---
tag: G9JMCHQD2M
kind: concept
name: Definable real number
aliases: [arithmetical-number, arithmetical-numbers, arithmetically-definable-number, definable-number, definable-numbers, definable-real-numbers, undefinable-number, undefinable-numbers]
sources:
  - wikipedia: en:8361@r1345380121
    locator: Definable real number
    note: adopted
    mention: wikipedia://en/8361
    sections: [Summary]
same_as:
  wikidata: Q3893614
  wikipedia: Definable real number
inventory: [E0B5EWR0W7]
---

## Summary

Informally, a **definable real number** is a [[real number|08J2M1JYAG]] that can be uniquely specified by its description. The description may be expressed as a construction or as a formula of a formal language. For example, the positive square root of 2, $\sqrt{2}$, can be defined as the unique positive solution to the equation $x^2 = 2$, and it can be constructed with a compass and straightedge.