File:Basilica Julia set with spine.svg

 /*  
batch file for Maxima CAS

maxima
batch("b.mac")



 */ 

start:elapsed_run_time ();

kill(all);
remvalue(all);

 /* --------------------------definitions of functions ------------------------------*/
 f(z,c):=z*z+c; /* Complex quadratic map */
 finverseplus(z,c):=sqrt(z-c)$
 finverseminus(z,c):=-sqrt(z-c)$ 

/* */
fn(p, z, c) :=
  if p=0 then z
  elseif p=1 then f(z,c)
  else f(fn(p-1, z, c),c)$

/*Standard polynomial F_p \, which roots are periodic z-points of period p and its divisors */
F(p, z, c) := fn(p, z, c) - z $

/* Function for computing reduced polynomial G_p\, which roots are periodic z-points of period p without its divisors*/
G[p,z,c]:=
block(
[f:divisors(p),
t:1], /* t is temporary variable = product of Gn for (divisors of p) other than p */
f:delete(p,f), /* delete p from list of divisors */
if p=1
then return(F(p,z,c)),
for i in f do 
 t:t*G[i,z,c],
g: F(p,z,c)/t,
return(ratsimp(g))
)$

GiveRoots(g):=
 block(
 [cc],
 cc:bfallroots(expand(%i*g)=0),
 cc:map(rhs,cc),/* remove string "c=" */
 cc:map('float,cc),
 return(cc)
  )$ 




/* endcons the complex point to list in the format for draw package */ 
endconsD(point,list):=endcons([realpart(point),imagpart(point)],list)$
consD(point,list):=cons([realpart(point),imagpart(point)],list)$

GiveForwardOrbit(z0,c,iMax):=
   /* 
   computes (without escape test)
   forward orbit of point z0
   and saves it to the list for draw package */
block(
 [z,orbit,temp],
 z:z0, /* first point = critical point z:0+0*%i */
 orbit:[[realpart(z),imagpart(z)]], 
 for i:1 thru iMax step 1 do
        ( z:expand(f(z,c)),
          orbit:endcons([realpart(z),imagpart(z)],orbit)),
         
 return(orbit) 
)$



 /* Gives points of backward orbit of z=repellor       */
 GiveBackwardOrbit(c,repellor,zxMin,zxMax,zyMin,zyMax,iXmax,iYmax):=
  block(
   hit_limit:4, /* proportional to number of details and time of drawing */
   PixelWidth:(zxMax-zxMin)/iXmax,
   PixelHeight:(zyMax-zyMin)/iYmax,
   /* 2D array of hits pixels . Hit > 0 means that point was in orbit */
   array(Hits,fixnum,iXmax,iYmax), /* no hits for beginning */
  /* choose repeller z=repellor as a starting point */
  stack:[repellor], /*save repellor in stack */
  /* save first point to list of pixels  */ 
  x_y:[repellor], 
 /* reversed iteration of repellor */
  loop,
  /* pop = take one point from the stack */
  z:last(stack),
  stack:delete(z,stack),
  /*inverse iteration - first preimage (root) */
  z:finverseplus(z,c),
  /* translate from world to screen coordinate */
  iX:fix((realpart(z)-zxMin)/PixelWidth),
  iY:fix((imagpart(z)-zyMin)/PixelHeight),
  hit:Hits[iX,iY],
  if hit<hit_limit   
   then 
    (
    Hits[iX,iY]:hit+1,
    stack:endcons(z,stack), /* push = add z at the end of list stack */
    if hit=0 then x_y:endcons( z,x_y)
    ),
  /*inverse iteration - second preimage (root) */
  z:-z,
 /* translate from world to screen coordinate, coversion to integer */
  iX:fix((realpart(z)-zxMin)/PixelWidth),
  iY:fix((imagpart(z)-zyMin)/PixelHeight),
  hit:Hits[iX,iY],
  if hit<hit_limit   
   then 
    (
     Hits[iX,iY]:hit+1,
     stack:endcons(z,stack), /* push = add z at the end of list stack to continue iteration */
     if hit=0 then x_y:endcons( z,x_y)
    ),
   if is(not emptyp(stack)) then go(loop), 
 return(x_y) /* list of pixels in the form [z1,z2] */
 )$

 
 
 /*-----------------------------------*/ 
 Psi_n(r,t,z_last, Max_R):=
 /*   */
 block(
  [iMax:200,
  iMax2:0],
  /* -----  forward iteration of 2 points : z_last and w --------------*/
  array(forward,iMax-1), /* forward orbit of z_last for comparison */
  forward[0]:z_last,
  i:0,
  while cabs(forward[i])<Max_R  and  i< ( iMax-2) do
  (     
  /* forward iteration of z in fc plane & save it to forward array */
  forward[i+1]:forward[i]*forward[i] + c, /* z*z+c */
  /* forward iteration of w in f0 plane :  w(n+1):=wn^2 */
  r:r*2, /* square radius = R^2=2^(2*r) because R=2^r */
  t:mod(2*t,1),
  /* */
  iMax2:iMax2+1,
  i:i+1
  ),
  /* compute last w point ; it is equal to z-point */
  R:2^r,
  /* w:R*exp(2*%pi*%i*t),       z:w, */
  array(backward,iMax-1),
  backward[iMax2]:rectform(ev(R*exp(2*%pi*%i*t))), /* use last w as a starting point for backward iteration to new z */
  /* -----  backward iteration point  z=w in fc plane --------------*/
  for i:iMax2 step -1 thru 1 do
  (
  temp:float(rectform(sqrt(backward[i]-c))), /* sqrt(z-c) */
  scalar_product:realpart(temp)*realpart(forward[i-1])+imagpart(temp)*imagpart(forward[i-1]),
  if (0>scalar_product) then temp:-temp, /* choose preimage */
  backward[i-1]:temp
  ),
  return(backward[0])
 )$
 
