Muckenhoupt inequality with three measures and applications to Sobolev   orthogonal polynomials
  We generalize the classical Muckenhoupt inequality with two measures to three under appropriate conditions. As a consequence, we prove a simple characterization of the undedness of the multiplication operator and thus of the boundedness of the zeros and the asymptotic behavior of the Sobolev orthogonal polynomials, for a large class of measures which includes the most usual examples in the literature. 
E. Colorado, D. Pestana, J. M. Rodriguez, and E. Romera
math.FA