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Hyperfinite set : ウィキペディア英語版
Hyperfinite set
In non-standard analysis, a branch of mathematics, a hyperfinite set or
*-finite set is a type of internal set. An internal set ''H'' of internal cardinality ''g'' ∈
*N (the hypernaturals) is hyperfinite if and only if there exists an internal bijection between ''G'' = and ''H''.〔 Hyperfinite sets share the properties of finite sets: A hyperfinite set has minimal and maximal elements, and a hyperfinite union of a hyperfinite collection of hyperfinite sets may be derived. The sum of the elements of any hyperfinite subset of
*R always exists, leading to the possibility of well-defined integration.
Hyperfinite sets can be used to approximate other sets. If a hyperfinite set approximates an interval, it is called a ''near interval'' with respect to that interval. Consider a hyperfinite set K = with a hypernatural ''n''. ''K'' is a near interval for () if ''k''1 = ''a'' and ''k''''n'' = ''b'', and if the difference between successive elements of ''K'' is infinitesimal. Phrased otherwise, the requirement is that for every ''r'' ∈ () there is a ''k''''i'' ∈ ''K'' such that ''k''''i'' ≈ ''r''. This, for example, allows for an approximation to the unit circle, considered as the set e^ for θ in the interval ().〔
In general, subsets of hyperfinite sets are not hyperfinite, often because they do not contain the extreme elements of the parent set.
== Ultrapower construction ==
In terms of the ultrapower construction, the hyperreal line
*R is defined as the collection of equivalence classes of sequences \langle u_n, n=1,2,\ldots \rangle of real numbers ''u''''n''. Namely, the equivalence class defines a hyperreal, denoted () in Goldblatt's notation. Similarly, an arbitrary hyperfinite set in
*R is of the form (), and is defined by a sequence \langle A_n \rangle of finite sets A_n \subset \mathbb, n=1,2,\ldots

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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