\begin{center}
\begin{forest}
for tree={circle, draw, no edge,
            minimum size=2em,
            inner sep=0pt,
            s sep=2mm,
            l sep=2mm}
[$f$, rectangle, draw,  tikz={\draw [line] () to (a.north);}
    [$a$, name=a, tikz={\draw [line] () to (c1.north east); \draw [dot=.5] () to (b1.north west);}
        [$c$, name=c1, tikz={\draw [line] () to (250:3); \draw [fulldot=.5] () to (265:3);}
        ]
        [$b$, name=b1, tikz={\draw [line] () to (270:3); \draw [fulldot=.5] () to (285:3);}
        ]
    ]
]
\end{forest}
\end{center}