Published online by Cambridge University Press: 12 March 2014
The basic setting of nonstandard analysis consists of a set-theoretical structure together with a map * from
into another structure *
of the same sort. The function * is taken to be an elementary embedding (in an appropriate sense) and is generally assumed to make *
into an enlargement of
[13]. The structures
and *
may be type-hierarchies as in [11] and [13] or they may be cumulative structures with ω levels as in [14]. The assumption that *
is an enlargement of
has been found to be the weakest hypothesis which allows for the familiar applications of nonstandard analysis in calculus, elementary topology, etc. Indeed, practice has shown that a smooth and useful theory can be achieved only by assuming also that *
has some stronger properties such as the saturation properties first introduced in nonstandard analysis by Luxemburg [11].
This paper concerns an entirely new family of properties, stronger than the saturation properties. For each cardinal number κ, * satisfies the κ-isomorphism property (as an enlargement of
) if the following condition holds:
For each first order language L with fewer than κ nonlogical symbols, if and
are elementarily equivalent structures for L whose domains, relations and functions are all internal (relative to *
and
), then
and
are isomorphic.