You are currently browsing the category archive for the ‘Random Walks on Finite Quantum Groups’ category.

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 $\nu$ being absolutely continuous (of the form $\nu(x)=\int_G xa_{\nu}$ for all $x\in C(G)$ and a unique $a_{\nu}\in C(G)$). 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 $S_n$ (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 $\mathcal{L}^2$-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 $\mathcal{L}^\infty$ 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 $S_n$ is effective at $k\sim (n-1)!$ while the upper bound shows the walk is random at time order $n!$.  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 $M_n(\mathbb{C})$ 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 $\alpha$ — 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 $n$-dependent driving probability and see if the cut-off phenomenon (Chapter 4) can be detected. This will need good lower bounds for $k\ll t_n$, some cut-off time.
• Convolutions 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 $Q$ 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.

## Distances between Probability Measures

Let $G$ be a finite quantum group and $M_p(G)$ be the set of states on the $\mathrm{C}^\ast$-algebra $F(G)$.

The algebra $F(G)$ has an invariant state $\int_G\in\mathbb{C}G=F(G)^\ast$, the dual space of $F(G)$.

Define a (bijective) map $\mathcal{F}:F(G)\rightarrow \mathbb{C}G$, by

$\displaystyle \mathcal{F}(a)b=\int_G ba$,

for $a,b\in F(G)$.

Then, where $\|\cdot\|_1^{F(G)}=\int_G|\cdot|$ and $\|\cdot\|_\infty^{F(G)}=\|\cdot\|_{\text{op}}$, define the total variation distance between states $\nu,\mu\in M_p(G)$ by

$\displaystyle \|\nu-\mu\|=\frac12 \|\mathcal{F}^{-1}(\nu-\mu)\|_1^{F(G)}$.

(Quantum Total Variation Distance (QTVD))

Standard non-commutative $\mathcal{L}^p$ machinary shows that:

$\displaystyle \|\nu-\mu\|=\sup_{\phi\in F(G):\|\phi\|_\infty^{F(G)}\leq 1}\frac12|\nu(\phi)-\mu(\phi)|$.

(supremum presentation)

In the classical case, using the test function $\phi=2\mathbf{1}_S-\mathbf{1}_G$, where $S=\{\nu\geq \mu\}$, we have the probabilists’ preferred definition of total variation distance:

$\displaystyle \|\nu-\mu\|_{\text{TV}}=\sup_{S\subset G}|\nu(\mathbf{1}_S)-\mu(\mathbf{1}_S)|=\sup_{S\subset G}|\nu(S)-\mu(S)|$.

In the classical case the set of indicator functions on the subsets of the group exhaust the set of projections in $F(G)$, and therefore the classical total variation distance is equal to:

$\displaystyle \|\nu-\mu\|_P=\sup_{p\text{ a projection}}|\nu(p)-\mu(p)|$.

(Projection Distance)

In all cases the quantum total variation distance and the supremum presentation are equal. In the classical case they are equal also to the projection distance. Therefore, in the classical case, we are free to define the total variation distance by the projection distance.

## Quantum Projection Distance $\neq$ Quantum Variation Distance?

Perhaps, however, on truly quantum finite groups the projection distance could differ from the QTVD. In particular, a pair of states on a $M_n(\mathbb{C})$ factor of $F(G)$ might be different in QTVD vs in projection distance (this cannot occur in the classical case as all the factors are one dimensional).

Just back from a great workshop at Seoul National University, I am just going to use this piece to outline in a relaxed manner my key goals for my work on random walks on quantum groups for the near future.

In the very short term I want to try and get a much sharper lower bound for my random walk on the Sekine family of quantum groups. I believe the projection onto the ‘middle’ of the $M_n(\mathbb{C})$ might 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 $\alpha$ — see Section 5.7).

I also want to understand how sharp (or otherwise) the order $n^n$ convergence for the random walk on the dual of $S_n$ is — $n^n$ sounds awfully high. Furthermore it should be possible to get a better lower bound that what I have.

