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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!

## 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.

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.*

In a recent preprint, Haonan Zhang shows that if (where is a Sekine Finite Quantum Group), then the convolution powers, , converges if

.

The algebra of functions is a multimatrix algebra:

.

As it happens, where , the counit on is given by , that is , dual to .

To help with intuition, making the incorrect assumption that is a classical group (so that is commutative — it’s not), because , the statement , implies that for a real coefficient ,

,

as for classical groups .

That is the condition is a quantum analogue of .

Consider a random walk on a classical (the algebra of functions on is commutative) *finite* group driven by a .

The following is a very non-algebra-of-functions-y proof that implies that the convolution powers of converge.

*Proof: *Let be the smallest subgroup of on which is supported:

.

We claim that the random walk on driven by is *ergordic* (see Theorem 1.3.2).

The driving probability is not supported on any proper subgroup of , by the definition of .

If is supported on a coset of proper normal subgroup , say , then because , this coset must be , but this also contradicts the definition of .

Therefore, converges to the uniform distribution on

Apart from the big reason — that this proof talks about points galore — this kind of proof is not available in the quantum case because there exist that converge, but not to the Haar state on any quantum subgroup. A quick look at the paper of Zhang shows that some such states have the quantum analogue of .

So we have some questions:

- Is there a proof of the classical result (above) in the language of the algebra of functions on , that necessarily bypasses talk of points and of subgroups?
- And can this proof be adapted to the quantum case?
- Is the claim perhaps true for all finite quantum groups but not all compact quantum groups?

## Quantum Subgroups

Let be a the algebra of functions on a finite or perhaps compact quantum group (with comultiplication ) and a state on . We say that a quantum group with algebra of function (with comultiplication ) is a quantum subgroup of if there exists a surjective unital *-homomorphism such that:

.

## The Classical Case

In the classical case, where the algebras of functions on and are commutative,

There is a natural embedding, in the classical case, if is open (always true for finite) (thanks UwF) of ,

,

with for , and otherwise.

Furthermore, is has the property that

,

which resembles .

In the case where is a probability on a classical group , supported on a subgroup , it is very easy to see that convolutions remain supported on . Indeed, is the distribution of the random variable

,

where the i.i.d. . Clearly and so is supported on .

We can also prove this using the language of the commutative algebra of functions on , . The state being supported on implies that

.

Consider now two probabilities on but supported on , say . As they are supported on we have

and .

Consider

,

that is is also supported on and inductively .

## Some Questions

Back to quantum groups with non-commutative algebras of functions.

- Can we embed in with a map and do we have , giving the projection-like quality to ?
- Is a suitable definition for being supported on the subgroup .

If this is the case, the above proof carries through to the quantum case.

- If there is no such embedding, what is the appropriate definition of a being supported on a quantum subgroup ?
- If does not have the property of , in this or another definition, is it still true that being supported on implies that is too?

## Edit

UwF has recommended that I look at this paper to improve my understanding of the concepts involved.

Slides of a talk given at the Irish Mathematical Society 2018 Meeting at University College Dublin, August 2018.

**Abstract** *Four generalisations are used to illustrate the topic. The generalisation from finite “classical” groups to finite quantum groups is motivated using the language of functors (“classical” in this context meaning that the algebra of functions on the group is commutative). The generalisation from random walks on finite “classical” groups to random walks on finite quantum groups is given, as is the generalisation of total variation distance to the quantum case. Finally, a central tool in the study of random walks on finite “classical” groups is the Upper Bound Lemma of Diaconis & Shahshahani, and a generalisation of this machinery is used to find convergence rates of random walks on finite quantum groups.*

In May 2017 I wrote down some problems that I hoped to look at in my study of random walks on quantum groups. Following work of Amaury Freslon, a number of these questions have been answered. In exchange for solving these problems, Amaury has very kindly suggested some other problems that I can work on. The below hopes to categorise some of these problems and their status.

