S-curves in polynomial external fields

Abstract

Curves in the complex plane that satisfy the S-property were first introduced by Stahl and they were further studied by Gonchar and Rakhmanov in the 1980s. Rakhmanov recently showed the existence of curves with the S-property in a harmonic external field by means of a max–min variational problem in logarithmic potential theory. This is done in a fairly general setting, which however does not include the important special case of an external field ReV where V is a polynomial of degree ≥2. In this paper we give a detailed proof of the existence of a curve with the S-property in the external field ReV within the collection of all curves that connect two or more pre-assigned directions at infinity in which ReV→+∞. Our method of proof is very much based on the works of Rakhmanov on the max–min variational problem and of Martínez-Finkelshtein and Rakhmanov on critical measures.