Diagram Panah

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows}
\definecolor{biru}{RGB}{56,94,141}
\usepackage{gentium}
\begin{document}
\begin{tikzpicture}[scale=.5]
\draw[very thick,biru]
(0,0) circle [x radius=1.5cm, y radius=5cm]
(6,0) circle [x radius=1.5cm, y radius=5cm];
\node (a1) at (0,3)  {\color{biru}$a_1$};
\node (a2) at (0,1)   {\color{biru}$a_2$};
\node (a3) at (0,-1)  {\color{biru}$a_3$};
\node (a4) at (0,-3)  {\color{biru}$a_4$};
\node (b1) at (6,2)  {\color{biru}$b_1$};
\node (b2) at (6,0)  {\color{biru}$b_2$};
\node (b3) at (6,-2) {\color{biru}$b_3$};
\draw[thick,-stealth,biru] (a1.east) -- (b1);
\draw[thick,-stealth,biru] (a2.east) -- (b1);
\draw[thick,-stealth,biru] (a3.east) -- (b3);
\draw[thick,-stealth,biru] (a4.east) -- (b2);
\node[font=\color{biru}\large\bfseries] at (0,5.5) {A};
\node[font=\color{biru}\large\bfseries] at (6,5.5) {B};
\node[font=\color{biru}\large\bfseries] at (3,3.5) {$f$};
\end{tikzpicture}
\end{document}