Multivariate Kawtchouk polynomials as Birth and Death polynomials

Abstract: Multivariate Krawtchouk polynomials are constructed explicitly as Birth and Death polynomials, which have the nearest neighbour interactions. They form the complete set of eigenpolynomials of a birth and death process with the birth and death rates at population $x=(x_1,\ldots,x_n)$ are $B_j(x)=\bigl(N-\sum_{i=1}^nx_i\bigr)$ and $D_j(x)=p_i^{-1}x_j$, $0<p_j$, $j=1,\ldots,n$. The corresponding stationary distribution is the multinomial distribution with the probabilities ${\eta_i}$, $\eta_i= p_i/(1+\sum_{j=1}^np_j)$. The polynomials, depending on $n+1$ parameters (${p_i}$ and $N$), satisfy the difference equation with the coefficients $B_j(x)$ and $D_j(x)$ $j=1,\ldots,n$, which is the straightforward generalisation of the difference equation governing the single variable Krawtchouk polynomials. The polynomials are truncated $(n+1,2n+2)$ hypergeometric functions of Aomoto-Gelfand. The divariate Rahman polynomials are identified as the dual polynomials with a special parametrisation.