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The homotopy interpretation of constructive dependent type theory, the univalence axiom, higher inductive types, internal languages of higher toposes, univalent foundations for mathematics, and implementations of such theories in proof assistants.

6 votes
1 answer
151 views

Univalence for weakly Tarski universes

In Martin-Löf type theory, a weakly Tarski universe is a type $\mathcal{U}$ with a type family $\mathcal{T}(A)$ indexed by terms $A:\mathcal{U}$, which is closed under identity types, dependent produc …
Madeleine Birchfield's user avatar
0 votes
0 answers
90 views

Univalence and higher inductive types in the lambda calculus model of type theory

In appendix A1 of the homotopy type theory book by the Univalent Foundations Project, the authors give a formal presentation of Martin-Löf type theory in lambda calculus. However, they did not give an …
Madeleine Birchfield's user avatar
2 votes
1 answer
545 views

Higher inductive types in higher observational type theory

Mike Shulman gave the following set of talks on higher observational type theory earlier this year (part 1, part 2, part 3). However, while he talked about how the identity types are defined and behav …
Madeleine Birchfield's user avatar
2 votes
1 answer
462 views

Path types and identity types in dependent type theory

There's been some debate at the nLab recently over the names of "identity type" and "path type" in certain dependent type theories. One user wrote that Many cubical type theorists make the distinctio …
Madeleine Birchfield's user avatar
1 vote
0 answers
76 views

Directly proving the extensionality principle for product types without quasi-inverses

In section 2.6 of the Univalent Foundations Project's Homotopy Type Theory book, the extensionality principle of product types is proven by showing that for all elements $a:A$, $a':A$, $b:A$, $b':A$, …
Madeleine Birchfield's user avatar
1 vote
1 answer
73 views

Constructing set-truncations of types from universes

This is a follow-up question from my previous question titled Constructing coproduct types and boolean types from universes, where I showed how every basic operation in set theory/topos theory could b …
Madeleine Birchfield's user avatar
1 vote
Accepted

Constructing set-truncations of types from universes

Given a universe $U$, the type of $U$-small propositions is given by $$\mathrm{Prop} \equiv \sum_{P:U} \prod_{x:P} \prod_{y:P} x = y$$ Given a type $A:U$, for $x:A$ and $y:A$, the type $$[x = y] \equi …
Madeleine Birchfield's user avatar
4 votes
1 answer
186 views

Predicativity and axiom $\mathbb{R}\flat$ in real cohesive homotopy type theory

In Mike Shulman's article Brouwer’s fixed-point theorem in real-cohesive homotopy type theory, the fundamental axiom adopted for his real-cohesive homotopy type theory (axiom $\mathbb{R}\flat$), which …
Madeleine Birchfield's user avatar
3 votes
0 answers
277 views

Principle of unique choice in homotopy type theory

In the MathOverflow thread Mathematics without the principle of unique choice, Mike Shulman defines the principle of unique choice to be if $R$ is a relation between two sets $A$, $B$, and for every …
Madeleine Birchfield's user avatar
3 votes
0 answers
74 views

Discreteness of the higher inductive-inductive Cauchy real numbers in real cohesive homotopy...

We work in cohesive homotopy type theory with propositional resizing, so that there is only one type of Dedekind real numbers $\mathbb{R}$ up to equivalence, and Mike Shulman's axiom $\mathbb{R}\flat$ …
Madeleine Birchfield's user avatar
5 votes
1 answer
169 views

Are lists in homotopy type theory free $A_\infty$-spaces?

Traditionally in dependent type theory with axiom K or uniqueness of identity proofs, every type $A$ is 0-truncated, and thus the type of lists on $A$, $\mathrm{List}(A)$, is 0-truncated and the free …
Madeleine Birchfield's user avatar
5 votes

Consistency of Generalised Continuum Hypothesis and univalence in HoTT

Dominik Kirst and Felix Rech managed to formalize the Generalized Continuum Hypothesis for sets in the HoTT library in Coq with univalence, which already shows the consistency of the GCH with univalen …
Madeleine Birchfield's user avatar