## Solved!

- Show that the total variation distance is equal to the projection distance. Amaury has an a third proof. Amaury suggests that this should be true in more generality than the case of being absolutely continuous (of the form for all and a unique ). If the Haar state is no longer tracial Amaury’s proof breaks down (and I imagine so do the two others in the link above). Amaury believes this is true in more generality and says perhaps the Jordan decomposition of states will be useful here.
- Prove the Upper Bound Lemma for compact quantum groups of Kac type. Achieved by Amaury.
- Attack random walks with conjugate invariant driving probabilitys: achieved by Amaury.
- Look at quantum generalisations of ‘natural’ random walks and shuffles. Solved is probably too strong a word, but Amaury has started this study by looking at a generalisation of the random transposition shuffle. As I suggested in Seoul, Amaury says: “One important problem in my opinion is to say something about analogues of classical random walks on (for instance the random transpositions or riffle shuffle)”. Amaury notes that “we are blocked by the counit problem. We must therefore seek bounds for other distances. As I suggest in my paper, we may look at the norm of the difference of the transition operators. The -estimate that I give is somehow the simplest thing one can do and should be thought of as a “spectral gap” estimate. Better norms would be the norms as operators on or even better, the completely bounded norm. However, I have not the least idea of how to estimate this.”

## Results to be Improved

- I have recently received an email from Isabelle Baraquin, a student of Uwe Franz, pointing out a small error in the thesis (a basis-error with the Kac-Paljutkin quantum groups).
- Recent calculations suggest that the lower bound for the random walk on the dual of is effective at while the upper bound shows the walk is random at time order . This is still a very large gap but at least the lower bound shows that this walk does converge very slowly.
- Get a much sharper lower bound for the random walk on the Sekine family of quantum groups studied in Section 5.7. Projection onto the ‘middle’ of the factor may provide something of use. On mature reflection, recognising that the application of the upper bound lemma is dominated by one set of terms in particular, it should be possible to use cruder but more elegant estimates to get the same upper bound except with lighter calculations (and also a smaller — see Section 5.7).

## More Questions on Random Walks

- Irreducibility is harder than the classical case (where ‘not concentrated’ on a subgroup is enough). Can anything be said about aperiodicity in the quantum case? (U. Franz).
- Prove an Ergodic Theorem (Theorem 1.3.2) for Finite Quantum Groups. Extend to Compact Quantum Groups. It is expected that the conditions may be more difficult than the classical case. However, it may be possible to use Diaconis-Van Daele theory to get some results in this direction. It should be possible to completely analyse some examples (such as the Kac-Paljutkin quantum group of order 8).This will involve a study of subgroups of quantum groups as well as
*normal*quantum subgroups and cosets. - Look at a random walk on the Sekine quantum groups with an -dependent driving probability and see if the cut-off phenomenon (Chapter 4) can be detected. This will need good lower bounds for , some cut-off time.
- Convolutions Factorisations of the Random Distribution: such a study may prove fruitful in trying to find the Ergodic Theorem. See Section 6.5.
**Amaury**mentions the problem of considering random walks associated to non-central states (in the compact case). “The difficulty is first to build non-central states (I do not have explicit examples at hand but Uwe Franz said he had some) and second to be able to compute their Fourier transform. Then, the computations will certainly be hard but may still be doable.”- A study of the Cesaro means: see Section 6.6.
- Spectral Analysis: it should be possible to derive crude bounds using the spectrum of the stochastic operator. More in Section 6.2.

## Future Work (for which I do not yet have the tools to attack)

**Amaury/Franz**Something perhaps more accessible is to investigate quantum homogeneous spaces. The free sphere is a noncommutative analogue of the usual sphere and a quantum homogeneous space for the free orthogonal quantum group. We can therefore define random walks on it and the whole machinery of Gelfand pairs might be available. In particular, Caspers gave a Plancherel theorem for Gelfand pairs of locally compact quantum groups which should apply here yielding an Upper Bound Lemma and then the problem boils down to something which should be close to my computations. There are probably works around this involving Adam Skalski and coauthors.**Amaury:**If one can prove a more general total variation distance equal to half one norm result, then Amaury suggests one can consider random walks on compact quantum groups which are not of Kac type. The Upper Bound Lemma will then involve matrices measuring the modular theory of the Haar state and some (but not all) dimensions in the formulas must be replaced by quantum dimensions. The main problem here is to define explicit central states since there is no Haar-state preserving conditional expectation onto the central algebra. However, there are tools from monoidal equivalence to do this.

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