You are currently browsing the category archive for the ‘Research’ category.

In my pursuit of an Ergodic Theorem for Random Walks on (probably finite) Quantum Groups, I have been looking at analogues of Irreducible and Periodic. I have, more or less, got a handle on irreducibility, but I am better at periodicity than aperiodicity.

The question of how to generalise these notions from the (finite) classical to noncommutative world has already been considered in a paper (whose title is the title of this post) of Fagnola and Pellicer. I can use their definition of periodic, and show that the definition of irreducible that I use is equivalent. This post is based largely on that paper.

## Introduction

Consider a random walk on a finite group driven by . The state of the random walk after steps is given by , defined inductively (on the algebra of functions level) by the associative

.

The convolution is also implemented by right multiplication by the stochastic operator:

,

where has entries, with respect to a basis . Furthermore, therefore

,

and so the stochastic operator describes the random walk just as well as the driving probabilty .

The random walk driven by is said to be *irreducible *if for all , there exists such that (if ) .

The *period *of the random walk is defined by:

.

The random walk is said to be *aperiodic *if the period of the random walk is one.

These statements have counterparts on the set level.

If is not irreducible, there exists a proper subset of , say , such that the set of functions supported on are -invariant. It turns out that is a proper subgroup of .

Moreover, when is irreducible, the period is the greatest common divisor of all the natural numbers such that there exists a partition of such that the subalgebras of functions supported in satisfy:

and (slight typo in the paper here).

In fact, in this case it is necessarily the case that is concentrated on a coset of a proper normal subgroup , say . Then .

Suppose that is supported on . We want to show that for . Recall that

.

This shows how the stochastic operator reduces the index .

A central component of Fagnola and Pellicer’s paper are results about how the decomposition of a stochastic operator:

,

specifically the maps can speak to the irreducibility and periodicity of the random walk given by . I am not convinced that I need these results (even though I show how they are applicable).

## Stochastic Operators and Operator Algebras

Let be a -algebra (so that is in general a virtual object). A -subalgebra is *hereditary *if whenever and , and , then .

It can be shown that if is a hereditary subalgebra of that there exists a projection such that:

.

All hereditary subalgebras are of this form.

In the case of a finite classical group , we can show that if we have i.i.d. random variables , that if , for a coset of a proper normal subgroup , that the random walk on driven by , the random variables:

,

exhibits a periodicity because

.

This shows that a necessary condition for ergodicity of a random walk on a finite classical group driven by is that the support of not be concentrated on the coset of a proper normal subgroup.

I had hoped that something similar might hold for the case of random walks on finite *quantum *groups but alas I think I have found a barrier.

Slides of a talk given at Munster Groups 2019, WIT.

**Abstract: ***It is a folklore theorem that necessary and sufficient conditions for a random walk on a finite group to converge in distribution to the uniform distribution – “to random” – are that the driving probability is not concentrated on a proper subgroup nor the coset of a proper normal subgroup. This is the Ergodic Theorem for Random Walks on Finite Groups. In this talk we will outline the rarely written down proof, and explain why, for example, adjacent transpositions can never mix up a deck of cards. From here we will, in a very leisurely and natural fashion, introduce (and motivate the definition of) finite quantum groups, and random walks on them. We will see how the group algebra of a finite group is the algebra of functions on a finite quantum group. Freslon has very recently proved the Ergodic Theorem in this setting, and we present ongoing work towards an Ergodic Theorem in the more general finite quantum group setting; a result that would generalise both the folklore and group algebra Ergodic Theorems.*

### Introduction

Every finite quantum group has finite dimensional algebra of functions:

.

At least one of the factors must be one-dimensional to account for the counit , and if this factor is denoted , the counit is given by the dual element . There may be more and so reorder the index so that for , and for :

,

Denote by the states of . The *pure *states of arise as pure states on single factors.

In the case of the Kac-Paljutkin and Sekine quantum groups, the convolution powers of pure states exhibit a periodicity of sorts. Recall for these quantum groups that consists of a single matrix factor.

In these cases, for pure states of the form , that is supported on (and we can say a little more than is necessary), the convolution remains supported on because

.

If we have a pure state supported on , then because

,

then must be supported on, because of , .

Inductively all of the are supported on and the are supported on . This means that the convolutions powers of a pure state, in these cases, cannot converge to the Haar state.

The question is, do the results above about the image of and under the coproduct hold more generally? I believe that the paper of Kac and Paljutkin shows that this is the case whenever consists of a single factor… but does it hold more generally?

To find out we go back and do some sandboxing with the paper of Kac and Paljutkin. Which is a pleasure because that paper is beautiful. The blue stuff is my own scribbling.

