Comptes Rendus. Mathématique (Jan 2021)
On the Fekete–Szegö type functionals for functions which are convex in the direction of the imaginary axis
Abstract
In this paper we consider two functionals of the Fekete–Szegö type: $\Phi _f(\mu ) = a_2 a_4-\mu a_3{}^2$ and $\Theta _f(\mu ) = a_4-\mu a_2a_3$ for analytic functions $f(z) = z+a_2z^2+a_3z^3+\ldots $, $z\in \Delta $, ($\Delta = \lbrace z\in \mathbb{C}:|z|<1\rbrace $) and for real numbers $\mu $. For $f$ which is univalent and convex in the direction of the imaginary axis, we find sharp bounds of the functionals $\Phi _f(\mu )$ and $\Theta _f(\mu )$. It is possible to transfer the results onto the class $\mathcal{K}_{\mathbb{R}}(i)$ of functions convex in the direction of the imaginary axis with real coefficients as well as onto the class $\mathcal{T}$ of typically real functions. As corollaries, we obtain bounds of the second Hankel determinant in $\mathcal{K}_{\mathbb{R}}(i)$ and $\mathcal{T}$.