On a Problem of Leont'ev and Representing Systems of Exponentials

We study whether an entire function of exponential type has totally regular growth if its derivative increases sufficiently fast on the zero set of the function itself. In particular, for a function with a trigonometrically convex (or positive) lower indicator, we obtain a solution of a well-known problem of Leont'ev. As an application, we refine some already known results concerning the characterization of exponents of the representing systems of exponentials.