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Nak Eun Cho

Publications and source records attributed to Nak Eun Cho.

4 recordsLinked to original sources

Sufficient conditions for strong starlikeness

Let $p$ be an analytic function defined on the open unit disc $\mathbb{D}$ with $p(0)=1$ and $0< α\leq 1$. The conditions on complex valued functions $C$, $D$ and $E$ are obtained for $p$ to be subordinate to $((1+z)/(1-z))^α$ when $C(z) z^{2}p''(z)+D(z)zp'(z) + E(z)p(z)=0$. Sufficient conditions for confluent (Kummer) hypergeometric function and generalized and normalized Bessel function of the first kind of complex order to be subordinate to $((1+z)/(1-z))^α$ are obtained as applications. The conditions on $α$ and $β$ are derived for $p$ to be subordinate to $((1+z)/(1-z))^α$ when $1+βzp'(z)/p^{n}(z)$ with $n=1,2$ is subordinate to $1+4z/3+2z^{2}/3=:φ_{CAR}(z)$. Similar problems were investigated for $\RE p(z)>0$ when the functions $p(z)+βzp'(z)/p^{n}(z)$ with $n=0,2$ is subordinate to $φ_{CAR}(z)$. The condition on $β$ is determined for $p$ to be subordinate to $((1+z)/(1-z))^α$ when $p(z)+βzp'(z)/p^{n}(z)$ with $n=0,1,2$ is subordinate to $((1+z)/(1-z))^α$.

math.CV

Sufficient Conditions for Starlike Functions Associated with the Lemniscate of Bernoulli

Let -1\leq B<A\leq 1. Condition on β, is determined so that 1+βzp'(z)/p^k(z)\prec(1+Az)/(1+Bz)\;(-1<k\leq3) implies p(z)\prec \sqrt{1+z}. Similarly, condition on βis determined so that 1+βzp'(z)/p^n(z) or p(z)+βzp'(z)/p^n(z)\prec\sqrt{1+z}\;(n=0, 1, 2) implies p(z)\prec(1+Az)/(1+Bz) or \sqrt{1+z}. In addition to that condition on βis derived so that p(z)\prec(1+Az)/(1+Bz) when p(z)+βzp'(z)/p(z)\prec\sqrt{1+z}. Few more problems of the similar flavor are also considered.

math.CV

A first-order differential double subordination with applications

Let $q_1$ and $q_2$ belong to a certain class of normalized analytic univalent functions in the open unit disk of the complex plane. Sufficient conditions are obtained for normalized analytic functions $p$ to satisfy the double subordination chain $q_1(z)\prec p(z)\prec q_2(z)$. The differential sandwich-type result obtained is applied to normalized univalent functions and to $Φ$-like functions.

math.CV