It should also be possible to redefine the quantum total variation distance as a supremum over projections $\sim$ subsets via $G \supset S\leftrightarrow \mathbf{1}_S$. If I can show that for a positive linear functional $\rho$ that $|\rho(a)|\leq \rho(|a|)$ then using these ideas I can. More on this soon hopefully. No, this approach won’t work. (I have since completed this objective with some help: see here).

The next thing I might like to do is look at a random walk on the Sekine quantum groups with an $n$-dependent driving probability and see if I can detect the cut-off phenomenon (Chapter 4). This will need good lower bounds for $k\ll t_n$, some cut-off time.

Going back to the start, the classical problem began around 1904 with the question of Markov:

Which card shuffles mix up a deck of cards and cause it to ‘go random’?

For example, the perfect riffle shuffle does not mix up the cards at all while a riffle shuffle done by an amateur will.

In the context of random walks on classical groups this question is answered by the Ergodic Theorem 1.3.2: when the driving probability is not concentrated on a subgroup (irreducibility) nor the coset of a normal subgroup (aperiodicity).

Necessary and sufficient conditions on the driving probability $\nu\in M_p(\mathbb{G})$ for the random walk on a quantum group to converge to random are required. 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.

It should be straightforward to extend the Upper Bound Lemma (Lemma 5.3.8) to the case of compact Kac algebras. Once that is done I will want to look at quantum generalisations of ‘natural’ random walks and shuffles.

I intend also to put the PhD thesis on the Arxiv. After this I have a number of options as regard to publishing what I have or maybe waiting a little while until I solve the above problems — this will all depend on how my further study progresses.

Slides of a talk given at the Topological Quantum Groups and Harmonic Analysis workshop at Seoul National University, May 2017.

Abstract A central tool in the study of ergodic random walks on finite groups is the Upper Bound Lemma of Diaconis & Shahshahani. The Upper Bound Lemma uses the representation theory of the group to generate upper bounds for the distance to random and thus can be used to determine convergence rates for ergodic walks. These ideas are generalised to the case of finite quantum groups.

After a long time I have finally completed my PhD studies when I handed in my hardbound thesis (a copy of which you can see here).

It was a very long road but thankfully now the pressure is lifted and I can enjoy my study of quantum groups and random walks thereon for many years to come.

I have finally finished the first draft of my PhD thesis. My advisor Dr Stephen Wills is presently reading through it and will get back to me with his comments in the next few weeks. The project was successful in that I managed to prove the Diaconis-Shahshahani Upper Bound Lemma for finite quantum groups… how successful my application of the Lemma to concrete examples is probably open to debate. First draft of abstract and introduction — without references — below the fold.

Let $\mathbb{G}$ be a finite quantum group described by $A=\mathcal{C}(\mathbb{G})$ with an involutive antipode (I know this is true is the commutative or cocommutative case. I am not sure at this point how restrictive it is in general. The compact matrix quantum groups have this property so it isn’t a terrible restriction.) $S^2=I_A$. Under the assumption of finiteness, there is a unique Haar state, $h:A\rightarrow \mathbb{C}$ on $A$.

# Representation Theory

A representation of $\mathbb{G}$ is a linear map $\kappa:V\rightarrow V\otimes A$ that satisfies

$\left(\kappa\otimes I_A\right)\circ\kappa =\left(I_V\otimes \Delta\right)\circ \kappa\text{\qquad and \qquad}\left(I_V\otimes\varepsilon\right)\circ \kappa=I_V.$

The dimension of $\kappa$ is given by $\dim\,V$. If $V$ has basis $\{e_i\}$ then we can define the matrix elements of $\kappa$ by

$\displaystyle\kappa\left(e_j\right)=\sum_i e_i\otimes\rho_{ij}.$

One property of these that we will use it that $\varepsilon\left(\rho_{ij}\right)=\delta_{i,j}$.

Two representations $\kappa_1:V_1\rightarrow V_1\otimes A$ and $\kappa_2:V_2\rightarrow V_2\otimes A$ are said to be equivalent, $\kappa_1\equiv \kappa_2$, if there is an invertible intertwiner between them. An intertwiner between $\kappa_1$ and $\kappa_2$ is a map $T\in L\left(V_1,V_2\right)$ such that

$\displaystyle\kappa_2\circ T=\left(T\otimes I_A\right)\circ \kappa_1.$

We can show that every representation is equivalent to a unitary representation.

