Über gewisse nichtfortsetzbare Potenzreihen. II.
We establish certain properties for the class of universal functions in with respect to the center , for certain types of connected non-simply connected domains . In the case where is discrete we prove that this class is -dense in , depends on the center and that the analog of Kahane’s conjecture does not hold.
A holomorphic function on a simply connected domain is said to possess a universal Taylor series about a point in if the partial sums of that series approximate arbitrary polynomials on arbitrary compacta outside (provided only that has connected complement). This paper shows that this property is not conformally invariant, and, in the case where is the unit disc, that such functions have extreme angular boundary behaviour.
We prove the existence of functions , the Fourier series of which being universally divergent on countable subsets of . The proof is based on a uniform estimate of the Taylor polynomials of Landau’s extremal functions on .