3,010 reputation
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bio website dorais.org
location Hanover, NH
age
visits member for 4 years, 1 month
seen 18 hours ago

I like math and a few other things...


Jul
20
awarded  Yearling
Jul
7
awarded  Nice Answer
Mar
8
comment Are there statements that are undecidable but not provably undecidable
@JDH's objection is correct but there is more. The implicit assumption is actually that ZFC+Con(ZFC) is $\Sigma^0_1$-sound: that this theory does not prove any false $\Sigma^0_1$-statement. If ZFC+Con(ZFC) is not $\Sigma^0_1$-sound then there is a Turing machine $M$ such that ZFC + Con(ZFC) proves that "$M$ halts" but $M$ doesn't actually halt...
Dec
23
comment Problems with introducing ordered pairs axiomatically
Andrej, this doesn't answer the question as posed. The OP is asking about a global pairing function, not about the existence of products, which only gives local pairing functions.
Dec
23
answered Problems with introducing ordered pairs axiomatically
Nov
22
awarded  Nice Answer
Aug
26
comment Special subgroup of a group of order $n$
@RobertM: When in doubt, flag for moderator attention and explain what you want to do. There are mechanisms set up to transfer questions from one site to another.
Aug
24
awarded  Good Answer
Aug
17
comment Category of profinite groups
If you want this question to be on MathOverflow, ask for it to be migrated. Please don't crosspost!
Jul
28
comment clearing doubt over a definition
The description on Wikipedia seems pretty understandable to me. The original paper could also be helpful - dx.doi.org/10.1109%2FSFCS.2000.892006 The bracket notation is a quirk of computer science and should ideally be described in any textbook that uses it.
Jul
20
awarded  Yearling
Jul
3
answered Definable sets à la Jech
May
14
awarded  Enlightened
May
14
awarded  Nice Answer
May
12
accepted A natural example in category theory
May
8
comment A natural example in category theory
Oh! This is excellent! I think this has finite products but not all finite limits. Right?
May
7
comment A natural example in category theory
This is not an explicit requirement but it is implied: if $X$ is itself inhabited then it is clearly isomorphic to a subobject of an inhabited object.
May
7
asked A natural example in category theory
May
7
awarded  Caucus
Apr
26
comment What is Baire's zero-dimensional metric space?
Additional context would help, but it's usually this one - en.wikipedia.org/wiki/Baire_space_(set_theory)