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Strict universes for Grothendieck topoi

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arxiv 2202.12012 v3 pith:Y6VBCSL7 submitted 2022-02-24 math.CT cs.LOmath.LO

classification math.CTcs.LOmath.LO
keywords theorytypegrothendiecktopoiuniversespropertyrealignmentcontext
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Hofmann and Streicher famously showed how to lift Grothendieck universes into presheaf topoi, and Streicher has extended their result to the case of sheaf topoi by sheafification. In parallel, van den Berg and Moerdijk have shown in the context of algebraic set theory that similar constructions continue to apply even in weaker metatheories. Unfortunately, sheafification seems not to preserve an important realignment property enjoyed by the presheaf universes that plays a critical role in models of univalent type theory as well as synthetic Tait computability, a recent technique to establish syntactic properties of type theories and programming languages. In the context of multiple universes, the realignment property also implies a coherent choice of codes for connectives at each universe level, thereby interpreting the cumulativity laws present in popular formulations of Martin-L\"of type theory. We observe that a slight adjustment to an argument of Shulman constructs a cumulative universe hierarchy satisfying the realignment property at every level in any Grothendieck topos. Hence one has direct-style interpretations of Martin-L\"of type theory with cumulative universes into all Grothendieck topoi. A further implication is to extend the reach of recent synthetic methods in the semantics of cubical type theory and the syntactic metatheory of type theory and programming languages to all Grothendieck topoi.

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  1. Canonicity for Cost-Aware Logical Framework via Synthetic Tait Computability

    cs.PL 2025-04 conditional novelty 7.0 of 10

    The authors prove canonicity for the cost-aware logical framework calf using synthetic Tait computability, for a modified version of the original signature.

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