# Questions tagged [type-theory]

The type-theory tag has no usage guidance.

**6**

votes

**2**answers

183 views

### When is a fold monomorphic/epimorphic

Given a functor $F : \mathcal C \to \mathcal C$ with initial algebra $\alpha : FA \to A$, and another algebra $\xi : FX \to X$, we obtain a unique morphism $\mathsf{fold}~\xi : A \to X$ such that $\...

**6**

votes

**2**answers

427 views

### How can the simply typed lambda calculus be Turing-incomplete, yet stronger than second-order logic?

It is well-known that the simply typed lambda calculus is strongly normalizing (for instance, Wikipedia). Hence, it is not strong enough to be Turing-complete, as also mentioned on the Wikipedia page ...

**3**

votes

**0**answers

202 views

### Formal foundations done properly [closed]

I would want to do mathematics properly, so that the proofs of results can be trusted on instead of them being just suggestions on which results could perhaps apply. This means formulating the math in ...

**12**

votes

**1**answer

515 views

### A peculiarity of Henkin's 1950 proof of completeness for higher order logic

My question concerns Henkin's original (1950) completeness proof https://projecteuclid.org/euclid.jsl/1183730860 for classical higher order logic and type theory relative to so-called general models.
...

**5**

votes

**1**answer

287 views

### Progress towards a computational interpretation of the univalence axiom?

I will preface this by saying that I am not an expert on type theory. I am just a curious outsider slowly making my way through the HoTT book when I (rarely) have some spare time.
I am just curious ...

**6**

votes

**1**answer

318 views

### Proof of ¬(¬1 ? ¬1) in tensorial logic

I believe I once had a proof of this proposition, but it's been lost to the mists of time and old hard drives, so who knows if it was correct, and try as I might I can't seem to reproduce it.
Is it ...

**3**

votes

**1**answer

98 views

### Internal equality for Eq-fibrations' morphisms

I have posted this question here on M.SE but since it received little attention and since it seems difficult to find helpfule references I reposting it here.
In Jacob's Categorical logic and Type ...

**10**

votes

**4**answers

917 views

### Formalizations of the idea that something is a function of something else?

I'll state my questions upfront and attempt to motivate/explain them afterwards.
Q1: Is there a direct way of expressing the relation "$y$ is a function of $x$" inside set theory?
More ...

**8**

votes

**0**answers

223 views

### What metatheory proves cut elimination for Simple Type Theory?

Gaisi Takeuti's book Proof Theory proves cut elimination for Simple Type Theory (a combined result of several researchers). Thus it proves consistency of Simple Type Theory with full comprehension in ...

**8**

votes

**0**answers

499 views

### What's the point of cubical type theory?

I have been following through the development of homotopy type theory since 2013 because I was really interested in the foundation of mathematics. The novel idea of combining programming with homotopy ...

**2**

votes

**1**answer

169 views

### Can a type in a lower universe be formed from types in higher universes?

A type universe is a type of small types that is closed under the basic type formation operations (dependent product, sum, coproduct etc.), that is to say for example that from
$A \colon U_i$ and
$x \...

**2**

votes

**0**answers

137 views

### What is the consistency limit of accumulative typing below $\omega_1^{CK}$?

Let $Th_\zeta$ be a Mono-sorted first order theory with $\zeta$ representing some recursive ordinal notation system.
Primitives: =, $\in$, $T_0, T_1, ..,T_i,..$ where i is an $\zeta$ ordinal, and ...

**2**

votes

**0**answers

61 views

### What is the relation of total functions in second order arithmetic and fast growing hierarchies?

Answer to this questions shows that fast growing hierarchies can grow arbitrarily fast for some definition of 'arbitrary'.
Can second order arithmetic define all these functions (for any ordinal) ...

**13**

votes

**2**answers

675 views

### What kind of category is generated by Cubical type theory?

What kind of ‘category’ is Cubical type theory the internal language of?
Its known that Martin-L?f type theories are the internal language of Locally cartesian closed categories, adding higher ...

**3**

votes

**2**answers

204 views

### Stability and complete types (in Model Theory)

I read the following statement in these slides of Saharon Shelah:
"$K$ is stable iff for every $M \in K$ there are only "few" complete types
over $M$." About the notation: here $K$ consists of all ...