Ismail's structure conjecture for orthogonal polynomials and the Askey–Wilson operator

Let (Pn)n0(P_n)_{n\geq 0} be a monic orthogonal polynomial sequence (OPS), and let π\pi be a polynomial independent of nn. For the first assertion, assume that π\pi has degree at most 22 and that

π(x)DqPn(x)=(anx+bn)Pn(x)+cnPn1(x).\pi(x)\mathcal{D}_q P_n(x)=(a_nx+b_n)P_n(x)+c_nP_{n-1}(x).

Ismail's structure conjecture. Then (Pn)n0(P_n)_{n\geq 0} are continuous qq-Jacobi polynomials, Al-Salam–Chihara polynomials, or special or limiting cases of these families. The same conclusion should hold when π\pi has degree s+1s+1 and, for positive integers rr and ss,

π(x)DqPn(x)=k=rscn,kPn+k(x),\pi(x)\mathcal{D}_qP_n(x)=\sum_{k=-r}^{s}c_{n,k}P_{n+k}(x),

with π\pi independent of nn. The first part is attributed to M. E. H. Ismail; the second part is disproved by a counterexample to the displayed general relation.

Sources & referencesView supporting material

Primary source

D. Mbouna, “A structure relation for some specific orthogonal polynomials”, arXiv:2206.10308 (2022).

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