The goal of this post is to compute the cohomology of the -torus in as many ways as I can think of. Below, if no coefficient ring is specified then the coefficient ring is by default. At the end we will interpret this computation in terms of cohomology operations.

## Posts Tagged ‘duality’

## The cohomology of the n-torus

Posted in algebraic topology, tagged computations, duality, fixed point theorems on October 12, 2013 | 9 Comments »

## Cartesian closed categories and the Lawvere fixed point theorem

Posted in category theory, logic and set theory, tagged abstract nonsense, adjoint functors, cartesian closed categories, duality, self-reference on September 28, 2013 | 1 Comment »

Previously we saw that Cantor’s theorem, the halting problem, and Russell’s paradox all employ the same diagonalization argument, which takes the following form. Let be a set and let

be a function. Then we can write down a function such that . If we **curry** to obtain a function

it now follows that there cannot exist such that , since .

Currying is a fundamental notion. In mathematics, it is constantly implicitly used to talk about function spaces. In computer science, it is how some programming languages like Haskell describe functions which take multiple arguments: such a function is modeled as taking one argument and returning a function which takes further arguments. In type theory, it reproduces function types. In logic, it reproduces material implication.

Today we will discuss the appropriate categorical setting for understanding currying, namely that of cartesian closed categories. As an application of the formalism, we will prove the Lawvere fixed point theorem, which generalizes the argument behind Cantor’s theorem to cartesian closed categories.

## Operations, pro-objects, and Grothendieck’s Galois theory

Posted in category theory, Galois theory, group theory, tagged abstract nonsense, adjoint functors, duality, filtered colimits, Lawvere theories, profinite groups on June 23, 2013 | 7 Comments »

Previously we looked at several examples of -ary operations on concrete categories . In every example except two, was a representable functor and had finite coproducts, which made determining the -ary operations straightforward using the Yoneda lemma. The two examples where was not representable were commutative Banach algebras and commutative C*-algebras, and it is possible to construct many others. Without representability we can’t apply the Yoneda lemma, so it’s unclear how to determine the operations in these cases.

However, for both commutative Banach algebras and commutative C*-algebras, and in many other cases, there is a sense in which a sequence of objects approximates what the representing object of “ought” to be, except that it does not quite exist in the category itself. These objects will turn out to define a pro-object in , and when is pro-representable in the sense that it’s described by a pro-object, we’ll attempt to describe -ary operations in terms of the pro-representing object.

The machinery developed here is relevant to understanding Grothendieck’s version of Galois theory, which among other things leads to the notion of étale fundamental group; we will briefly discuss this.

## Operations and Lawvere theories

Posted in category theory, differential geometry, functional analysis, topology, universal algebra, tagged abstract nonsense, adjoint functors, duality, Lawvere theories, pointless topology, reconstruction theorems on June 9, 2013 | 10 Comments »

Groups are in particular sets equipped with two operations: a binary operation (the group operation) and a unary operation (inverse) . Using these two operations, we can build up many other operations, such as the ternary operation , and the axioms governing groups become rules for deciding when two expressions describe the same operation (see, for example, this previous post).

When we think of groups as objects of the category , where do these operations go? They’re certainly not morphisms in the corresponding categories: instead, the morphisms are supposed to preserve these operations. But can we recover the operations themselves?

It turns out that the answer is yes. The rest of this post will describe a general categorical definition of -ary operation and meander through some interesting examples. After discussing the general notion of a Lawvere theory, we will then prove a reconstruction theorem and then make a few additional comments.

## Four flavors of Schur-Weyl duality

Posted in invariant theory, Lie theory, representation theory, tagged duality, MaBloWriMo, representation theory of the symmetric group, symmetric functions on November 13, 2012 | 17 Comments »

If is a finite-dimensional complex vector space, then the symmetric group naturally acts on the tensor power by permuting the factors. This action of commutes with the action of , so all permutations are morphisms of -representations. This defines a morphism , and a natural question to ask is whether this map is surjective.

Part of Schur-Weyl duality asserts that the answer is yes. The double commutant theorem plays an important role in the proof and also highlights an important corollary, namely that admits a canonical decomposition

where runs over partitions, are some irreducible representations of , and are the Specht modules, which describe all irreducible representations of . This gives a fundamental relationship between the representation theories of the general linear and symmetric groups; in particular, the assignment can be upgraded to a functor called a Schur functor, generalizing the construction of the exterior and symmetric products.

The proof below is more or less from Etingof’s notes on representation theory (Section 4.18). We will prove four versions of Schur-Weyl duality involving , and (in the special case that is a complex inner product space) .

