In mathematics, particularly in nonstandard analysis, overspill is a widely used proof technique. It is based on the fact that N is not an internal subset of the nonstandard integers *N. Indeed, by applying the induction principle and transfer principle we get the following general principle Wikibooks Wikiversity has more about this subject: School of Mathematics Wikiquote has a collection of quotations related to: Mathematics Look up Mathematics in Wiktionary, the free dictionary Wikimedia Commons has media related to: Mathematics Interactive Mathematics Miscellany and Puzzles â€” A collection of articles on various math topics, with interactive Java...
Nonstandard analysis is that branch of mathematics that formulates analysis using a rigorous notion of infinitesimal, where an element of an ordered field F is infinitesimal if and only if its absolute value is smaller than any element of F of the form 1/n, for n a natural...
In mathematical logic, in particular in model theory and nonstandard analysis, an internal set is a set that is a member of a model. ...
In mathematics particularly in nonstandard analysis, the transfer principle is a rule which transforms assertions about standard sets, mappings etc. ...
for any internal subset A of *N, if  1 is an element of A and
 for every element n of A, n+1 also belongs to A
then  A= *N
Instantiating this general principle with N, it would follow N=*N which we know not to be the case. This principle has a number of extremely useful consequences:  The set of standard hyperreals is not internal.
 The set of bounded hyperreals is not internal.
 The set of infinitesimal hyperreals is not internal.
In particular:  If an internal set contains all infinitesimal nonnegative hyperreals, it contains a positive noninfinitesimal (or appreciable) hyperreal.
 If an internal set contains N it contains an unbounded element of *N.
Example
We can use these facts to prove equivalence of the following two conditions for an internal hyperrealvalued function f defined on *R. and The proof that the second fact implies the first uses overspill, since given a noninfinitesimal positive ε By overspill a positive appreciable δ with the requisite properties exists. These equivalent conditions express the property known in nonstandard analysis as Scontinuity of f at x. Scontinuity is referred to as an external property, since its extension (e.g. the set of pairs (f, x) such that f is Scontinuous at x) is not an internal set. In metaphysics, extension is the property of taking up space; see Extension (metaphysics). ...
