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Beth number : ウィキペディア英語版
Beth number
In mathematics, the infinite cardinal numbers are represented by the Hebrew letter \aleph (aleph) indexed with a subscript that runs over the ordinal numbers (see aleph number). The second Hebrew letter \beth (beth) is used in a related way, but does not necessarily index all of the numbers indexed by \aleph.
== Definition ==
To define the beth numbers, start by letting
:\beth_0=\aleph_0
be the cardinality of any countably infinite set; for concreteness, take the set \mathbb of natural numbers to be a typical case. Denote by ''P''(''A'') the power set of ''A''; i.e., the set of all subsets of ''A''. Then define
:\beth_=2^ is the cardinality of ''A''.
Given this definition,
:\beth_0,\ \beth_1,\ \beth_2,\ \beth_3,\ \dots
are respectively the cardinalities of
:\mathbb,\ P(\mathbb),\ P(P(\mathbb)),\ P(P(P(\mathbb))),\ \dots.
so that the second beth number \beth_1 is equal to \mathfrak c, the cardinality of the continuum, and the third beth number \beth_2 is the cardinality of the power set of the continuum.
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:
:\beth_=\sup\.
One can also show that the von Neumann universes V_ \! have cardinality \beth_ \!.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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