Recently my friend Elias started his own math blog adventure, and his first post gave a nice introduction to spectral sequences. Reading it I remembered that I should really understand some of the parts better myself, because a lot of the arguments one makes in chromatic homotopy theory are based on spectral sequences. There is a framework for constructing spectral sequences that are not covered in my old post on them, as well as Elias’ post, and that is creating spectral sequences from exact couples....

February 17, 2022 · 13 min · Torgeir Aambø

## Bousfield localization

Topology, particularly homotopy theory, is hard. The scenes where these kind of mathematics happen are immensely complicated; the category of topological spaces; the category of spectra. The problem is that there is simply too much information to try to capture by using simple tools that we can actually understand properly. Trying to classify topological spaces or spectra is a feat that many deem impossible, it is simply too difficult. So, how can we try to fix this?...

December 2, 2021 · 19 min · Torgeir Aambø

## Complex cobordism cohomology

In the next couple years I will need to understand the ins and outs of different cohomology theories and the spectra that represents them. Some of the most important of these (for my research) can be described using \$MU\$ — the complex cobordism spectrum. We briefly met this spectrum — or at least its cohomology theory — when we discussed formal group laws. There we explained briefly a theorem of Quillen, stating that the universal formal group law over the Lazard ring corresponds to complex cobordism cohomology....

November 18, 2021 · 16 min · Torgeir Aambø

## The homotopy groups of the spheres. Part 1

Recently I gave a talk about the homotopy groups of spheres, and as usual, I try to collect my thoughts on this blog before (or after) presenting. The homotopy groups of spheres have featured several times on this blog, and we have made some effort into calculating them for some small dimensions. In the talk I wanted to showcase some methods used to calculate these groups, as well as doing some of the “calculations”....

October 26, 2021 · 10 min · Torgeir Aambø

## Formal group laws

Recently we have covered a lot of heavy topology and abstract mathematics, so today I thought we would cover something else — something maybe a bit easier to grasp. We will introduce the concept of formal group laws, and a bit on why they are interesting. Introduction and definition To not just spew out the definition straight away, we look at a situation where formal group laws arise very naturally. Let \$G\$ be a one-dimensional commutative Lie group (Think here of the real numbers \$\mathbb{R}\$ or the circle group \$S^1\$)....

September 3, 2021 · 13 min · Torgeir Aambø

## A first look at spectra

Even though this blog is not centered around a specific topic, we have during the last year looked more frequently at certain topics than others, such as (co)homology theory, homotopy theory and category theory. We will continue this trend today as we will try to find a solid reason for a particular object to exist. These objects were briefly mentioned in the earlier post on tensor triangulated categories, namely spectra. These objects are hugely important to the field of algebraic topology, one reason being that they are intimately linked to cohomology....

August 20, 2021 · 10 min · Torgeir Aambø

## On formal DG-algebras

I have recently handed in and defended my master thesis in mathematics, so I though I would go through its abstract and try to explain what it’s all about. We look at formality of DG-algebras, Massey products, A_infinity-algebras and how we can use these to some interesting results.

July 19, 2021 · 14 min · Torgeir Aambø

## The associating homotopy

In the two last posts we have been discussing operations that are associative up to homotopy, and where such operations might arise naturally in topology. One claim I made, which I later realized was maybe a bit unmotivated and in need of some clarification was how some higher arity maps actually defined (or were defined by) homotopies between combinations of the lower arity maps. We also purely looked at this in a topological setting, but in algebraic topology we often translate to algebraic structures, so I also wanted to see clearly that the same constructions hold in that setting....

April 3, 2021 · 12 min · Torgeir Aambø

## Spaces with operations

In the most recent blog post we discussed homotopy associativity and how to transfer algebraic structures on topological spaces. There we in particular used topological groups, which are topological spaces with group structures. That said, any group is a topological group by equipping it with the discrete topology. So if we want to study some actual topology, and not just glorified group theory, we need to look at where multiplications and binary operations arise naturally in topology....

March 4, 2021 · 6 min · Torgeir Aambø

## Homotopy associativity

Imagine we have a system of two topological spaces \$f:T\longrightarrow G\$. We are often interested in knowing if a certain property on the space \$G\$ can be transferred through f such that we have the same property on \$T\$. If f is a nice enough morphism an example could be a topological invariant of \$G\$, for example its Euler characteristic. In this post we are more interested in transferring other things than invariants, more specifically structures....

February 12, 2021 · 8 min · Torgeir Aambø