Not sure what you are trying to do on Lines 32 & 33.. ?? 

and I think that your set function isn't going to work - check the reference for how to use it (and colors) in Processing.

thanks for the help. I think the main problem is to translate this math formula in a Processing Code:

Hi Jesse,

now I don't have any errors, but the code work in a strange manner, it does'nt still work correctly. Maybe are the lines 36&37, but I don't know:

float a,b,xn1,yn1,xn,yn,G1,G2,x,i;

float s,iterations;

color bla;

void setup() 

{

  size (800,800);

  frameRate(30);

  bla= color(0,150,15);

  a=0.02;

  b=0.9998;

  xn1=5;

  yn1=0;

  s=10;

  iterations=20000;

  i=0;

  G1= a * x + (2 * (((1 - a) * x*x) / (1 + x*x)));

  G2= a * (x+i) + (2 * (((1 - a) * ((x+i)*(x+i)) )/ (1 + ((x+i)*(x+i)))));

}

void draw() 

{   

  background(255); 

  fill(175,40);

  a = mouseY / 1000;

  xn1 = mouseX / 30;

  yn1 = 0;

  for (int i= 0; i<iterations; i++){

    xn = xn1;

    yn = yn1;

    xn1 = b * yn + G1;

    yn1 =  -xn + G2;

    set(int(280 + xn1 * s),int( 300 + yn1 * s), bla);

    //ellipse(int(280 + xn1 * s),int( 300 + yn1 * s),10,10);

  }

}