# A modular proof of the straightening theorem

Abstract: The development of ∞-category theory in Lurie's book 'Higher Topos Theory' is founded on a series of rectification theorems, the first of which is the (unmarked) Straightening Theorem. This theorem states that, for each simplicial set A, the straightening--unstraightening adjunction is a Quillen equivalence between the category of simplicial presheaves over the homotopy coherent realization of A (equipped with the projective Kan model structure) and the category of simplicial sets over A (equipped with the contravariant model structure, whose fibrant objects are the right fibrations over A). Lurie's proof of this theorem is notoriously difficult; alternative proofs -- substantially different from Lurie's proof and from each other -- have since been given by Stevenson and by Heuts and Moerdijk.

In this talk, I will present a new, simple proof of the Straightening Theorem. This proof is based on an idea which may be found in §51 of Joyal's 'Notes on quasi-categories': we factorize the striaghtening-unstraightening adjunction as the composite of three adjunctions (in fact, two adjunctions and an equivalence), each of which we show to be a Quillen equivalence. One of these adjunctions (the equivalence) is easily seen to be a Quillen equivalence (in fact, an equivalence of model categories). To prove that the remaining two adjunctions are Quillen equivalences, I will use my recent proof of Joyal's Cylinder Conjecture.

In person attendance is available in HN 1.33 for up to 52 people.

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Zoom attenence is also avalible.