1 results
INTEGRAL MEANS AND DIRICHLET INTEGRAL FOR CERTAIN CLASSES OF ANALYTIC FUNCTIONS
- Part of
-
- Journal:
- Journal of the Australian Mathematical Society / Volume 99 / Issue 3 / December 2015
- Published online by Cambridge University Press:
- 16 July 2015, pp. 315-333
- Print publication:
- December 2015
-
- Article
-
- You have access
- Export citation
-
For a normalized analytic function
$f(z)=z+\sum _{n=2}^{\infty }a_{n}z^{n}$ in the unit disk 
$\mathbb{D}:=\{z\in \mathbb{C}:|z|<1\}$, the estimate of the integral means is an important quantity for certain problems in fluid dynamics, especially when the functions
$$\begin{eqnarray}L_{1}(r,f):=\frac{r^{2}}{2{\it\pi}}\int _{-{\it\pi}}^{{\it\pi}}\frac{d{\it\theta}}{|f(re^{i{\it\theta}})|^{2}}\end{eqnarray}$$
$f(z)$ are nonvanishing in the punctured unit disk 
$\mathbb{D}\setminus \{0\}$. Let 
${\rm\Delta}(r,f)$ denote the area of the image of the subdisk 
$\mathbb{D}_{r}:=\{z\in \mathbb{C}:|z|<r\}$ under 
$f$, where 
$0<r\leq 1$. In this paper, we solve two extremal problems of finding the maximum value of 
$L_{1}(r,f)$ and 
${\rm\Delta}(r,z/f)$ as a function of 
$r$ when 
$f$ belongs to the class of 
$m$-fold symmetric starlike functions of complex order defined by a subordination relation. One of the particular cases of the latter problem includes the solution to a conjecture of Yamashita, which was settled recently by Obradović et al. [‘A proof of Yamashita’s conjecture on area integral’, Comput. Methods Funct. Theory13 (2013), 479–492].
