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The long real line pastes together ℵ 1 * + ℵ 1 copies of the real line plus a single point (here ℵ 1 * denotes the reversed ordering of ℵ 1) to create an ordered set that is "locally" identical to the real numbers, but somehow longer; for instance, there is an order-preserving embedding of ℵ 1 in the long real line but not in the real ...
Including 0, the set has a semiring structure (0 being the additive identity), known as the probability semiring; taking logarithms (with a choice of base giving a logarithmic unit) gives an isomorphism with the log semiring (with 0 corresponding to ), and its units (the finite numbers, excluding ) correspond to the positive real numbers.
In summary, a set of the real numbers is an interval, if and only if it is an open interval, a closed interval, or a half-open interval. [4] [5] A degenerate interval is any set consisting of a single real number (i.e., an interval of the form [a, a]). [6] Some authors include the empty set in this definition.
A mathematical symbol is a figure or a combination of figures that is used to represent a mathematical object, ... Denotes the set of real numbers.
The mathematical symbol for the set of all natural numbers is N, ... The symbol for the real numbers is R, also written as . They include all the measuring numbers. ...
Computable number: A real number whose digits can be computed by some algorithm. Period: A number which can be computed as the integral of some algebraic function over an algebraic domain. Definable number: A real number that can be defined uniquely using a first-order formula with one free variable in the language of set theory.
The set of real algebraic numbers is countably infinite (assign to each formula its Gödel number.) So the cardinality of the real algebraic numbers is ℵ 0 {\displaystyle \aleph _{0}} . Furthermore, the real algebraic numbers and the real transcendental numbers are disjoint sets whose union is R {\displaystyle \mathbb {R} } .
The rationals are a dense subset of the real numbers; every real number has rational numbers arbitrarily close to it. [6] A related property is that rational numbers are the only numbers with finite expansions as regular continued fractions. [18] In the usual topology of the real numbers, the rationals are neither an open set nor a closed set. [19]