On the zeros of asymptotically extremal polynomial sequences in the plane

Abstract

Let E be a compact set of positive logarithmic capacity in the complex plane and let {Pn(z)}1∞ be a sequence of asymptotically extremal monic polynomials for E in the sense that lim supn→∞‖Pn‖E1/n≤cap(E). The purpose of this note is to provide sufficient geometric conditions on E under which the (full) sequence of normalized counting measures of the zeros of {Pn} converges in the weak-star topology to the equilibrium measure on E, as n→∞. Utilizing an argument of Gardiner and Pommerenke dealing with the balayage of measures, we show that this is true, for example, if the interior of the polynomial convex hull of E has a single component and the boundary of this component has an “inward corner” (more generally, a “non-convex singularity”). This simple fact has thus far not been sufficiently emphasized in the literature. As applications we mention improvements of some known results on the distribution of zeros of some special polynomial sequences.