We investigate effective Kan fibrations in the context of the semantics of Homotopy Type Theory (HoTT). Effective Kan fibrations were proposed by Benno van den Berg and Eric Faber as constructive alternatives to classical Kan fibrations for modeling HoTT. Our work specifically explores their interaction with W-types in HoTT, which are inductive types representing well-founded trees, and extends this exploration to variants such as M-types. By using the categorical properties of W-types, we show that effective Kan fibrations model them. Additionally, we examine the behavior of quotient maps and discuss that certain cases can also be classified as effective Kan fibrations.
@InProceedings{tanaka:LIPIcs.TYPES.2024.8, author = {Tanaka, Shinichiro}, title = {{Effective Kan Fibrations for W-Types in Homotopy Type Theory}}, booktitle = {30th International Conference on Types for Proofs and Programs (TYPES 2024)}, pages = {8:1--8:20}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-376-8}, ISSN = {1868-8969}, year = {2025}, volume = {336}, editor = {M{\o}gelberg, Rasmus Ejlers and van den Berg, Benno}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.TYPES.2024.8}, URN = {urn:nbn:de:0030-drops-233707}, doi = {10.4230/LIPIcs.TYPES.2024.8}, annote = {Keywords: Homotopy Type Theory, Effective Kan Fibrations, W-types} }
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