Butterfly Fractal

$\begin{figure}\begin{center}\BoxedEPSF{Butterfly_Fractal.epsf}\end{center}\end{figure}$

The Fractal-like curve generated by the 2-D function

$\begin{displaymath} f(x,y)={(x^2-y^2)\sin\left({x+y\over a}\right)\over x^2+y^2}. \end{displaymath}$