The orthogonality properties of certain subspaces associated with bivariate
Bernstein-Szeg\H{o} measures are considered. It is shown that these spaces
satisfy more orthogonality relations than expected from the relations that
define them. The results are used to prove a Christoffel-Darboux like formula
for these measures.
We interpret the Rahman polynomials in terms of the Lie algebra $sl_3(C)$.
Using the parameters of the polynomials we define two Cartan subalgebras for
$sl_3(C)$, denoted $H$ and $\tilde{H}$. We display an antiautomorphism
$\dagger$ of $sl_3(C)$ that fixes each element of $H$ and each element of
$\tilde{H}$.
We construct a commutative algebra A_z, generated by d algebraically
independent q-difference operators acting on variables z_1, z_2,..., z_d, which
is diagonalized by the multivariable Askey-Wilson polynomials P_n(z) considered
by Gasper and Rahman [6]. Iterating Sears' transformation formula, we show that
the polynomials P_n(z) possess a certain duality between z and n.