Kernel identities for van Diejen's $q$-difference operators and   transformation formulas for multiple basic hypergeometric series
  In this paper, we show that the kernel function of Cauchy type for type $BC$ intertwines the commuting family of van Diejen's $q$-difference operators. This result gives rise to a transformation formula for certain multiple basic hypergeometric series of type $BC$. We also construct a new infinite family of commuting $q$-difference operators for which the Koornwinder polynomials are joint eigenfunctions. 
Yasuho Masuda
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