User Peter LeFanu Lumsdaine

46 votes

8 answers

10k views

Non-examples of model structures, that fail for subtle/surprising reasons?

asked May 2, 2010 at 18:07

4 votes

0 answers

4k views

Pronunciation of ¡ (inverted exclamation mark, historically used for subfactorial)

asked Jan 13, 2016 at 23:40

15 votes

3 answers

2k views

History of the pullback corner notation

asked Mar 26, 2017 at 18:58

45 votes

2 answers

2k views

Applications of Zorn’s lemma that aren’t chain-complete/directed-complete?

asked Apr 18, 2021 at 14:48

22 votes

3 answers

2k views

Has the Isbell–Freyd criterion ever been used to check that a category is concretisable?

asked Mar 3, 2019 at 22:01

19 votes

5 answers

1k views

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

asked Jun 11, 2018 at 8:23

15 votes

2 answers

1k views

Does foundation/regularity have any categorical/structural consequences, in ZF?

asked May 12, 2018 at 14:28

13 votes

0 answers

1k views

Has anyone read/debunked Yessenin-Volpin–Hennix “Beware of the Gödel-Wette paradox”?

asked Jan 5, 2015 at 9:56

15 votes

3 answers

968 views

Consequences of foundation/regularity in ordinary mathematics (over ZF–AF)?

asked May 30, 2018 at 16:46

12 votes

3 answers

875 views

Can we always make a strictly functorial choice of pullbacks/re-indexing?

asked Oct 11, 2013 at 22:37

14 votes

2 answers

739 views

Is every connected space equivalent to some B(Aut(X))?

asked Nov 15, 2013 at 22:04

24 votes

1 answer

729 views

Is Lemma D4.5.3 in the Elephant correct? (“In a topos, weakly projective implies internally projective.”)

asked Mar 4, 2016 at 16:02

9 votes

4 answers

682 views

Pullback-stable model of fibrewise suspension of fibrations (in simplicial sets, or similar setting)

asked Sep 29, 2015 at 18:06

8 votes

1 answer

545 views

Does this “flipping lexicographic” ordering have a standard name?

asked Feb 5, 2011 at 0:34

11 votes

2 answers

526 views

Historical quotation search: Equations/formulae in (Latin?) prose, before modern symbolic notation

asked Feb 24, 2014 at 19:14

9 votes

1 answer

435 views

Free models of finitely presented essentially algebraic theories in elementary toposes?

asked Nov 22, 2019 at 15:47

3 votes

1 answer

388 views

Monoidal closed structure(s) on the category "bicategories, with strict functors"?

asked Apr 15, 2010 at 5:01

3 votes

1 answer

282 views

Reference request: eliminating function symbols in predicate logic

asked Jan 27, 2016 at 11:32

9 votes

0 answers

266 views

Is “simplicity is elementary” still hard? (Felgner’s 1990 theorem on simple groups, and subsequent work)

asked Sep 12, 2022 at 19:47

3 votes

3 answers

260 views

Ref request: making a homotopy equivalence fibrewise, in an abstract setting (eg fibration categories)

asked Sep 19, 2016 at 14:19

8 votes

1 answer

223 views

Reference for precise form of Mostowski: “sets correspond to rooted, well-founded, extensional graphs”?

asked Jul 5, 2023 at 12:13

3 votes

2 answers

170 views

Terminology: product on strict preorders corresponding to direct product of preorders?

asked Mar 27, 2018 at 21:47

4 votes

0 answers

112 views

Ref request: modelling regular theories as an injectivity condition

asked Feb 3, 2016 at 12:30

7 votes

0 answers

95 views

Terminology: a certain semicategory with objects mor(C) (not the usual or twisted arrow category)

asked Mar 26, 2019 at 9:51

Stack Exchange Network

24 Questions

Non-examples of model structures, that fail for subtle/surprising reasons?

Pronunciation of ¡ (inverted exclamation mark, historically used for subfactorial)

History of the pullback corner notation

Applications of Zorn’s lemma that aren’t chain-complete/directed-complete?

Has the Isbell–Freyd criterion ever been used to check that a category is concretisable?

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

Does foundation/regularity have any categorical/structural consequences, in ZF?

Has anyone read/debunked Yessenin-Volpin–Hennix “Beware of the Gödel-Wette paradox”?

Consequences of foundation/regularity in ordinary mathematics (over ZF–AF)?

Can we always make a strictly functorial choice of pullbacks/re-indexing?

Is every connected space equivalent to some B(Aut(X))?

Is Lemma D4.5.3 in the Elephant correct? (“In a topos, weakly projective implies internally projective.”)

Pullback-stable model of fibrewise suspension of fibrations (in simplicial sets, or similar setting)

Does this “flipping lexicographic” ordering have a standard name?

Historical quotation search: Equations/formulae in (Latin?) prose, before modern symbolic notation

Free models of finitely presented essentially algebraic theories in elementary toposes?

Monoidal closed structure(s) on the category "bicategories, with strict functors"?

Reference request: eliminating function symbols in predicate logic

Is “simplicity is elementary” still hard? (Felgner’s 1990 theorem on simple groups, and subsequent work)

Ref request: making a homotopy equivalence fibrewise, in an abstract setting (eg fibration categories)

Reference for precise form of Mostowski: “sets correspond to rooted, well-founded, extensional graphs”?

Terminology: product on strict preorders corresponding to direct product of preorders?

Ref request: modelling regular theories as an injectivity condition

Terminology: a certain semicategory with objects mor(C) (not the usual or twisted arrow category)