Extensionx values determine frequencies; y values determine amplitudes. Stable ranges for a & b tend to be between -3 to + 3. c & d between 0.5 and 1.5. There are combinations within these ranges that are unstable, so be prepared to tweak this oscillator.
| minfreq, maxfreq | 
 iteration frequency in Hertz  | 
| a, b, c, d | 
 equation variables  | 
| x0 | 
 initial value of x  | 
| y0 | 
 initial value of y  |