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Limit cardinal : ウィキペディア英語版
Limit cardinal
In mathematics, limit cardinals are certain cardinal numbers. A cardinal number λ is a weak limit cardinal if λ is neither a successor cardinal nor zero. This means that one cannot "reach" λ from another cardinal by repeated successor operations. These cardinals are sometimes called simply "limit cardinals" when the context is clear.
A cardinal λ is a strong limit cardinal if λ cannot be reached by repeated powerset operations. This means that λ is nonzero and, for all κ < λ, 2κ < λ. Every strong limit cardinal is also a weak limit cardinal, because κ+ ≤ 2κ for every cardinal κ, where κ+ denotes the successor cardinal of κ.
The first infinite cardinal, \aleph_0 (aleph-naught), is a strong limit cardinal, and hence also a weak limit cardinal.
== Constructions ==

One way to construct limit cardinals is via the union operation: \aleph_ is a weak limit cardinal, defined as the union of all the alephs before it; and in general \aleph_ for any limit ordinal λ is a weak limit cardinal.
The ב operation can be used to obtain strong limit cardinals. This operation is a map from ordinals to cardinals defined as
:\beth_ = \aleph_0,
:\beth_ = 2^ = \bigcup \.
The cardinal
:\beth_ = \bigcup \, \beth_, \ldots \} = \bigcup_ \beth_
is a strong limit cardinal of cofinality ω. More generally, given any ordinal α, the cardinal
:\beth_ = \bigcup_ \beth_
is a strong limit cardinal. Thus there are arbitrarily large strong limit cardinals.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
ウィキペディアで「Limit cardinal」の詳細全文を読む



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