An analysis of relationships between Craig-style interpolation, compactness, and other related model-theoretic properties is carried out in the softer framework of categories of pre-institutions. While the equivalence between sentence interpolation and the Robinson property under compactness and Boolean closure is well known, a similar result under different assumptions (not involving compactness) is newly established for presentation interpolation. The standard concept of naturality of model transformation is enriched by a new property, termed restriction adequacy, which proves useful for the reduction of interpolation along pre-institution transformations. A distinct reduction theorem for the Robinson property is presented as well. A variant of the ultraproduct concept is further introduced, and the related closure property for pre-institutions is shown to be equivalent to compactness.* resting at some floor of 'palais Grothendieck'. * REL is the category of sets with binary relations as morphisms, |C| is the set of objects of category C, while ||C|| is the set of morphisms of category C.
A. Salibra and G. Scollo 276Proposition 4.7. If 9~ :J-+f is adequate, then (ET)^-£ E$-%$-for all signature morphisms T:XI->^2 and I^-presentations E in ,/.Prao/ E(=T(ET) by Proposition 4.5(vi), hence E^-|=(T(ET))^ by Proposition 2.7(ii) since 3~ is adequate, therefore E#-\=Ty-(Er)y-by naturality of presentation transformation. Proposition 4.5(vi) also entails E$-\= T$-{E$-r$-), but in fact, according to Definition 4.4(ii), EJTTT is the largest set