On harmonic univalent functions defined by a generalized Ruscheweyh derivatives operator

Let $\mathcal{S_H}$ denote the class of functions $f=h+\overline g$ which are harmonic univalent and sense preserving in the unit disk $\mathbf U$. Al-Shaqsi and Darus [7] introduced a generalized Ruscheweyh derivatives operator denoted by $D^n_\lambda$ where $D^n_\lambda f(z)=z+\sum\limits_{k=2}^\infty[1+\lambda(k-1)]C(n,k)a_kz^k$, where $C(n,k)={{k + n-1}\choose n}$. The authors, using this operators, introduce the class $\mathcal H^n_\lambda$ of functions which are harmonic in $\mathbf U$. Coefficient bounds, distortion bounds and extreme points are obtained.