The Poupard polynomials are polynomials in one variable with integer coefficients,
with some close relationship to Bernoulli and tangent numbers. They also have a
combinatorial interpretation. We prove that every Poupard polynomial has all its
roots on the unit circle. We also obtain the same property for another sequence of
polynomials introduced by Kreweras and related to Genocchi numbers. This is
obtained through a general statement about some linear operators acting on
palindromic polynomials.
Keywords
palindromic polynomial, unit circle, complex root, linear
operator, Bernoulli number