Extrema for gap power series of positive real part

G. Pólya and I. J. Schoenberg, Remarks on de la Vallée Poussin means and convex conformal maps of the circle,Pacific Journal of Mathemtics,8 (1958), 295–334. M. S. Robertson, Variational methods for funcions with positive real part,Transactions of American Mathematical Society,102 (1962), 82–93. I. J. Schoenberg, Extrema for gap power series of positive real part,Proceedings of the National Academy of Sciences,48 (1962), 1151–1154.