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Peter Smith's user avatar
Peter Smith's user avatar
Peter Smith's user avatar
Peter Smith
  • Member for 11 years, 11 months
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36 votes
3 answers
2k views

Simple/natural questions in group theory whose answers depend on set theory

26 votes
5 answers
2k views

Alternative set theories

20 votes
3 answers
3k views

What's a good motivating example for the concept of a slice category?

19 votes
1 answer
1k views

Primitive recursive function which isn't $\Delta_0$

18 votes
2 answers
811 views

You've got your head round Basic Category Theory: why look at monads next?

17 votes
2 answers
1k views

The (un)decidability of Robinson-Arithmetic-without-Multiplication?

17 votes
2 answers
464 views

Two styles of semantics for a first-order language: what's to choose?

16 votes
1 answer
681 views

Mendelson's $\mathit{Mathematical\ Logic}$ and the missing Appendix on the consistency of PA

16 votes
2 answers
360 views

Natural isomorphisms: what is the status now of "the Eilenberg/Mac Lane Thesis"?

14 votes
1 answer
327 views

Is there a quicker argument from the HBL derivability conditions to the equivalence of fixed points of $\neg\Box$ to $\mathsf{Con}$?

13 votes
1 answer
573 views

Olympiad-style question about functions satisfying condition $f(f(f(n))) = f(n+1) + 1$

11 votes
0 answers
260 views

Elementary references on Robinson Arithmetic, Prim. Recursive fns etc.

10 votes
1 answer
218 views

Who first explicitly noted that second-order logic is unaxiomatizable?

10 votes
1 answer
915 views

How to prove adjunctions compose (via units and counits)?

9 votes
1 answer
674 views

Do hom-sets really live in the category Set?

9 votes
1 answer
1k views

Introductions to intuitionistic logic?

8 votes
1 answer
256 views

What is a nice "naturally occurring" example of an arrow category?

7 votes
1 answer
144 views

Survey of varieties of non-standard analysis?

7 votes
1 answer
1k views

Symbol for assignment of a truth-value?

7 votes
1 answer
317 views

Recent sets of notes/newly available online books on category theory?

6 votes
2 answers
419 views

Examples of functors that preserves products but not equalizers, and vice versa.

6 votes
3 answers
165 views

Categories with some but not all exponentials

6 votes
1 answer
302 views

Why is a split monomorphism (or split epimorphism) so called?

6 votes
1 answer
148 views

Elementary proof that, in a topos, every epimorphism is regular?

5 votes
2 answers
833 views

A proof that right adjoints preserve limits?

5 votes
2 answers
230 views

Why is $\forall x\exists y(Fy \to Gx) \therefore \exists y\forall x(Fy \to Gx)$ valid, intuitively?

5 votes
0 answers
245 views

The counted is to the countable as the ??? is to the (order)-isomorphic.

5 votes
2 answers
1k views

Introductory example(s) of a functor that is full but not faithful

4 votes
0 answers
82 views

Symbol for function composition [duplicate]

4 votes
0 answers
175 views

Cutest proof that PA proves its finitely axiomatised subtheories are consistent?