Sufficiency for Nephroid Starlikeness using Hypergeometric Functions

Abstract: Let $\mathcal{A}$ consists of analytic functions $f:\mathbb{D}\to\mathbb{C}$ satisfying $f(0)=f'(0)-1=0$. Let $\mathcal{S}^*_{Ne}$ be the recently introduced Ma-Minda type functions family associated with the $2$-cusped kidney-shaped {\it nephroid} curve $\left((u-1)^2+v^2-\frac{4}{9}\right)^3-\frac{4 v^2}{3}=0$ given by \begin{align*} \mathcal{S}^*_{Ne}:= \left{f\in\mathcal{A}:\frac{zf'(z)}{f(z)}\prec\varphi_{\scriptscriptstyle {Ne}}(z)=1+z-z^3/3\right}. \end{align*} In this paper, we adopt a novel technique that uses the geometric properties of {\it hypergeometric functions} to determine sharp estimates on $\beta$ so that each of the differential subordinations \begin{align*} p(z)+\beta zp'(z)\prec \begin{cases} \sqrt{1+z}; 1+z; e^z; \end{cases} \end{align*} imply $p(z)\prec\varphi_{\scriptscriptstyle{Ne}}(z)$, where $p(z)$ is analytic satisfying $p(0)=1$. As applications, we establish conditions that are sufficient to deduce that $f\in\mathcal{A}$ is a member of $\mathcal{S}^*_{Ne}$.