Faculty of Mechanical Engineering

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    Certain properties of the class of univalent functions with real coefficients
    (2021-12-29)
    Milutin Obradović
    ;
    Nikola Tuneski
    Let ${\mathcal U}^+$ be the class of analytic functions $f$ such that $\frac{z}{f(z)}$ has real and positive coefficients and $f^{-1}$ be its inverse. In this paper we give sharp estimates of the initial coefficients and initial logarithmic coefficients for $f$, as well as, sharp estimates of the second and the third Hankel determinant for $f$ and $f^{-1}$. We also show that the Zalcman conjecture holds for functions $f$ from ${\mathcal U}^+$.
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    Some properties of the class $\mathcal{U}$
    (2018-12-20)
    Milutin Obradovic
    ;
    Nikola Tuneski
    In this paper we study the class $\mathcal{U}$ of functions that are analytic in the open unit disk ${\mathbb D}=\{z:|z|<1\}$, normalized such that $f(0)=f'(0)-1=0$ and satisfy \[\left|\left [\frac{z}{f(z)} \right]^{2}f'(z)-1 \right|<1\quad\quad (z\in {\mathbb D}).\] For functions in the class $\mathcal{U}$ we give sharp estimate of the second ant the third Hankel determinant, its relationship with the class of $\alpha$-convex functions, as well as certain starlike properties.
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    Hankel determinant for a class of analytic functions
    (2019-03-19)
    Milutin Obradovic
    ;
    Nikola Tuneski
    Let $f$ be analutic in the unit disk $\mathbb D$ and normalized so that $f(z)=z+a_2z^2+a_3z^3+\cdots$. In this paper we give sharp bound of Hankel determinant of the second order for the class of analytic unctions satisfying \[ \left|\arg \left[\left(\frac{z}{f(z)}\right)^{1+\alpha}f'(z) \right] \right|<\gamma\frac{\pi}{2} \quad\quad (z\in\mathbb D),\] for $0<\alpha<1$ and $0<\gamma\leq1$.
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    Hankel determinant of second order for some classes of analytic functions
    (2019-03-19)
    Milutin Obradovic
    ;
    Nikola Tuneski
    Let $f$ be analytic in the unit disk $\mathbb D$ and normalized so that $f(z)=z+a_2z^2+a_3z^3+\cdots$. In this paper, we give upper bounds of the Hankel determinant of second order for the classes of starlike functions of order $\alpha$, Ozaki close-to-convex functions and two other classes of analytic functions. Some of the estimates are sharp.