Timmermann shows that if $\{\kappa_\alpha\}_{\alpha}$ is a maximal family of pairwise inequivalent irreducible representation that $\{\rho_{ij}^\alpha\}_{\alpha,i,j}$ is a basis of $A$. When we refer to “the matrix elements” we always refer to such a family. We define the span of $\{\rho_{ij}\}$ as $\mathcal{C}\left(\kappa\right)$, the space of matrix elements of $\kappa$.

Given a representation $\kappa$, we define its conjugate, $\overline{\kappa}:\overline{V}\rightarrow\overline{V}\otimes A$, where $\overline{V}$ is the conjugate vector space of $V$, by

$\displaystyle\overline{\kappa}\left(\bar{e_j}\right)=\sum_i \bar{e_i}\otimes\rho_{ij}^*,$

so that the matrix elements of $\overline{\kappa}$ are $\{\rho_{ij}^*\}$.

Timmermann shows that the matrix elements have the following orthogonality relations:

• If $\alpha$ and $\beta$ are inequivalent then $h\left(a^*b\right)=0,$ for all $a\in \mathcal{C}\left(\kappa_\alpha\right)$ and $b\in\mathcal{C}\left(\kappa_\beta\right)$.
• If $\kappa$ is such that the conjugate, $\overline{\kappa}$, is equivalent to a unitary matrix (this is the case in the finite dimensional case), then we have

$\displaystyle h\left(\rho_{ij}^*\rho_{kl}\right)=\frac{\delta_{i,k}\delta_{j,l}}{d_\alpha}.$

This second relation is more complicated without the $S^2=I_A$ assumption and refers to the entries and trace of an intertwiner $F$ from $\kappa$ to the coreprepresention with matrix elements $\{S^2\left(\rho_{ij}\right)\}$. If $S^2=I_A$, then this intertwiner is simply the identity on $V$ and so the the entries $\left[F\right]_{ij}=\delta_{i,j}$ and the trace is $d=\dim V$.

Denote by $\text{Irr}(\mathbb{G})$ the set of unitary equivalence classes of irreducible unitary representations of $\mathbb{G}$. For each $\alpha\in\text{Irr}(\mathbb{G})$, let $\kappa_\alpha:V_{\alpha}\rightarrow V_{\alpha}\otimes A$ be a representative of the class $\alpha$ where $V_\alpha$ is the finite dimensional vector space on which $\kappa_\alpha$ acts.

# Diaconis-Van Daele Fourier Theory

In Group Representations in Probability and Statistics, Diaconis presents his celebrated Upper Bound Lemma for a random walk on a finite  group $G$ driven by $\nu\in M_p(G)$. It states that

$\displaystyle \|\nu^{\star k}-\pi\|^2\leq \frac{1}{4}\sum d_\rho \text{Tr }(\widehat{\nu(\rho)}^k(\widehat{\nu(\rho)}^\ast)^k)$,

where the sum is over all non-trivial irreducible representations of $G$.

In this post, we begin this study by looking a the (co)-representations of a quantum group $A$. The first thing to do is to write down a satisfactory definition of a representation of a group which we can quantise later. In the chapter on Diaconi-Fourier Theory here we defined a representation of a group as a group homomorphism

$\rho:G\rightarrow GL(V)$

While this was perfectly adequate for when we are working with finite groups, it might not be as transparently quantisable. Instead we define a representation of a group as an action

$\Phi:V\times G\rightarrow V$.

such that the map $\rho(g):V\rightarrow V$$\rho(g)x=\Phi(x,g)$ is linear.

Let $G$ be a group and let $A:=C^*(G)$  be the C*-algebra of the group $G$. This is a C*-algebra whose elements are complex-valued functions on the group $G$. We define operations on $A$ in the ordinary way save for multiplication

$\displaystyle (fg)(s)=\sum_{t\in G}f(t)g(t^{-1}s)$,

and the adjoint $f^*(s)=\overline{f(s^{-1})}$. Note that the above multiplication is the same as defining $\delta_s\delta_t=\delta_{st}$ and extending via linearity. Thence $A$ is abelian if and only if $G$ is.

