# 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 = {1,2,3,...,g} and H.[1][2] 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.[2]

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 ${\displaystyle K={k_{1},k_{2},\dots ,k_{n}}}$ with a hypernatural n. K is a near interval for [a,b] if k1 = a and kn = b, and if the difference between successive elements of K is infinitesimal. Phrased otherwise, the requirement is that for every r ∈ [a,b] there is a kiK such that kir. This, for example, allows for an approximation to the unit circle, considered as the set ${\displaystyle e^{i\theta }}$ for θ in the interval [0,2π].[2]

In general, subsets of hyperfinite sets are not hyperfinite, often because they do not contain the extreme elements of the parent set.[3]

## Ultrapower construction

In terms of the ultrapower construction, the hyperreal line *R is defined as the collection of equivalence classes of sequences ${\displaystyle \langle u_{n},n=1,2,\ldots \rangle }$ of real numbers un. Namely, the equivalence class defines a hyperreal, denoted ${\displaystyle [u_{n}]}$ in Goldblatt's notation. Similarly, an arbitrary hyperfinite set in *R is of the form ${\displaystyle [A_{n}]}$, and is defined by a sequence ${\displaystyle \langle A_{n}\rangle }$ of finite sets ${\displaystyle A_{n}\subset \mathbb {R} ,n=1,2,\ldots }$[4]

## Notes

1. ^ J. E. Rubio (1994). Optimization and nonstandard analysis. Marcel Dekker. p. 110. ISBN 0-8247-9281-5.
2. ^ a b c R. Chuaqui (1991). Truth, possibility, and probability: new logical foundations of probability and statistical inference. Elsevier. pp. 182–3. ISBN 0-444-88840-3.
3. ^ L. Ambrosio; et al. (2000). Calculus of variations and partial differential equations: topics on geometrical evolution problems and degree theory. Springer. p. 203. ISBN 3-540-64803-8.
4. ^ R. Goldblatt (1998). Lectures on the hyperreals. An introduction to nonstandard analysis. Springer. p. 188. ISBN 0-387-98464-X.