## Finite Ring Groups

Let be a finite quantum group with notation on the algebra of functions as above. Note that is commutative. Let

,

which is a central idempotent.

### Lemma 8.1

.

*Proof: *If , then for some , and , the mapping is a non-zero homomorphism from into commutative which is impossible.

If , then one of the , with ‘something’ in . Using the centrality and projectionality of , we can show that the given map is indeed a homomorphism.

It follows that , and so

### Lemma 8.2

*Proof: *Suppose that for some non-commutative . This means that there exists an index such that . Then for that factor,

is a non-null homomorphism from the non-commutative into the commutative.

We see that for all . Putting we get the result

The following says that is a group-like projection. We know from previous work that if a state is supported on a group-like projection that it will remain supported on it. In particular, any state supported on will remain there.

### Lemma 8.3

.

*Proof: *Since is a homomorphism, is an idempotent in . I do not understand nor require the rest of the proof.

### Lemma 8.4

* is the algebra of functions on finite group with elements , and we write . The coproduct is given by .*

We have:

,

,

,

as .

The element is a sum of four terms, lying in the subalgebras:

.

We already know what is going on with the first summand. Denote the second by . From the group-like-projection property, the last two summands are zero, so that

=\sum_{t\in G_1}\delta_{t}\otimes \delta_{t^{-1}i}+P_i$.

Since the are symmetric () mutually orthogonal idempotents, has similar properties:

for .

At this point Kac and Paljutkin restrict to , that is there is only one summand. Here we *try *to keep arbitrarily (finitely) many summands in .

Let the summand have matrix units , where . Kac and Paljutkin now do something which I think is a little dodgy, but basically that the integral over is equal on each of the , equal on each of the , and then zero off the diagonal.

It does follow from above that each is a projection.

Now I am stuck!

*This sandbox is going to take from a variety of sources, mostly Shuzhou Wang.*

## C*-Ideals

Let be a closed (two-sided) ideal in a non-commutative unital -algebra . Such an ideal is self-adjoint and so a non-commutative -algebra . The quotient map is given by , , where is the equivalence class of under the equivalence relation:

.

Where we have the product

,

and the norm is given by:

,

the quotient is a -algebra.

Consider now elements and . Consider

.

The tensor product . Now note that

,

by the nature of the Tensor Product (). Therefore .

### Definition

A WC*-ideal (W for *Woronowicz*) is a C*-ideal such that , where is the quotient map .

Let be the algebra of functions on a classical group . Let . Let be the set of functions which vanish on : this is a C*-ideal. The kernal of is .

Let so that . Note that

and so

.

Note that if . It is not possible that both and are in : if they were , but , which is not in by assumption. Therefore one of or is equal to zero and so:

,

and so by linearity, if vanishes on a subgroup ,

.

In this way, WC*-ideals generalise functions which vanish on distinguished subgroups. In fact, without checking all the details, I imagine that first isomorphism theorem can show that . Let be the ring homomorphism

.

Then , , and so we have the isomorphism of rings, which presumably carries forward to the algebras of functions level…

*Just some notes on section 1 of this paper. Flags and notes are added but mistakes are mine alone.*

#### Definition

Let be the algebra of continuous functions on a compact matrix quantum group. Such an object is given by a matrix which generates as a C*-algebra. Furthermore, there exists a C*-algebra homomorphism such that

,

and both and are invertible in .

Any subgroup is such an object, with the given by . Furthermore

.

We say that is a *representation *if it is invertible and

.

The transpose is also invertible and so we have:

#### Proposition

The C*algebra generated by the is also the algebra of continuous functions on a compact matrix quantum group.

## Background

I am trying to prove an Ergodic Theorem for Random Walks on Finite Quantum Groups. A random walk on a quantum group driven by is *ergodic* if the convolution powers converge to the Haar state .

The classical theorem for finite groups:

#### Ergodic Theorem for Random Walks on Finite Groups

A random walk on a finite group driven by a probability is ergodic if and only if is not concentrated on a proper subgroup nor the coset of a proper normal subgroup.

Not concentrated on a proper subgroup gives *irreducibility*. A random walk is *irreducible *if for all , there exists such that .

Not concentrated on the coset of a proper normal subgroup gives *aperiodicity. *Something which should be equivalent to aperiodicity is if

is equal to one (perhaps via invariance ).

If is concentrated on the coset a proper normal subgroup , specifically on , then we have periodicity (), and , the order of .

In Markov chain theory, ergodicity is equivalent to irreduciblity and aperiodicity.

The theorem in the quantum case should look like:

#### Ergodic Theorem for Random Walks on Finite Quantum Groups

A random walk on a finite quantum group driven by a state is ergodic if and only if “X”.

