\begin{center}
\begin{forest}
for tree={circle, draw, no edge,
            minimum size=2em,
            inner sep=0pt,
            s sep=5mm,
            l sep=5mm}   
[$f$, rectangle, draw,  tikz={\draw [line] () to (a.north);}
    [$a$, name=a, tikz={\draw [line, bend right] () to (c.north west); \draw [fulldot=.5, bend left] () to (c.north east);}
        [$c$, name=c, tikz={\draw [line] () to (260:3.4); \draw [fulldot=.5] () to (280:3.4);}
        ]
    ]
]
\end{forest}
\end{center}