Laguerre’s theorem regarding the number of nonreal zeros of a polynomial and its
image under certain linear operators is generalized. This generalization is
then used to (1) exhibit a number of previously undiscovered complex zero
decreasing sequences for the Jacobi, ultraspherical, Legendre, Chebyshev, and
generalized Laguerre polynomial bases and (2) simultaneously generate a basis
and a corresponding complex zero decreasing sequence for the basis
.
An extension to transcendental entire functions in the Laguerre–Pólya
class is given, which, in turn, gives a new and short proof of a previously
known result due to Piotrowski. The paper concludes with several open
questions.
Keywords
complex zero decreasing sequences, diagonalizable linear
operators, zeros of polynomials, orthogonal polynomials