To give the structure of a quantum group we define the following linear maps:

$\Delta:A\rightarrow A\otimes A$$\Delta(\delta_s)=\delta_s\otimes\delta_s$.

$\displaystyle \varepsilon:A\rightarrow \mathbb{C}$$\varepsilon(\delta_s)=1.$

$S:A\rightarrow A$$S(\delta_s)=\delta_{s^{-1}}$.

The functional $h:A\rightarrow \mathbb{C}$ defined by $h=\mathbf{1}_{\{\delta_e\}}$ is the Haar state on $A$. It is very easy to write down the $j_n$:

$\displaystyle j_n(\delta_s)=\bigotimes_{i=0}^n\delta_s$.

To do probability theory consider states $\varepsilon,\,\phi$ on $A$ and form the product state:

$\displaystyle \Psi=\varepsilon\otimes\bigotimes_{i=1}^\infty \phi$.

Whenever $\phi$ is a state of $A$ such that $\phi(\delta_s)=1$ implies that $s=e$, then the distribution of the random variables $j_n$ converges to $h$.

At the moment we will use the one-norm to measure the distance to stationary:

$d(\Psi\circ j_n,h)=\|\Psi\circ j_n-h\|_1$.

A quick calculation shows that:

$d(\Psi\circ j_n,h)=\sum_{s\neq e}|\phi(\delta_s)|^n$.

When, for example, $\phi(\delta_s)=2/m^2$ when $s$ are transpositions in $S_m$, then we have

$d(\Psi\circ j_n,h)={m\choose 2}\left(\frac{2}{m^2}\right)^n$.

Taken from Random Walks on Finite Quantum Groups by Franz & Gohm.

## Theorem

Let $\phi$ be a state on a finite quantum group $A$. Then the Cesaro mean

$\displaystyle \phi_n=\frac{1}{n}\sum_{k=1}^n\phi^{\star n}$$n\in\mathbb{N}$

converges to an idempotent state on $A$, i.e. to a state $\pi$ such that $\pi\star\pi=\pi$.

Proof : Let $\phi'$ be an accumulation point of $\{\phi_n\}_{n\geq0}$, this exists since the states on $A$ form a compact set. We have

$\|\phi_n-\phi\star \phi_n\|=\frac{1}{n}\|\phi-\phi^{n+1}\|\leq \frac{2}{n}$.

I have no idea where the equality comes from.

Choose sequence $\{n_k\}_{k\geq 0}$ such that $\phi_{n_k}\rightarrow \phi'$, we get $\phi\star\phi'=\phi'$ and similarly $\phi'\star \phi=\phi'$. By linearity this implies $\phi_n\star\phi'=\phi'=\phi'\star \phi_n$. If $\phi''$ is another accumulation point of $\{\phi_n\}_{n\geq 0}$ and $\{m_{\ell}\}_{\ell\geq 0}$ a sequence such that $\phi_{m_\ell}\rightarrow\phi''$, then we get $\phi''\star\phi'=\phi'=\phi'\star\phi''$ and thus $\phi'=\phi''$ by symmetry (??). Therefore the sequence $\{\phi_n\}_{n\geq0}$ has a unique accumulation point, i.e. it converges $\bullet$

### Remark

If $\phi$ is faithful, then the Cesaro limit $\pi$ is the Haar state on $A$ (prove this).

### Remark

Due to cyclicity the sequence $\{\phi^{\star n}\}_{n\geq 0}$ does not converge in general. Take, for example, the state $\phi=\eta_2$ (p.28) on the Kac-Paljutkin quantum group $A$, then we have

$\eta_2^{\star n}=\left\{\begin{array}{ccc}\eta_2&\text{if}& n\text{ is odd}\\ \varepsilon&\text{if}&n\text{ is even}\end{array}\right.$,

but

$\displaystyle \lim_{n\rightarrow\infty}\frac1{n}\sum_{k=1}^n\eta_2^{\star k}=\frac{\varepsilon+\eta_2}2$.