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Internal set : ウィキペディア英語版
Internal set

In mathematical logic, in particular in model theory and non-standard analysis, an internal set is a set that is a member of a model.
The concept of internal sets is a tool in formulating the transfer principle, which concerns the logical relation between the properties of the real numbers R, and the properties of a larger field denoted
*R called the hyperreal numbers. The field
*R includes, in particular, infinitesimal ("infinitely small") numbers, providing a rigorous mathematical justification for their use. Roughly speaking, the idea is to express analysis over R in a suitable language of mathematical logic, and then point out that this language applies equally well to
*R. This turns out to be possible because at the set-theoretic level, the propositions in such a language are interpreted to apply only to internal sets rather than to all sets (note that the term "language" is used in a loose sense in the above).
Edward Nelson's internal set theory is an axiomatic approach to non-standard analysis (see also Palmgren at constructive non-standard analysis). Conventional infinitary accounts of non-standard analysis also use the concept of internal sets.
==Internal sets in the ultrapower construction==
Relative to the ultrapower construction of the hyperreal numbers as equivalence classes of sequences \langle u_n\rangle, an internal subset () of
*R is one defined by a sequence of real sets \langle A_n \rangle, where a hyperreal () is said to belong to the set ()\subset \; ^
*\! if and only if the set of indices n such that u_n \in A_n, is a member of the ultrafilter used in the construction of
*R.
More generally, an internal entity is a member of the natural extension of a real entity. Thus, every element of
*R is internal; a subset of
*R is internal if and only if it is a member of the natural extension ^
* \mathcal(\mathbb) of the power set \mathcal(\mathbb) of R; etc.

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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