# Homotopy Type Theory open problems (Rev #2, changes)

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A list of (believed to be) open problems in homotopy type theory. To add more detail about a problem (such as why it is hard or interesting, or what ideas have been tried), make a link to a new page.

## Semantics

• Is there a Quillen model structure on semi-simplicial sets? (with semi-simplicial Kan fibrations) making it Quillen equivalent to simplicial sets (with Kan fibrations)?
• Construct a model of type theory in an (infinity,1)-topos in general.
• Construct an (infinity,1)-topos? out of (the syntactic category of) a system of type theory.
• Define a notion of elementary (infinity,1)-topos and show that any such admits a model of homotopy type theory.

## Metatheory

• Give a computational interpretation? of univalence and HITs.
• Is there a cubical type theory? that describes more exactly what happens in the model of type theory in cubical sets??
• Show in HoTT that the $n^{th}$ universe is a model of HoTT with $(n-1)$ universes.
• Consider the type theory with a sequence of universes plus for each $n\gt 0$, an axiom asserting that there are n-types? that are not $(n-1)$-types. Does adding axioms asserting that each universe is univalent increase the logical strength?

## Homotopy theory

• Prove that the torus? (with the HIT definition involving a 2-dimensional path) is equivalent to a product of two circles.
• What is the loop space of a wedge of circles indexed by a set without decidable equality?
• Calculate more homotopy groups of spheres.
• Show that the homotopy groups of spheres are all finitely generated, and are finite with the same exceptions as classically.
• Construct the “super Hopf fibration” $S^3 \to S^7 \to S^4$.

## Higher algebra and higher category theory

• Define semi-simplicial types? in type theory.
• Define a weak omega-category in type theory?.

## Other Mathematics

• What axioms of set theory are satisfied by the HIT model of ZFC? constructed in chapter 10 of the book?
• Are any of the stronger forms of the axiom of choice? mentioned in the book consistent with univalence?
• Do the higher inductive-inductive real numbers? from the book coincide with the Escardo-Simpson reals?

## Formalization

• Formalize the construction of models of type theory using contextual categories.
• Formalize what remains to be done from chapters 8, 10, and 11 of the book.

Revision on February 19, 2014 at 11:40:39 by Steve Awodey?. See the history of this page for a list of all contributions to it.