# Bled'11 - 7th Slovenian International Conference on Graph Theory

19-25 June 2011
Bled, Slovenia
Europe/Ljubljana timezone
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# A characterization of Leonard pairs using the parameters $\{a_{i}\}_{i=0}^{d}$

Presented by Edward HANSON
Type: Oral presentation
Track: Representations of Graphs

## Content

Let $V$ denote a vector space with finite positive dimension. We consider an ordered pair of linear transformations $A: V\rightarrow V$ and $A^*: V\rightarrow V$ that satisfy (i) and (ii) below: \begin{enumerate} \item There exists a basis for $V$ with respect to which the matrix representing $A$ is irreducible tridiagonal and the matrix representing $A^*$ is diagonal. \item There exists a basis for $V$ with respect to which the matrix representing $A^*$ is irreducible tridiagonal and the matrix representing $A$ is diagonal. \end{enumerate} We call such a pair a {\it Leonard pair} on $V$. Arlene Pascasio recently obtained a characterization of the $Q$-polynomial distance-regular graphs using the intersection numbers $a_{i}$. In this talk, we extend her results to a linear algebraic level and obtain a characterization of Leonard pairs. Pascasio's argument appears to rely on the underlying combinatorial assumptions, so we take a different approach that is algebraic in nature.

## Place

Location: Bled, Slovenia
Address: Best Western Hotel Kompas Bled

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