Chris Purcell @ccppurcell@mathstodon.xyz

Update: source
https://xkcd.com/2286/

Or have I got it wrong and 2m is the distance in America too? I'm sure I heard 6ft a lot when I was there last month...

In Europe the recommended distance is 2m, in America 6ft (which is approx 90% of 2m). Are humans good enough at distance estimation for this to make a difference? Probably there are too many other factors to figure this out from data.

Sorry if this turns out to be a silly question, but I couldn't find (or think of) an answer.

Suppose I add edges at random to a set of vertices until I have a connected graph. What is the expected number of cut vertices?

I'd be interested to know if there's a spike in sci-hub traffic what with universities being closed. It's often quicker to paste a doi into sci-hub than go through the remote access rigmarole. Or so I'm told...

Drawing a blank:

Is there a name for the "all-or-none" logical operator? Or more specifically its negation, which is XOR for 2-inputs but I want the general case.

Met a woman who seemed to think of herself as a non-math person (though she didn't say it out loud), whose son had asked her how many points were in a circle. I didn't want to give a boring closed answer, so, thinking of eg the ℓ₁ norm, I started to say "it depends..." and she finished the sentence saying "..on how many dimensions you have?" :D

Needless to say, I congratulated her on an excellent observation!

A silly fact: 987654312/123456789 = 8

On my next birthday I'll be 25... sorry I mean 2^5

Oh I found it https://hog.grinvin.org/ViewGraphInfo.action?id=25187

Came across quite a nice little graph that I feel should be "known" if anyone is interested:

I'm getting tired of living in cities that are growing. It means living in, and navigating around, building sites all the time. But what's the alternative? Places that are on the way down are pretty grim and difficult to live in as well. There can't be many cities whose population is stable. Maybe the thing to look at is the second derivative.

https://www.ams.org/journals/notices/202001/rnoti-o1.pdf

I don't see why insisting our colleagues have inclusive values is just as bad as insisting our colleagues have exclusive values. In fact I don't see why insisting on some values is bad per se, it clearly depends on the values in question.

The argument that diversity statements harm the very people they are intended to help is much more convincing to me.

Could be a good puzzle either to prove, or if I am wrong to find a counter example.

Oh and probably n should be at least 3. So far I've mainly played games where n=3

I've been playing "binary sudoku". You have a 2nx2n grid, with some 0s and 1s and you have to fill it in according to some rules:
* each row and column has equal number of 0s and 1s
* no row or column has a run of three 0s or 1s
* no pair of rows is the same
* no pair of columns is the same

conjecture: there must be a pair of rows (or columns) that are "inverse" i.e. a pair of strings s t where s[i]=/=t[i] for all i.

I can see a boring case analysis proof but is there an "at a glance" proof?

Has anyone here ever seen, in a paper or book that covers boolean satisfiability, a neat way to refer to the "sign" of a variable appearing in a clause, in such a way that I could take the product of two signs (with several variables appearing positively and negatively perhaps in the same expresssion). I am tying myself in notational knots here. I have previously sometimes used \(x^\alpha\) and α should be -1 or 1. But that gets messy if you have multiple clauses and variables

Perhaps "class" is a better word than category...

I'm sure many of you have seen this, it's a couple of years old - there's a sort of lattice of quadrilaterals (a square is a rectangle is a parallelogram etc) but it's a bit ugly if you stick to standard named ones. Here a "kitoid" is introduced to make a really beautiful diagram

https://www.hambrecht.ch/blog/2017/7/26/the-quest-for-the-lost-quadrilateral

But it got me thinking, is there any well defined sense in which these categories are exhaustive (for convex quads)? And if so how many are there for n-gons

When n=2k-1 the graph is isomorphic to the Kneser graph \(KG_{n,n-k}\) ..but I am still interested in the general case

Anyone know the name of the class of graphs on \({n \choose k}\) vertices, each vertex is a \(k\) element subset of \(\{1,\ldots,n\}\) and there is an edge \(ST\) if and only if the intersection of \(S\) and \(T\) is a singleton? When n=5 k=3 you get the Petersen graph for instance. In fact I am interested in the case n=2k-1 anyway...