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Definable real number information


The square root of 2 is equal to the length of the hypotenuse of a right triangle with legs of length 1 and is therefore a constructible 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.

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Definable real number

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notion of definable numbers has at most countably many definable real numbers. However, by Cantor's diagonal argument, there are uncountably many real numbers...

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Definable

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Look up definable in Wiktionary, the free dictionary. In mathematical logic, the word definable may refer to: A definable real number A definable set A...

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Real number

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axioms of ZF, a well ordering of the real numbers can be shown to be explicitly definable by a formula. A real number may be either computable or uncomputable;...

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Hyperreal number

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shows that there is a definable, countably saturated (meaning ω-saturated but not countable) elementary extension of the reals, which therefore has a...

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Extended real number line

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In mathematics, the extended real number system is obtained from the real number system R {\displaystyle \mathbb {R} } by adding two infinity elements:...

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Number line

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advanced mathematics, the number line can be called the real line or real number line, formally defined as the set R of all real numbers. It is viewed as...

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Berry paradox

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therefore it is definable in under sixty letters, and is not the smallest positive integer not definable in under sixty letters, and is not defined by this expression...

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List of mathematical logic topics

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language Principia Mathematica Hilbert's program Impredicative Definable real number Algebraic logic Boolean algebra (logic) Dialectica space categorical...

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Computable number

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arithmetically definable, but not vice versa. There are many arithmetically definable, noncomputable real numbers, including: any number that encodes the...

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Arithmetical set

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\mathbb {N} } is called arithmetically definable if the graph of f {\displaystyle f} is an arithmetical set. A real number is called arithmetical if the set...

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Imaginary number

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imaginary number is the product of a real number and the imaginary unit i, which is defined by its property i2 = −1. The square of an imaginary number bi is...

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Totally real number field

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In number theory, a number field F is called totally real if for each embedding of F into the complex numbers the image lies inside the real numbers....

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Complex number

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In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted i, called the imaginary...

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Constructible number

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compass construction problem put forth by Pappus. Computable number Definable real number Kazarinoff (2003, pp. 10 & 15); Martin (1998), Corollary 2.16...

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Glossary of set theory

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"Dedekind infinite".) def The set of definable subsets of a set definable A subset of a set is called definable set if it is the collection of elements...

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Definable set

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parameters in the defining formula). A function is definable in M {\displaystyle {\mathcal {M}}} (with parameters) if its graph is definable (with those parameters)...

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Real Madrid CF

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Real Madrid Club de Fútbol (Spanish pronunciation: [reˈal maˈðɾið ˈkluβ ðe ˈfuðβol] ), commonly referred to as Real Madrid, is a Spanish professional...

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Rational number

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rational number is a real number. The real numbers that are rational are those whose decimal expansion either terminates after a finite number of digits...

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Aleph number

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measure the sizes of sets, while infinity is commonly defined either as an extreme limit of the real number line (applied to a function or sequence that "diverges...

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Construction of the real numbers

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In mathematics, there are several equivalent ways of defining the real numbers. One of them is that they form a complete ordered field that does not contain...

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Absolute value

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In mathematics, the absolute value or modulus of a real number x {\displaystyle x} , denoted | x | {\displaystyle |x|} , is the non-negative value of...

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Projectively extended real line

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values 0, 1 and ∞. The projectively extended real number line is distinct from the affinely extended real number line, in which +∞ and −∞ are distinct. Unlike...

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Fractional calculus

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that studies the several different possibilities of defining real number powers or complex number powers of the differentiation operator D {\displaystyle...

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Transcendental number

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In mathematics, a transcendental number is a real or complex number that is not algebraic – that is, not the root of a non-zero polynomial of finite degree...

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Completeness of the real numbers

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property of the real numbers that, intuitively, implies that there are no "gaps" (in Dedekind's terminology) or "missing points" in the real number line. This...

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Function of a real variable

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For every real number r, the constant function ( x ) ↦ r {\displaystyle (x)\mapsto r} , is everywhere defined. For every real number r and every function...

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Natural number

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the number 1 differently than larger numbers, sometimes even not as a number at all. Euclid, for example, defined a unit first and then a number as a...

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Surreal number

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In mathematics, the surreal number system is a totally ordered proper class containing not only the real numbers but also infinite and infinitesimal numbers...

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