A Non-standard Class of Sobolev Orthogonal Polynomials

When τ is a quasi-definite moment functional onP , the vector space of all real polynomials, we consider a symmetric bilinear form φ(·, ·) on P ×P defined by φ(p, q) = λp(a)q(a)+ μp(b)q(b)+ 〈τ, p′q ′〉, where λ,μ, a, and b are real numbers. We first find a necessary and sufficient condition for φ(·, ·) to be quasi-definite. When τ is a semi-classical moment functional, we discuss algebraic properties of the orthogonal polynomials relative to φ(·, ·) and show that such orthogonal polynomials satisfy a fifth order differential equation with polynomial coefficients.