Set theory

Definable real number

Informally, a definable real number is a real number 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, , can be defined as the unique positive solution to the equation , and it can be constructed with a compass and straightedge. Different choices of a formal language or its interpretation give rise to different notions of definability. Specific varieties of definable numbers include the constructible numbers of geometry, the algebraic numbers, and the computable numbers. Because formal languages can have only countably many formulas, every notion of definable numbers has at most countably many definable real numbers. However, by Cantor's diagonal argument, there are uncountably many real numbers, so almost every real number is undefinable. (Wikipedia).

Definable real number
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Related pages

Set theory | Countable set | Chaitin's constant | Almost everywhere | Algebraic number | Ordinal definable set | Polynomial | Arithmetical hierarchy | Model theory | Analytical hierarchy | Formal language | E (mathematical constant) | Entscheidungsproblem | Doubling the cube | Algorithmically random sequence | Specker sequence | Zermelo–Fraenkel set theory | Field extension | Alan Turing | Natural number | Constructible universe | Tarski's undefinability theorem | Real number | Second-order arithmetic | Pi | Computable number | Dedekind cut | Class (set theory) | Cardinality | Von Neumann universe | Cantor's diagonal argument | Constructible number