Definition: Finite
A set is finite if there exists a natural number such that .
Definition: Countable
A set is countable if .

You may also find it helpful to read the entry on Diagonalization and the proofs linked from that entry.