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The logarithm of an infinite cardinal number κ is defined as the least cardinal number μ such that κ ≤ 2 μ. Logarithms of infinite cardinals are useful in some fields of mathematics, for example in the study of cardinal invariants of topological spaces , though they lack some of the properties that logarithms of positive real numbers possess.
Cantor introduced the cardinal numbers, and showed—according to his bijection-based definition of size—that some infinite sets are greater than others. The smallest infinite cardinality is that of the natural numbers ( ℵ 0 {\displaystyle \aleph _{0}} ).
In linguistics, and more precisely in traditional grammar, a cardinal numeral (or cardinal number word) is a part of speech used to count.Examples in English are the words one, two, three, and the compounds three hundred [and] forty-two and nine hundred [and] sixty.
In set theory, a regular cardinal is a cardinal number that is equal to its own cofinality. More explicitly, this means that κ {\displaystyle \kappa } is a regular cardinal if and only if every unbounded subset C ⊆ κ {\displaystyle C\subseteq \kappa } has cardinality κ {\displaystyle \kappa } .
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.
Existence of a cardinal number κ of a given type implies the existence of cardinals of most of the types listed above that type, and for most listed cardinal descriptions φ of lesser consistency strength, V κ satisfies "there is an unbounded class of cardinals satisfying φ".
The smallest infinite cardinal number is ().The second smallest is ().The continuum hypothesis, which asserts that there are no sets whose cardinality is strictly between and , means that =. [2]
Two important generalizations of natural numbers arise from the two uses of counting and ordering: cardinal numbers and ordinal numbers. A natural number can be used to express the size of a finite set; more precisely, a cardinal number is a measure for the size of a set, which is even suitable for infinite sets.