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INFINITARY GENERALIZATIONS OF DELIGNE’S COMPLETENESS THEOREM

Published online by Cambridge University Press:  04 September 2020

CHRISTIAN ESPÍNDOLA*
Affiliation:
DEPARTMENT OF MATHEMATICS AND STATISTICS MASARYK UNIVERSITY, FACULTY OF SCIENCES KOTLÁŘSKÁ 2, 611 37BRNOCZECH REPUBLICE-mail: espindolach@math.muni.cz

Abstract

Given a regular cardinal $\kappa $ such that $\kappa ^{<\kappa }=\kappa $ (or any regular $\kappa $ if the Generalized Continuum Hypothesis holds), we study a class of toposes with enough points, the $\kappa $ -separable toposes. These are equivalent to sheaf toposes over a site with $\kappa $ -small limits that has at most $\kappa $ many objects and morphisms, the (basis for the) topology being generated by at most $\kappa $ many covering families, and that satisfy a further exactness property T. We prove that these toposes have enough $\kappa $ -points, that is, points whose inverse image preserve all $\kappa $ -small limits. This generalizes the separable toposes of Makkai and Reyes, that are a particular case when $\kappa =\omega $ , when property T is trivially satisfied. This result is essentially a completeness theorem for a certain infinitary logic that we call $\kappa $ -geometric, where conjunctions of less than $\kappa $ formulas and existential quantification on less than $\kappa $ many variables is allowed. We prove that $\kappa $ -geometric theories have a $\kappa $ -classifying topos having property T, the universal property being that models of the theory in a Grothendieck topos with property T correspond to $\kappa $ -geometric morphisms (geometric morphisms the inverse image of which preserves all $\kappa $ -small limits) into that topos. Moreover, we prove that $\kappa $ -separable toposes occur as the $\kappa $ -classifying toposes of $\kappa $ -geometric theories of at most $\kappa $ many axioms in canonical form, and that every such $\kappa $ -classifying topos is $\kappa $ -separable. Finally, we consider the case when $\kappa $ is weakly compact and study the $\kappa $ -classifying topos of a $\kappa $ -coherent theory (with at most $\kappa $ many axioms), that is, a theory where only disjunction of less than $\kappa $ formulas are allowed, obtaining a version of Deligne’s theorem for $\kappa $ -coherent toposes from which we can derive, among other things, Karp’s completeness theorem for infinitary classical logic.

Type
Articles
Copyright
© The Association for Symbolic Logic 2020

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References

REFERENCES

Butz, C. and Johnstone, P., Classifying toposes for first-order theories . Annals of Pure and Applied Logic , vol. 91 (1998), no. 1, pp. 3358.CrossRefGoogle Scholar
Dickmann, M. A., Large Infinitary Languages , North-Holland Publishing Company, New York, 1975.Google Scholar
Espíndola, C., Infinitary first-order categorical logic . Annals of Pure and Applied Logic , vol. 170 (2019), no. 2, pp. 137162.CrossRefGoogle Scholar
Jech, T., Set theory . The Third Millenium Edition, revised and expanded, Springer Monographs in Mathematics, Springer-Verlag, Berlin, 2003.Google Scholar
Johnstone, P., Sketches of an Elephant (Volume 2). A Topos Theory Compendium , Oxford University Press, New York, 2002.Google Scholar
Karp, C., Languages with Expressions of Infinite Length , North-Holland Publishing Company, Amsterdam, Netherlands, 1964.Google Scholar
Makkai, M., A theorem on Barr-exact categories, with an infinitary generalization . Annals of Pure and Applied Logic , vol. 47 (1990), no. 3, pp. 225268.CrossRefGoogle Scholar
Makkai, M. and Reyes, G., First-Order Categorical Logic. Model-Theoretical Methods in the Theory of Topoi and Related Categories , Springer, New York, 1977.CrossRefGoogle Scholar
Specker, E., Sur un problème de Sikorski . Colloquium Mathematicum , vol. 2 (1949), pp. 912.CrossRefGoogle Scholar