indiscernible sequence?
Morley sequence?
Ramsey theorem?
Erdos-Rado theorem?
Ehrenfeucht-Fraïssé games (back-and-forth games)
Hrushovski construction?
generic predicate?
The Łoś ultraproduct theorem characterizes the first-order theory of an ultraproduct of -structures (for some signature). It generalizes the transfer principle from nonstandard analysis, by replacing the hyperreals with an ultraproduct and formulas from the standard reals with formulas which are true on an “ultrafilter-large” subset of factors of the ultraproduct.
The theorem is usually given in this form:
let be a family of nonempty -structures, and let be an ultrafilter on . Let be the ultraproduct of with respect to . Since each was nonempty, is the quotient of the product by the equivalence relation which identifies sequences which coincide on a subset of indices belonging to . Let be such a sequence, and let denote its equivalence class. Then for each -formula ,
The above follows from a more fundamental fact:
Any functor
which specifies a choice of ultraproduct (and hence comparison maps, since ultraproducts are colimits) is elementary (logical in the terminology of Makkai-Reyes, i.e. a pretopos morphism.)
Recall that the ultraproduct sends the family of sets to the colimit of , indexed by . (the opposite category of the sub-poset ) is filtered because is a filter.
A functor is a pretopos morphism precisely when it preserves finite limits, initial objects, disjoint sums, and quotients by internal equivalence relations.
Finite limits: a -indexed product of -indexed limits is a -indexed limit of -indexed products, and finite limits commute with filtered colimits.
Initial objects: a -indexed product of empty sets is empty (as each is nonempty by properness of ), and a colimit of empty sets is empty.
Disjoint sums: a -indexed product of disjoint sums is a disjoint sum of products . For any in this set, there is a -large subset such that the restriction of to is constantly (). Therefore the restriction map identifies with , and the -indexed colimit of the is equivalent to the -indexed colimit of the , and colimits commute with colimits.
Quotients: Given and equivalence relations , a -indexed product of quotients is a quotient of products (because the natural map from the latter to the former is an iso, using the axiom of choice), and colimits commute with colimits.
Now we can show the theorem:
Since the ultraproduct functor is elementary, then the process of taking points of a definable set inside models commutes with taking ultraproducts. In symbols,
Since this is a filtered colimit, a sequence satisfies that its germ is in if and only if there is some such that the restriction of to is in . i.e. if for each .
An immediate consequence of the Łoś theorem is the transfer principle for the hyperreals.
The compactness theorem also follows quickly from the Łoś theorem, so anything that you build using compactness can be realized a bit more concretely as an ultraproduct.
The proposition should be interpreted as saying that Set simultaneously carries a pretopos structure and an ultracategory structure, and that these two structures commute. Indeed, one can view Set as a dualizing object for a generalized Stone duality between pretoposes and ultracategories; this is the starting point of Makkai duality.
There is an analogous statement for ultraproducts of structures in continuous logic.
Hisashi Aratake, Sheaves of Structures, Heyting-Valued Structures, and a Generalization of Łoś’s Theorem , arXiv:2012.04317 (2020). (abstract)
Michael Makkai, Stone duality for first order logic, Advances in Mathematics, 65(2):97–170, 1987.
Last revised on May 17, 2022 at 17:51:54. See the history of this page for a list of all contributions to it.