A set is called countable if it can be placed into a one-to-one correspondence with some subset of the natural numbers. Thus every finite set is countable, and many infinite sets.
- [MLA] “countable.” Platonic Realms Interactive Mathematics Encyclopedia. Platonic Realms, 10 Apr 2014. Web. 25 May 2018. <http://platonicrealms.com/>
- [APA] countable (10 Apr 2014). Retrieved 25 May 2018 from the Platonic Realms Interactive Mathematics Encyclopedia: http://platonicrealms.com/encyclopedia/countable/