3,555 reputation
1332
bio website peterlefanulumsdaine.com
location Stockholm, Sweden
age 32
visits member for 5 years, 1 month
seen 2 hours ago

Mathematician, math.LO/math.CT, currently postdoc at Stockholm University. Mainly working in categorical logic, especially homotopy type theory and higher categories.

Previously worked at Institute for Advanced Study, Princeton; Dalhousie University, Halifax, Nova Scotia; and Carnegie Mellon University, Pittsburgh.


Jan
6
awarded  Nice Question
Jan
5
answered Theorem versus Proposition
Jan
5
comment How to refer to plural of mathematical symbols - with or without an apostrophe
@YemonChoi: very good point; and the answers/discussion so far bear it out. You’ve converted me to the close-vote camp, though as “primarily opinion-based” rather than “off-topic”.
Jan
5
comment How to refer to plural of mathematical symbols - with or without an apostrophe
@YemonChoi: for PDE’s vs. PDEs, it’s certainly a general English usage question — and indeed it’s already asked and well-answered on english.stackexchange. But for $x_i$ vs. $x_i$s vs. $x_i$’s, the usage and conventions are pretty specific to mathematical writing, so it seems reasonably on-topic here to me.
Jan
5
comment Has anyone read/debunked Yessenin-Volpin–Hennix “Beware of the Gödel-Wette paradox”?
@AsafKaragila: Sure, I don’t particularly want to do it myself either, hence asking if someone already has. But when cranks accuse mathematicians of being hidebound reactionaries ignoring their work, we say that no, there are clear standards of correctness that they fail to live up to. And to substantiate this, we need (as a community) to give attention from time to time to some of the more coherent “outsider” papers, and show how they are in error. (And occasionally we may find that they’re not — though, again, I highly doubt that in this case.)
Jan
5
comment How to refer to plural of mathematical symbols - with or without an apostrophe
@YuichiroFujiwara (continuing the possibly unnecessary seriousness): A less prescriptivist approach is exactly what I’m trying to advocate! I believe that in speech, the explicitly-pluralised-with-a-[z]-phoneme version is more common/natural than the zero-pluralised version (it’s at least a common choice), and so saying “you can’t write that, because it looks terrible” (or “…is wrong”) is an unreasonably prescriptive stance. Unless you read the zero-pluralised written form as representing the explicitly-pluralised spoken form — but I don’t think most people read it that way.
Jan
5
comment How to refer to plural of mathematical symbols - with or without an apostrophe
@YuichiroFujiwara: sure, yes. I’m not advocating spelling reforms. But when there are multiple existing conventions (as in this case), closeness to speech can reasonably be a factor in the choice between them.
Jan
5
asked Has anyone read/debunked Yessenin-Volpin–Hennix “Beware of the Gödel-Wette paradox”?
Jan
5
comment How to refer to plural of mathematical symbols - with or without an apostrophe
In speaking, many mathematicians would pluralise it. Why not in writing also? Both options suggested are visually awkward, true; but it’s also awkward, in a different way, when orthography fails to follow speech.
Dec
25
comment Gluings and collages along profunctors
I would consider this just as the category of elements of $\varphi$, unless I’m missing something. How you would you see this as differing from any other construction of the category of elements?
Dec
6
comment Is the defining bijection for a pullback of topological spaces a homeomorphism?
@AndrejBauer: in your last-but-one comment, should the domain of the step function be $(-\infty,0) \cup [0,\infty)$, i.e. with one of the intervals open at 0?
Dec
4
comment Is the defining bijection for a pullback of topological spaces a homeomorphism?
@Andrej: my guess is that many people think of "constructive => continuous" just as a heuristic, and don't know there are formal statements; so your un-elaborated answer looked (to the downvoter) like invoking a heuristic as a proof.
Dec
3
comment Small objects vs Compact objects
This shows what goes wrong if you try to prove that they are equivalent. But it would still be good to see a specific counterexample, as the question asks for!
Dec
3
awarded  Yearling
Nov
4
reviewed Approve Is there some Riemannian manifold's version of Whitney theorem?
Oct
25
comment A problem on chains of squares — can one find an easy combinatorial proof?
“I can show that there is no simple algorithm for your problem” — the complexity argument shows there can be no quick algorithm for the problem, but it doesn’t show there can’t be a simple one. Many problems have naïve exponential-time algorithms, which aren’t good for practical computation but can be great for understanding existence proofs easily.
Oct
12
comment Concise definition of subobjects
I think the well-poweredness is beside the point: the main thing is that the groupoid core of the the category of subobjects is essentially discrete, and so we are quotienting by unique isomorphisms, which is generally well-behaved.
Oct
2
comment What is the Complete Set of Shortest Axioms of Classical Conditional-Negation Propositional Calculus?
In infix notation, perhaps slightly more readable than prefix: [(((p→q)→(¬r→¬s))→r)→t] → [(t→p)→(s→p)].
Sep
29
comment What is the most useful non-existing object of your field?
This function exists; it just isn’t computable. A program computing this function could arguably be an answer.
Sep
29
comment What is the most useful non-existing object of your field?
One can also argue that this one does exist, perfectly happily: it’s just a class that isn’t a set.