On the sine polynomials of Fejér and Lukács

Abstract

The sine polynomials of Fejér and Lukács are defined by

$$\begin{aligned} F_n(x)=\sum _{k=1}^n\frac{\sin (kx)}{k} \quad \text{ and } \quad L_n(x)=\sum _{k=1}^n (n-k+1)\sin (kx), \end{aligned}$$

respectively. We prove that for all $n\ge 2$ and $x\in (0,\pi )$, we have

$$\begin{aligned} F_n(x)\le \lambda \, L_n(x) \quad \text{ and } \quad \mu \le \frac{1}{F_n(x)}-\frac{1}{L_n(x)} \end{aligned}$$

with the best possible constants

$$\begin{aligned} \lambda = \frac{8-3\sqrt{2}}{12(2-\sqrt{2})} \quad \text{ and } \quad \mu =\frac{2}{9}\sqrt{3}. \end{aligned}$$

An application of the first inequality leads to a class of absolutely monotonic functions involving the arctan function.