Let $A=F(\mathbb{G})$ be the algebra of functions on a finite quantum group with a Haar state $$h=:\int\_\mathbb{G}:F(\mathbb{G})\rightarrow \mathbb{C}.$$


There is a convolution product on $A=F(\mathbb{G})$ given by 


$$a\star\_A b=b\_{(2)}\int\_{\mathbb{G}}S\left(b\_{(1)}\right)a,$$


where 


$$\Delta(b)=\sum b\_{1,i}\otimes b\_{2,i}=:b\_{(1)}\otimes b\_{(2)}.$$


There is a link between the convolution in $F(\mathbb{G})$ and the convolution product in $A'=F(\mathbb{G})'=\mathbb{CG}$. The convolution product in $\mathbb{CG}$, $\star$ (with no subscript), is the dual of the comultiplication of $F(\mathbb{G})$ and thus for $\varphi,\,\nu\in\mathbb{CG}$
$$\varphi\star\nu=(\varphi\otimes\nu)\Delta.$$


Let $\mathcal{F}:F(\mathbb{G})\rightarrow \mathbb{CG}$ be the map $b\mapsto \mathcal{F}(b)$ where


$$\mathcal{F}(b)a=\int\_{\mathbb{G}}ab.$$


What could be called the Van Daele convolution theorem says:


$$\mathcal{F}(a\star\_A b)=\mathcal{F}(a)\star \mathcal{F(b)}.$$


I am wondering 



> 
> 1. Is there any connection between projections in the algebra $(A,\star\_A)$ and group-like elements in the coalgebra $(A,\Delta)$:
> 
> 
> $$p\star\_A p=p\qquad\overset{?}{\sim}\qquad\Delta(p)=p\otimes p.$$
> 
> 
> 2. Also is there any connection between matrix units in the algebra $(A,\star\_A)$ and matrix elements in the coalgebra $(A,\Delta)$:
> 
> 
> $$e\_{ij}\star\_Ae\_{k\ell}=\delta\_{jk}e\_{i\ell}\qquad\overset{?}{\sim}\qquad\Delta(e\_{ij})=\sum\_ke\_{ik}\otimes e\_{kj}.$$
> 
> 
> 


A [question](https://mathoverflow.net/questions/181117/the-convolution-on-a-semisimple-finite-quantum-groupoid?rq=1) in a similar vein.


**Context**


I had thought that I had [found what I was looking for](https://mathoverflow.net/questions/220201/the-irreducible-representations-of-the-sekine-quantum-groups) but in fact I was mistaken.


I thought [Lemmas 4, 5, 8 and 9](http://www.ams.org/journals/proc/1996-124-04/S0002-9939-96-03199-1/S0002-9939-96-03199-1.pdf) were the matrix elements of the irreducible representations of the Sekine quantum group. 


Calculations using a 'spectral-type formula' --- involving a sum over irreducible representations --- were giving me nonsensical answers. Unable to find any error in the derivation of the 'spectral-type formula', and seeing the formula fail to give the correct answers, it dawned on me to check were those elements of the algebra of functions actually matrix elements of irreducible representations... no they were not. Specifically, for $k$ odd,


$$\varepsilon\left(e^{u,v}\_{11}\right)=2\qquad\text{and}\qquad h\left(\left(e\_{11}^{u,v}\right)^\*e\_{11}^{u,v}\right)=2,$$


when the first should be one and the second should be $\displaystyle \frac12$.


Looking at $k$ odd in particular --- Lemmas 4 and 5 --- I have a feeling that the $p\_{l,\pm}$ are indeed the matrix elements of the one dimensional representations.


I could try and adjust the functions in Lemma 5 according to:


$$\rho\_{ij}^{u,v}=\frac{c\_{ij}^{u,v}}{2}e\_{kk}^{u,v}$$


with $c\_{ij}^{u,v}\in\mathbb{C}$ --- equal to one for $i=j$ and of modulus one otherwise. This would clean up the issues with the counit and the Haar state but this cannot be correct. My 'spectral-type formula' is still failing in a simple case with this adjustment.


Rather than going back to the drawing board and essentially starting again on the last question, I was wondering is there a way to exploit the fact that in the convolution algebra, the $p\_{l,\pm}$ are projections and the $e\_{ij}^{u,v}$ are matrix units.



Quantum group representations from (convolution) matrix units?
As explained [here](https://mathoverflow.net/a/235111/35482), with help from Sébastien I was able to show that, in the [context of what I was working on](https://mathoverflow.net/questions/220201/the-irreducible-representations-of-the-sekine-quantum-groups), the one-dimensional minimal central projections satisfied


$$\Delta(p)=p\otimes p,$$


and the matrix units in the range of the two-dimensional central minimal projections satisfied


$$\Delta(E\_{i\ell})=\sum\_{k=1}^2\frac12 \left(E\_{ik}\otimes E\_{k\ell}\right).$$


There wasn't a problem with the 'spectral-type formula' when I made this 


$$\rho\_{ij}^{u,v}=\frac{1}{2}e\_{ij}^{u,v}$$


adjustment --- I incorrectly assumed that the random walk driven by $\nu=(\delta^{(0,1)}+\delta^{(1,0)})/2$ on $\mathbb{Z}\_n\times\mathbb{Z}\_n$ was ergodic --- it is not.


