## Free notes on rigidity of groups acting on manifolds

A final version of a 160-page text written by Aaron Brown and others appeared on the arXiv today. The abstract: This text is an expanded series of lecture notes based on a 5-hour course given at the workshop entitled “Workshop for young researchers: Groups acting on manifolds” held in Teresópolis, Brazil in June 2016. The […]

## Weak dimension one rings are axiomatizable

Let $R$ be a ring. In the previous post on pure exact sequences, we called an exact sequence $0\to A\to B\to C\to 0$ of left $R$-modules pure if its image under any functor $X\otimes -$ is an exact sequence of abelian groups for any right $R$-module $X$. Here is yet another characterization of purity: Theorem. […]

## Pure exact sequences

Over the next few posts, I’ll talk more about axiomatizability of algebraic structures in first-order logic. Before I do that, we need to know about purity of exact sequences. So let’s fix a ring $R$. An exact sequence $$0\to A\to B\to C\to 0$$ in the category of left $R$ modules is called pure if for […]

## Axiomatizability of classes of structures

Let’s talk about axiomatizability in first-order logic, with examples in ring theory. Let’s call a class $\Ccl$ of rings axiomatizable if there exists a set $T$ of first order sentences such that $C\in\Ccl$ if and only if $C$ is a model of $T$ (that is, satisfies every sentence in $T$.) What are some examples? The […]

## Fun with principal ideal domains

A commutative ring $R$ is called a principal ideal domain (PID) if every ideal of $R$ can be generated by a single element. If $R$ is a principal ideal domain, is every subring of $R$ a principal ideal domain? No, definitely not. That is because you can take any integral domain that is not a […]

## A quick intro to Galois descent for schemes

This is a very quick introduction to Galois descent for schemes defined over fields. It is a very special case of faithfully flat descent and other topos-descent theorems, which I won’t go into at all. Typically, if you look up descent in an algebraic geometry text you will quickly run into all sorts of diagrams […]

## Dividing a square into triangles of equal area

Take a square and divide it down a diagonal, dividing the square into two triangles. Drawing the opposite diagonal now divides it into four triangles. In these two examples, we divided a square into an even number of triangles, all with equal area. Can we divide a square into an odd number of nonoverlapping triangles, […]

## Finite-dimensional k[x]-modules: projective or not?

Let’s suppose $M$ is a nonzero projective $\Z$-module. Can it be finite? Nope. I’m sure there are plenty ways to prove it, but one way is to observe that a projective $\Z$-module is free, and hence if $M$ is nonzero it must have at least one copy of $\Z$. So, $M$ is infinite. What’s the […]

## Explicit example showing non-residual finiteness

This is mostly a continuation on the group I gave in the last post, which is given by the presentation $$G = \langle a,t ~|~ t^{-1}a^2t = a^3\rangle.$$ At the risk of beating a dead horse, I proved that the homomorphism $f:G\to G$ given on generators by $f(t) = t$ and $f(a) = a^2$ is […]

## Yet another group that is not Hopfian

A few weeks ago I gave an example of a non-Hopfian finitely-presented group. Recall that a group $G$ is said to be Hopfian if every surjective group homomorphism $G\to G$ is actually an isomorphism. All finitely-generated, residually finite groups are Hopfian. So for example, the group of the integers $\Z$ is Hopfian. Another example of […]