Editing Transfer principle (section)

==Transfer principle for the hyperreals==
{{See also|Hyperreal number#Transfer principle}}
The transfer principle concerns the logical relation between the properties of the real numbers '''R''', and the properties of a larger field denoted *'''R''' called the [[hyperreal number]]s.  The field *'''R''' includes, in particular, infinitesimal ("infinitely small") numbers, providing a rigorous mathematical realisation of a project initiated by Leibniz.

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 set]]s rather than to all sets. As [[Abraham Robinson|Robinson]] put it, ''the sentences of [the theory] are interpreted in *'''R''' in [[Leon Henkin|Henkin]]'s sense.''<ref>Robinson, A. The metaphysics of the calculus, in Problems in the Philosophy of Mathematics, ed. Lakatos (Amsterdam: North Holland), pp. 28–46, 1967. Reprinted in the 1979 Collected Works. Page 29.</ref>

The theorem to the effect that each proposition valid over '''R''', is also valid over *'''R''', is called the transfer principle.

There are several different versions of the transfer principle, depending on what model of nonstandard mathematics is being used. 
In terms of model theory, the transfer principle states that a map from a standard model to a nonstandard model is an [[elementary equivalence|elementary embedding]] (an embedding preserving the [[truth value]]s of all statements in a language), or sometimes a ''bounded'' elementary embedding (similar, but only for statements with [[bounded quantifier]]s).{{clarification needed|date=November 2022|reason=Why only "sometimes"? When, or when not?}}

The transfer principle appears to lead to contradictions if it is not handled correctly.
For example, since the hyperreal numbers form a non-[[Archimedean property|Archimedean]] [[ordered field]] and the reals form an Archimedean ordered field, the property of being Archimedean ("every positive real is larger than <math>1/n</math> for some positive integer <math>n</math>") seems at first sight not to satisfy the transfer principle. The statement "every positive hyperreal is larger than <math>1/n</math> for some positive integer <math>n</math>" is false; however the correct interpretation is "every positive hyperreal is larger than <math>1/n</math> for some positive [[hyperinteger]] <math>n</math>". In other words, the hyperreals appear to be Archimedean to an internal observer living in the nonstandard universe, but appear
to be non-Archimedean to an external observer outside the universe.

A freshman-level accessible formulation of the transfer principle is [[Howard Jerome Keisler|Keisler's]] book ''[[Elementary Calculus: An Infinitesimal Approach]]''.

===Example===
Every real <math>x</math> satisfies the inequality
<math display=block>x \geq \lfloor x \rfloor,</math>
where <math>\lfloor \,\cdot\, \rfloor</math> is the [[floor and ceiling functions|integer part]] function.  By a typical application of the transfer principle, every hyperreal <math>x</math> satisfies the inequality
<math display=block>x \geq {}^{*}\! \lfloor x \rfloor,</math>
where <math>{}^{*}\! \lfloor \,\cdot\, \rfloor</math> is the natural extension of the integer part function.  If <math>x</math> is infinite, then the [[hyperinteger]] <math>{}^{*}\! \lfloor x \rfloor</math> is infinite, as well.