In mathematics, the infinite cardinal numbers are represented by the Hebrew letter
Contents
Definition
To define the beth numbers, start by letting
be the cardinality of any countably infinite set; for concreteness, take the set
which is the cardinality of the power set of A if
Given this definition,
are respectively the cardinalities of
so that the second beth number
Because of Cantor's theorem each set in the preceding sequence has cardinality strictly greater than the one preceding it. For infinite limit ordinals λ the corresponding beth number is defined as the supremum of the beth numbers for all ordinals strictly smaller than λ:
One can also show that the von Neumann universes
Relation to the aleph numbers
Assuming the axiom of choice, infinite cardinalities are linearly ordered; no two cardinalities can fail to be comparable. Thus, since by definition no infinite cardinalities are between
Repeating this argument (see transfinite induction) yields
The continuum hypothesis is equivalent to
The generalized continuum hypothesis says the sequence of beth numbers thus defined is the same as the sequence of aleph numbers, i.e.,
Beth null
Since this is defined to be
Beth one
Sets with cardinality
Beth two
Sets with cardinality
Beth omega
Generalization
The more general symbol
So
In ZF, for any cardinals κ and μ, there is an ordinal α such that:
And in ZF, for any cardinal κ and ordinals α and β:
Consequently, in Zermelo–Fraenkel set theory absent ur-elements with or without the axiom of choice, for any cardinals κ and μ, the equality
holds for all sufficiently large ordinals β (that is, there is an ordinal α such that the equality holds for every ordinal β ≥ α).
This also holds in Zermelo–Fraenkel set theory with ur-elements with or without the axiom of choice provided the ur-elements form a set which is equinumerous with a pure set (a set whose transitive closure contains no ur-elements). If the axiom of choice holds, then any set of ur-elements is equinumerous with a pure set.