## String diagrams, duality, and trace

Posted in category theory, linear algebra, topology, tagged diagrammatic algebra, duality, MaBloWriMo, monoidal categories on November 6, 2012 | Leave a Comment »

Previously we introduced string diagrams and saw that they were a convenient way to talk about tensor products, partial compositions of multilinear maps, and symmetries. But string diagrams really prove their use when augmented to talk about duality, which will be described topologically by bending input and output wires. In particular, we will be able to see topologically the sense in which the following four pieces of information are equivalent:

- A linear map ,
- A linear map ,
- A linear map ,
- A linear map .

Using string diagrams we will also give a diagrammatic definition of the trace of an endomorphism of a finite-dimensional vector space, as well as a diagrammatic proof of some of its basic properties.

Below all vector spaces are finite-dimensional and the composition convention from the previous post is still in effect.

## Banach algebras, the Gelfand representation, and the commutative Gelfand-Naimark theorem

Posted in functional analysis, operator algebras, quantum mechanics, tagged c*-algebras, dagger categories, duality, spectral theory on July 17, 2012 | 4 Comments »

Banach algebras abstract the properties of closed algebras of operators on Banach spaces. Many basic properties of such operators have elegant proofs in the framework of Banach algebras, and Banach algebras also naturally appear in areas of mathematics like harmonic analysis, where one writes down Banach algebras generalizing the group algebra to study topological groups.

Today we will develop some of the basic theory of Banach algebras, our goal being to discuss the Gelfand representation of a commutative Banach algebra and the fact that, for commutative C*-algebras, this representation is an isometric isomorphism. This implies in particular a spectral theorem for self-adjoint operators on a Hilbert space.

This material can be found in many sources; I am working from Dales, Aiena, Eschmeier, Laursen and Willis’ *Introduction to Banach Algebras, Operators, and Harmonic Analysis*.

Below all vector spaces are over , all algebras are unital, and all algebra homomorphisms preserve units unless otherwise stated. In the context of Banach algebras, the last two assumptions are not standard, but in practice non-unital Banach algebras are studied by adjoining units first, so we do not lose much generality.

## Hilbert spaces (and dagger categories)

Posted in category theory, functional analysis, quantum mechanics, tagged abstract nonsense, adjoint functors, dagger categories, duality on June 25, 2012 | 10 Comments »

Hilbert spaces are a particularly nice class of Banach spaces. They axiomatize ideas from Euclidean geometry such as orthogonality, projection, and the Pythagorean theorem, but the ideas apply to many infinite-dimensional spaces of functions of interest to various branches of mathematics. Hilbert spaces are also fundamental to quantum mechanics, as vectors in Hilbert spaces (up to phase) describe (pure) states of quantum systems.

Today we’ll develop and discuss some of the basic theory of Hilbert spaces. As with the theory of Banach spaces, there are (at least) two types of morphisms we might want to talk about (unitary operators and bounded operators), and we will discuss an elegant formalism that allows us to talk about both. Things written by John Baez will be cited excessively.

## Boolean rings, ultrafilters, and Stone’s representation theorem

Posted in category theory, commutative algebra, logic and set theory, order theory, topology, tagged adjoint functors, axiom of choice, duality, ultrafilters, universal properties on November 22, 2010 | 10 Comments »

Recently, I have begun to appreciate the use of ultrafilters to clean up proofs in certain areas of mathematics. I’d like to talk a little about how this works, but first I’d like to give a hefty amount of motivation for the definition of an ultrafilter.

Terence Tao has already written a great introduction to ultrafilters with an eye towards nonstandard analysis. I’d like to introduce them from a different perspective. Some of the topics below are also covered in these posts by Todd Trimble.

## Hecke algebras and the Kazhdan-Lusztig polynomials

Posted in algebraic combinatorics, representation theory, tagged Coxeter groups, duality, finite fields, Hecke algebras, Kazhdan-Lusztig, q-analogues on July 12, 2010 | 2 Comments »

The **Hecke algebra** attached to a Coxeter system is a deformation of the group algebra of defined as follows. Take the free -module with basis , and impose the multiplicative relations

if , and

otherwise. (For now, ignore the square root of .) Humphreys proves that these relations describe a unique associative algebra structure on with as the identity, but the proof is somewhat unenlightening, so I will skip it. (Actually, the only purpose of this post is to motivate the definition of the Kazhdan-Lusztig polynomials, so I’ll be referencing the proofs in Humphreys rather than giving them.)

The motivation behind this definition is a somewhat long story. When is the Weyl group of an algebraic group with Borel subgroup , the above relations describe the algebra of functions on which are bi-invariant with respect to the left and right actions of under a convolution product. The representation theory of the Hecke algebra is an important tool in understanding the representation theory of the group , and more general Hecke algebras play a similar role; see, for example MO question #4547 and this Secret Blogging Seminar post. For example, replacing and with and gives the Hecke operators in the theory of modular forms.