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Register a new userIdempotents in intensional type theory
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Added by
J\\\"urgen Koslowski
Publication date
2015
fields
Informatics Engineering
and research's language is
English
Authors
Michael Shulman
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We study idempotents in intensional Martin-Lof type theory, and in particular the question of when and whether they split. We show that in the presence of propositional truncation and Voevodskys univalence axiom, there exist idempotents that do not split; thus in plain MLTT not all idempotents can be proven to split. On the other hand, assuming only function extensionality, an idempotent can be split if and only if its witness of idempotency satisfies one extra coherence condition. Both proofs are inspired by parallel results of Lurie in higher category theory, showing that ideas from higher category theory and homotopy theory can have applications even in ordinary MLTT. Finally, we show that although the witness of idempotency can be recovered from a splitting, the one extra coherence condition cannot in general; and we construct the type of fully coherent idempotents, by splitting an idempotent on the type of partially coherent ones. Our results have been formally verified in the proof assistant Coq.
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