Some matrices associated with the split decomposition for a Q-polynomial distance-regular graph
Abstract
Description
We consider a $Q$-polynomial distance-regular graph $Γ$ with vertex set $X$ and diameter $D \geq 3$. For $μ, ν\in \lbrace \downarrow, \uparrow \rbrace$ we define a direct sum decomposition of the standard module $V=\C X$, called the $(μ,ν)$--split decomposition. For this decomposition we compute the complex conjugate and transpose of the associated primitive idempotents. Now fix $b,β\in \mathbb C$ such that $b \neq 1$ and assume $Γ$ has classical parameters $(D,b,α,β)$ with $α= b-1$. Under this assumption Ito and Terwilliger displayed an action of the $q$-tetrahedron algebra $\boxtimes_q$ on the standard module of $Γ$. To describe this action they defined eight matrices in
$\hbox{Mat}_X(\mathbb C)$, called \begin{eqnarray*} \label{eq:list} A,\quad A^*,\quad B,\quad B^*, \quad K,\quad K^*,\quad Φ,\quad Ψ. \end{eqnarray*}
For each matrix in the above list we compute the transpose and complex conjugate. Using this information we compute the transpose and complex conjugate for each generator of $\boxtimes_q$ on $V$.