---
tag: X2VQ566AVR
kind: concept
name: Cocountability
aliases: [co-countability, co-countable, co-countably, cocountable, cocountable-set, cocountable-subset, cocountably, countable-cocountable-algebra]
sources:
  - wikipedia: en:801119@r1360631332
    locator: Cocountability
    note: adopted
    mention: wikipedia://en/801119
    sections: [Summary]
same_as:
  wikidata: Q5139907
  wikipedia: Cocountability
inventory: [CEHCB984JK]
---

## Summary

In mathematics, a **cocountable** [[subset|0BJR9QR08E]] of a set $X$ is a subset $Y$ whose [[complement|0RKF1ZT5JJ]] in $X$ is a [[countable set|01TQB88H4C]]. In other words, $Y$ contains all but countably many elements of $X$. Since the rational numbers are a countable subset of the reals, for example, the irrational numbers are a cocountable subset of the reals. If the complement is finite, then one says $Y$ is cofinite.