Ultraproducts the Category Theoretic Way January 21, 2009Posted by dcorfield in Uncategorized.
Unlike limits and colimits, an ultraproduct is not defined by any universal mapping property. Of course, if the category has limits and (filtered) colimits, then it has ultraproducts constructed as colimits of products…But usually the category of models of a coherent theory (such as the theory of fields) lacks products and hence does not have categorical ultraproducts.
In fact the category theoretic definition is slightly different from the usual one, and comes with an advantage, according to Barr:
The only argument for banning the empty model that has any force comes from the observation that if is a collection of models and is a nonprincipal ultraproduct of the , then one wants and expects that will be empty if and only if the set of for which is null belongs to the ultrafilter. If one takes the traditional definition of an ultraproduct as a quotient of the product, the ultraproduct will be empty as soon as one factor is.
Defined in terms of a colimit of products this problem goes away.
But how to cope with the problem Barr mentions with fields?