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Modal descent

Published online by Cambridge University Press:  26 October 2020

Felix Cherubini Open the ORCID record for Felix Cherubini [Opens in a new window]  and
Egbert Rijke
Felix Cherubini*
Affiliation:
Softwareschneiderei GmbH
Egbert Rijke
Affiliation:
University of Ljubljana
*
*Corresponding author. Email: felix.cherubini@posteo.de
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Abstract

Any modality in homotopy type theory gives rise to an orthogonal factorization system of which the left class is stable under pullbacks. We show that there is a second orthogonal factorization system associated with any modality, of which the left class is the class of ○-equivalences and the right class is the class of ○-étale maps. This factorization system is called the modal reflective factorization system of a modality, and we give a precise characterization of the orthogonal factorization systems that arise as the modal reflective factorization system of a modality. In the special case of the n-truncation, the modal reflective factorization system has a simple description: we show that the n-étale maps are the maps that are right orthogonal to the map $${\rm{1}} \to {\rm{ }}{{\rm{S}}^{n + 1}}$$ . We use the ○-étale maps to prove a modal descent theorem: a map with modal fibers into ○X is the same thing as a ○-étale map into a type X. We conclude with an application to real-cohesive homotopy type theory and remark how ○-étale maps relate to the formally etale maps from algebraic geometry.

Keywords

Homotopy type theorymodal homotopy type theorycohesive homotopy type theoryalgebraic geometryfactorization systems
Type
Paper
Information
Mathematical Structures in Computer Science , Volume 31 , Issue 4 , April 2021 , pp. 363 - 391
Copyright
© The Author(s), 2020. Published by Cambridge University Press

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