## Archive for September, 2011

### MO is two

September 28, 2011

As advertised on meta, MO is two today.  Quick stats very much like last year :

– “active users” (i.e. those with rep$\geq 11$): about 4900, so that’s about 2200 more in a year

– 55 users are at or above 10k, compared to 18 a year ago

– most prominent senior users who joined roughly in the past year are Joseph O’Rourke with 20k, Andreas Blass with 15k, Mark Sapir with 14k, Sándor Kovács with 12k and Francesco Polizzi with 11k

– first user to 30k and 40k was Joel David Hamkins (currently at 47k), while most gold medals so far is 7 by Mariano Suárez-Alvarez

– there have been about 23 240 questions asked in total, so that’s about 12 200 more than a year ago (i.e. the average is roughly stable, around 11 500/year). Most don’t get as much votes as in the early days, but it’s still possible to ask highly appreciated ones, this recent one got 37 upvotes in 15 hours.

– it’s fair to say some areas of math are still vastly under-represented, especially dynamical systems broadly speaking, (a few new senior users would probably add much-needed traction there…)

So overall, a great past year, hoping for even more interesting stuff in the next one!

And to add a few words on little brother math.stackexchange.com  who is now about 1 year and 2 months old, it is doing fantastic too: about 22 480 questions asked,   many excellent contributors (32 of them above 10k) including  Arturo Magidin (74k, 6 gold), Qiaochu Yuan  (57k, 2 gold) and Bill Dubuque (47k, 3 gold).

### 3 recent themes in logic

September 27, 2011

There are 3 recent things that I’d very much like to work on and digest in the next year or so:

– Edward Nelson’s ongoing work (summary here), where he claims he found an inconsistency in Peano Arithmetic (namelly that there are superexponential recursions which do not terminate) , and proposes another weaker system as a foundation for elaborated mathematics, together with a new proof checker program which outputs human-readable proofs ; [Update sept 28: there might well be a flaw in Nelson’s proof, since in comments elsewhere both Terry Tao and David Madore have independently questioned the same part of the argument… Update sept 29: looks like a more detailed discussion between Terry Tao and Edward Nelson is taking place at the n-Category Café]

– the ongoing project started by Voevodsky, Awodey and Coquand on a new foundation for mathematics based on homotopy theory, called Univalent Type Theory (the website contains lots of things, an IAS program is also planned in 2012-2013) ;

– Jean-Yves Girard’s latest paper (in french) very much the culmination of decades of his deep work on “la syntaxe transcendantale” (started long ago with linear logic , and eventually dealing recently with subject/object distinction and normativity).