## Irreducibility

At the moment I have some part of X;the irreducibility bit. As is well known since Pal (1996), it is possible to have a probability not concentrated on a quantum subgroup be reducible. This led Franz & Skalski to generalise quantum subgroups to *group-like-projections,* which I will say correspond to *quasi-subgroups *following Kasprzak & Sołtan.

I have shown that if is concentrated on a proper quasi-subgroup , in the sense that for a group-like-projection , that so are the . The analogue of irreducible is that for all projections in , there exists such that . If is concentrated on a quasi-subgroup , then for all , , where .

I have also shown on the other hand that if the random walk is reducible that it must be concentrated on a proper quasi-subgroup. Franz & Skalski show that group-like-projections also have a correspondence with idempotent states. The Césaro Means

,

converge to an idempotent state . If for all then the also, so that (as the Haar state is faithful). I was able to prove that is supported on the quasi-subgroup given by the idempotent .

I believe this result — irreducible if and only if not concentrated on a quasi-subgroup — holds more generally than just in finite quantum groups.

*Just some notes on the pre-print. I am looking at this paper to better understand this pre-print. In particular I am hoping to learn more about the support of a probability on a quantum group. Flags and notes are added but mistakes are mine alone.*

#### Abstract

From this paper I will look at:

- lattice operations on , for a LCQG (analogues of intersection and generation)

## 1. Introduction

Idempotent states on quantum groups correspond with “subgroup-like” objects. In this work, on LCQG, the correspondence is with *quasi-subgroups *(the work of Franz & Skalski the correspondence was with *pre-subgroups *and *group-like projections*).

Let us show the kind of thing I am trying to understand better.

Let be the algebra of function on a finite quantum group. Let be concentrated on a pre-subgroup . We can associate to a group like projection .

Let, and this is another thing I am trying to understand better, this support, the support of be ‘the smallest’ (?) projection such that . Denote this projection by . Define similarly. That are concentrated on is to say that and .

Define a map by

(or should this be or ?)

We can decompose, in the finite case, .

**Claim: **If is concentrated on , … I don’t have a proof but it should fall out of something like together with the decomposition of above. It may also require that is a trace, I don’t know. Something very similar in the preprint.

From here we can do the following. That is a group-like projection means that:

Hit both sides with to get:

.

By the fact that are supported on , the right-hand side equals one, and by the as-yet-unproven claim, we have

.

However this is the same as

,

in other words , that is remains supported on . As a corollary, a random walk driven by a probability concentrated on a pre-subgroup remains concentrated on .

Slides of a talk given to the Functional Analysis seminar in Besancon.

Some of these problems have since been solved.

### “e in support” implies convergence

Consider a on a *finite* quantum group such that where

,

with . This has a positive density of trace one (with respect to the Haar state ), say

,

where is the Haar element.

An element in a direct sum is positive if and only if both elements are positive. The Haar element is positive and so . Assume that (if , then for all and we have trivial convergence)

Therefore let

be the density of .

Now we can explicitly write

.

This has stochastic operator

.

Let be an eigenvalue of of eigenvector . This yields

and thus

.

Therefore, as is also an eigenvector for , and is a stochastic operator (if is an eigenvector of eigenvalue , then , contradiction), we have

.

This means that the eigenvalues of lie in the ball and thus the only eigenvalue of magnitude one is , which has (left)-eigenvector the stationary distribution of , say .

If is symmetric/reversible in the sense that , then is self-adjoint and has a basis of (left)-eigenvectors and we have, if we write ,

,

which converges to (so that ).

If is not reversible, it is a standard argument to show that when put in Jordan normal form, that the powers converge and thus so do the

### Total Variation Decrasing

Uses Simeng Wang’s . Result holds for compact Kac if the state has a density.

### Periodic is concentrated on a coset of a proper normal subgroup of

is a minimal projection (coset) in the quotient space of the normal subgroup (to be double checked) given by

### Supported on Subgroup implies Reducible

I have a proof that reducible is equivalent to supported on a pre-subgroup.

*Diaconis–Shahshahani Upper Bound Lemma for Finite Quantum Groups, *Journal of Fourier Analysis and Applications, doi: 10.1007/s00041-019-09670-4 (earlier preprint available here)

**Abstract**

*A central tool in the study of ergodic random walks on finite groups is the Upper Bound Lemma of Diaconis and Shahshahani. The Upper Bound Lemma uses Fourier analysis on the group to generate upper bounds for the distance to random and thus can be used to determine convergence rates for ergodic walks. The Fourier analysis of quantum groups is remarkably similar to that of classical groups. This allows for a generalisation of the Upper Bound Lemma to an Upper Bound Lemma for finite quantum groups. The Upper Bound Lemma is used to study the convergence of ergodic random walks on the dual group as well as on the truly quantum groups of Sekine.*

## Recent Comments