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The cardinality of the continuum is the size of the set of real numbers. The continuum hypothesis is sometimes stated by saying that no cardinality lies between that of the continuum and that of the natural numbers , ℵ 0 {\displaystyle \aleph _{0}} , or alternatively, that c = ℵ 1 {\displaystyle {\mathfrak {c}}=\aleph _{1}} .
Cantor's diagonal argument shows that is strictly greater than , but it does not specify whether it is the least cardinal greater than (that is, ).Indeed the assumption that = is the well-known Continuum Hypothesis, which was shown to be consistent with the standard ZFC axioms for set theory by Kurt Gödel and to be independent of it by Paul Cohen.
Explicitly including the definition of the limit of a function, we obtain a self-contained definition: Given a function : as above and an element of the domain , is said to be continuous at the point when the following holds: For any positive real number >, however small, there exists some positive real number > such that for all in the domain ...
The continuum hypothesis asserts that is also the second aleph number, . [2] In other words, the continuum hypothesis states that there is no set A {\displaystyle A} whose cardinality lies strictly between ℵ 0 {\displaystyle \aleph _{0}} and c {\displaystyle {\mathfrak {c}}}
The continuum hypothesis was advanced by Georg Cantor in 1878, [1] and establishing its truth or falsehood is the first of Hilbert's 23 problems presented in 1900. The answer to this problem is independent of ZFC, so that either the continuum hypothesis or its negation can be added as an axiom to ZFC set theory, with the resulting theory being ...
The definition of implies (in ZF, Zermelo–Fraenkel set theory without the axiom of choice) that no cardinal number is between and . If the axiom of choice is used, it can be further proved that the class of cardinal numbers is totally ordered , and thus ℵ 1 {\displaystyle \aleph _{1}} is the second-smallest infinite cardinal number.
A subset A of a continuum X such that A itself is a continuum is called a subcontinuum of X. A space homeomorphic to a subcontinuum of the Euclidean plane R 2 is called a planar continuum . A continuum X is homogeneous if for every two points x and y in X , there exists a homeomorphism h : X → X such that h ( x ) = y .
In the mathematical field of order theory, a continuum or linear continuum is a generalization of the real line.. Formally, a linear continuum is a linearly ordered set S of more than one element that is densely ordered, i.e., between any two distinct elements there is another (and hence infinitely many others), and complete, i.e., which "lacks gaps" in the sense that every nonempty subset ...