On extremal problems related to the Krzyż conjecture

Abstract: We study general extremal problems similar or closely related to the Krzyż conjecture and establish the structural properties of extremal functions, pointing out some differences with the original Krzyż case. We also introduce a family of more specific problems of this kind, interpolating between the Krzyż problem and the problem of maximizing the constant term, and show that they are closely related to the Krzyż conjecture: the conjecture implies the analogous statement for all of them and, conversely, if one of them has the same extremal value as the Krzyż problem, then this value is $2/e$. This may represent a new approach to attacking the original problem. Finally, we show that for these problems an extremal function whose Taylor coefficients $a_1,\ldots,a_{\lfloor n/3\rfloor}$ vanish must be the conjectured one; as a special case, we obtain a new proof of a recent result of MacDonald.