Tuesday, March 25th, 2014 | Author:

In this lightheaded post (written long time ago) I want to share with you some fundamental adjunctions that are the "source" of various other adjunctions that pop up all over in mathematics (well, at least all over algebraic topology).

# Magmas, Loops and Monoids

Monday, February 24th, 2014 | Author:

This is about the mathematical concepts of a "magma", a "loop" and a "monoid", which are descriptions of certain properties that the combining of things may enjoy.

# 9 Questions on Information Theory

Thursday, February 13th, 2014 | Author:

Back in 2010 I had a series of posts going about questions in information theory that arose from a 2-week seminar with a bunch of students coming from various scientific disciplines (a wonderful event!). Here I picked those that I still find particularly compelling:

1. Is the mathematical definition of Kullback-Leibler distance the key to understand different kinds of information?
2. Can we talk about the total information content of the universe?
3. Is hypercomputation possible?
4. Can we tell for a physical system whether it is a Turing machine?
5. Given the fact that every system is continually measured, is the concept of a closed quantum system (with unitary time evolution) relevant for real physics?
6. Can we create or measure truly random numbers in nature, and how would we recognize that?
7. Would it make sense to adapt the notion of real numbers to a limited (but not fixed) amount of memory?
8. Can causality be deﬁned without reference to time?
9. Should we re-deﬁne “life”, using information-theoretic terms?

What do you think?

# Motivic Cell Structure of Toric Surfaces

Wednesday, April 17th, 2013 | Author:

In this post I'll do a few very explicit computations for motivic cell structures of smooth projective toric varieties coming from the Białynicki-Birula decomposition, namely $\mathbb{P}^1, \mathbb{P}^1 \times \mathbb{P}^1, \mathbb{P}^2$ and Hirzebruch surfaces. It is a bit lengthy but maybe helpful to anyone who wants to do some explicit calculations with BB-decompositions. I hope you're accustomed to toric varieties, but I won't do anything fancy. You can safely skip the motivic part of this post.

# Białynicki-Birula and Motivic Decompositions

Thursday, April 04th, 2013 | Author:

This is about Białynicki-Birula's paper from '72 on actions of reductive linear algebraic groups on non-singular varieties, in particular Gm-operations on smooth projective varieties. I give a proof sketch of Theorem 4.1 therein and explain a little bit how Brosnan applied these results in 2005 to get decompositions of the Chow motive of smooth projective varieties with Gm-operation. Wendt used these methods in 2010 to lift such a decomposition on the homotopy-level, to prove that smooth projective spherical varieties admit stable motivic cell decompositions. Most of this blogpost consists of an outline of the B-B paper.