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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notion of definable numbers has at most countably many definablereal numbers. However, by Cantor's diagonal argument, there are uncountably many real numbers...
Look up definable in Wiktionary, the free dictionary. In mathematical logic, the word definable may refer to: A definablerealnumber A definable set A...
axioms of ZF, a well ordering of the real numbers can be shown to be explicitly definable by a formula. A realnumber may be either computable or uncomputable;...
shows that there is a definable, countably saturated (meaning ω-saturated but not countable) elementary extension of the reals, which therefore has a...
In mathematics, the extended realnumber system is obtained from the realnumber system R {\displaystyle \mathbb {R} } by adding two infinity elements:...
advanced mathematics, the number line can be called the real line or realnumber line, formally defined as the set R of all real numbers. It is viewed as...
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...
language Principia Mathematica Hilbert's program Impredicative Definablerealnumber Algebraic logic Boolean algebra (logic) Dialectica space categorical...
arithmetically definable, but not vice versa. There are many arithmetically definable, noncomputable real numbers, including: any number that encodes the...
\mathbb {N} } is called arithmetically definable if the graph of f {\displaystyle f} is an arithmetical set. A realnumber is called arithmetical if the set...
imaginary number is the product of a realnumber and the imaginary unit i, which is defined by its property i2 = −1. The square of an imaginary number bi is...
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....
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...
compass construction problem put forth by Pappus. Computable numberDefinablerealnumber Kazarinoff (2003, pp. 10 & 15); Martin (1998), Corollary 2.16...
"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...
parameters in the defining formula). A function is definable in M {\displaystyle {\mathcal {M}}} (with parameters) if its graph is definable (with those parameters)...
Real Madrid Club de Fútbol (Spanish pronunciation: [reˈal maˈðɾið ˈkluβ ðe ˈfuðβol] ), commonly referred to as Real Madrid, is a Spanish professional...
rational number is a realnumber. The real numbers that are rational are those whose decimal expansion either terminates after a finite number of digits...
measure the sizes of sets, while infinity is commonly defined either as an extreme limit of the realnumber line (applied to a function or sequence that "diverges...
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...
In mathematics, the absolute value or modulus of a realnumber x {\displaystyle x} , denoted | x | {\displaystyle |x|} , is the non-negative value of...
values 0, 1 and ∞. The projectively extended realnumber line is distinct from the affinely extended realnumber line, in which +∞ and −∞ are distinct. Unlike...
that studies the several different possibilities of definingrealnumber powers or complex number powers of the differentiation operator D {\displaystyle...
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...
property of the real numbers that, intuitively, implies that there are no "gaps" (in Dedekind's terminology) or "missing points" in the realnumber line. This...
For every real number r, the constant function ( x ) ↦ r {\displaystyle (x)\mapsto r} , is everywhere defined. For every realnumber r and every function...
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...
In mathematics, the surreal number system is a totally ordered proper class containing not only the real numbers but also infinite and infinitesimal numbers...