Skip to main content
Search type Search syntax
Tags [tag]
Exact "words here"
Author user:1234
user:me (yours)
Score score:3 (3+)
score:0 (none)
Answers answers:3 (3+)
answers:0 (none)
isaccepted:yes
hasaccepted:no
inquestion:1234
Views views:250
Code code:"if (foo != bar)"
Sections title:apples
body:"apples oranges"
URL url:"*.example.com"
Saves in:saves
Status closed:yes
duplicate:no
migrated:no
wiki:no
Types is:question
is:answer
Exclude -[tag]
-apples
For more details on advanced search visit our help page
Results tagged with
Search options not deleted user 31233

Constructive mathematics in the style of Bishop, including its semantics using realizabilty or topological methods.

16 votes
Accepted

Locales as spaces of ideal/imaginary points

I can only answer some of your questions. Yes, the Zariski locale is extensively studied. It's one of the ways of setting up scheme theory in a constructive context: Don't define schemes as locally r …
Ingo Blechschmidt's user avatar
13 votes
1 answer
626 views

Constructively correct notion of unique factorization domain

Recall the well-known proof that a unique factorization domain is a GCD domain: Let $x, y \in R \setminus \{ 0 \}$. Factor $x$ and $y$ into pairwise non-associated irreducible elements: $$\begin{a …
Ingo Blechschmidt's user avatar
12 votes

Tight apartness relations in toposes

I'm not precisely sure what you're looking for. Here is an example for the external interpretation of an apartness relation: Recall that the object of Dedekind reals $\mathbb{R}$ in a sheaf topos $\m …
Ingo Blechschmidt's user avatar
11 votes
Accepted

Constructive proof that a kernel consists of nilpotent elements

This answer provides a scheme how to construct a constructive proof, though I'm still working to actually explicitly extract the constructive proof, so please don't accept the answer just yet. (Update …
Ingo Blechschmidt's user avatar
9 votes
0 answers
374 views

Reflection principle for intuitionistic Zermelo–Fraenkel?

The well-known reflection principle for classical Zermelo–Fraenkel states: For any formula $\varphi(x_1,\ldots,x_n)$ of the language of ZFC with free variables $x_1,\ldots,x_n$, ZFC proves $$ \f …
Ingo Blechschmidt's user avatar
7 votes
2 answers
1k views

Explaining the consistency of PRA and ZF from predicative foundations

Recently I got interested in predicative foundations, mostly because of Laura Crosilla's work and because Agda employs a predicative type theory. From the point of view of a predicative foundation to …
Ingo Blechschmidt's user avatar
6 votes

Constructively, is the unit of the “free abelian group” monad on sets injective?

Yes! Here is a proof which is slightly different from both your proof and the proof in Mines–Richman–Ruitenberg. First define the similarity relation on $\mathrm{List}(R \times X)$ as in Mines–Richma …
Ingo Blechschmidt's user avatar
5 votes

What does overtness mean for metric spaces?

The spectrum of a commutative ring, defined as the classifying locale of its prime filters, is overt if and only if any element is nilpotent or not nilpotent (Proposition 12.51 in these notes of mine) …
Ingo Blechschmidt's user avatar
4 votes

constructive Serre classes

Any subclass $\mathcal{C}$ of an abelian category determines a smallest Serre class containing it, by iteratively adding (the zero object and) the object $Y$ for any exact sequence $X \to Y \to Z$ whe …
Ingo Blechschmidt's user avatar
4 votes

Pure first order logic formulations of Markov's principle

This is not an answer to the question, but rather a comment which benefits from proper formatting. For me personally, Markov's principle is specifically associated with the natural numbers and not wi …
Ingo Blechschmidt's user avatar
3 votes

Explaining the consistency of PRA and ZF from predicative foundations

Last week, I learned from Ulrik Buchholtz that there are proof-theoretic reductions from certain impredicative systems to certain predicative ones. While these fall short of predicatively explaining t …
Ingo Blechschmidt's user avatar