The oscillation of differential transforms – entire Jacobi expansions

Let L=(1−x2)D2−((β−α)−(α+β+2)x)D with , and . Let f∈C∞[−1,1], , with normalized Jacobi polynomials and the Cn decrease sufficiently fast. Set Lk=L(Lk−1), k⩾2. Let ρ>1. If the number of sign changes of (Lkf)(x) in (−1,1) is O(k1/(ρ+1)), then f extends to be an entire function of logarithmic order . For Legendre expansions, the result holds with replaced with .