 
 GiveRay(t,c):=
 block(
  [r],
  /* range for drawing  R=2^r ; as r tends to 0 R tends to 1 */
  rMin:1E-5, /* 1E-4;  rMin > 0  ; if rMin=0 then program has infinity loop !!!!! */
  rMax:2, 
  caution:0.9330329915368074, /* r:r*caution ; it gives smaller r */
  /* upper limit for iteration */
  R_max:300,
  /* */
  zz:[], /* array for z points of ray in fc plane */
  /*  some w-points of external ray in f0 plane  */
  r:rMax,
  while 2^r<R_max do r:2*r, /* find point w on ray near infinity (R>=R_max) in f0 plane */
  R:2^r,
  w:rectform(ev(R*exp(2*%pi*%i*t))),
  z:w, /* near infinity z=w */
  zz:cons(z,zz),
  unless r<rMin do
  (     /* new smaller R */
  r:r*caution,  
  R:2^r,
  /* */
  w:rectform(ev(R*exp(2*%pi*%i*t))),
  /* */
  last_z:z,
  z:Psi_n(r,t,last_z,R_max), /* z=Psi_n(w) */
  zz:cons(z,zz)
  ),
  return(zz)
 )$





  


/* 
converts complex number z = x*y*%i 
to the list in a draw format:  
[x,y] 
*/
d(z):=[float(realpart(z)), float(imagpart(z))]$

ToPoints(myList):= points(map(d,myList))$


/* give Draw List from one point*/
ToPoint(z):=points([d(z)])$


GiveSpine(zc, Alfa,c, iMax):=block(
  	[s, center, cut], 
  	
  	/* first center of component, only one !!! */
  	center: zc, 
  	s:[center],
  	
  	/* first pair of cut points  */
  	s:   cons(  Alfa,s),
  	cut : -Alfa, 
  	s:endcons( cut,s),
  	
  	for i: 1  thru iMax step 1 do (
  	
  	/* pair of component's centers  */
  	center: finverseminus(center,c), 
  	s: cons(center, s),
  	center : - center,
  	s: endcons (center, s),
  	
  	/* pair of cut points  */
  	cut : finverseminus(cut,c),  
  	s:   cons(  cut,s),
  	cut : - cut,
  	s:endcons( cut,s)
  	
  	),
  	
  	
  	
  	
  	
  	/* convert to draw format and return list */  
  	s: ToPoints(s) 
  
  
  )$
  
  
  





compile(all)$

 /* ----------------------- main ----------------------------------------------------*/


period:3$

  

 /* external angle in turns */
 /* resolution is proportional to number of details and time of drawing */
 iX_max:1000$
 iY_max:1000$
 /* define z-plane ( dynamical ) */
 ZxMin:-2.0$
 ZxMax:2.0$
 ZyMin:-2.0$
 ZyMax:2.0$

 

/* give c a value */
 c:-1$ /* center of period 2 component */

 

 /* compute fixed points */
 Beta:float(rectform((1+sqrt(1-4*c))/2))$ /* compute repelling fixed point beta */
 alfa:float(rectform((1-sqrt(1-4*c))/2))$ /* other fixed point */

 /* compute backward orbit of repelling fixed point */
 xy: GiveBackwardOrbit(c,Beta,ZxMin,ZxMax,ZyMin,ZyMax,iX_max,iY_max)$ 


  /* compute ray points & save to zz list */
 eRayZero:GiveRay(0/1,c)$
 
 eRay1o2:GiveRay(1/2, c)$
   

 spine: GiveSpine(0, alfa,c, 4)$

 /* time of computations */
 time:fix(elapsed_run_time ()-start)$

 /* draw it using draw package by */
 
 
 
 load(draw)$ 

 path:"~/maxima/batch/julia/spine/basilica/"$ /*  if empty then file is in a home dir */

 /* if graphic  file is empty (= 0 bytes) then run draw2d command again */
 
 draw2d(
  terminal  = 'svg,
  file_name = sconcat(path,"BasilicaSpine"),
  user_preamble="set size square;set key top right",
  title= concat("Dynamical plane for fc(z)=z*z",string(c)),
  dimensions = [iX_max, iY_max],
  yrange = [ZyMin,ZyMax],
  xrange = [ZxMin,ZyMax],
  xlabel     = "Z.re ",
  ylabel     = "Z.im",
  point_type = filled_circle,
  points_joined =true,
  point_size    =  0.2,
  color         = red,
    
  
  
  points_joined =false,
  color         = black,
  key = "backward orbit of z=beta",
  ToPoints(xy),
  
  
  
 
  points_joined =true,
  point_size    =  0.2,
  color         = red,
  key = "external ray 0",
  ToPoints(eRayZero),
  
  
  key = "external ray 1/2",
  color = magenta,
  ToPoints(eRay1o2),
  
  
  
  
  points_joined =true,
  point_size    =  0.8,
  color         = gray,
  key = "spine",
  spine, 
  
  
  

  points_joined =false,
  
  color         = black,
  point_size    =  1.4,
  key = "critical point z = 0.0",
  ToPoint(0.0),
  
  
  color         = red,
  point_size    =  1.4,
  key = "repelling fixed point z= beta",
  ToPoint(Beta),
  
  color = magenta,
  key = "minus beta",
  ToPoint(-Beta),
  
  
  color         = yellow,
  key = "parabolic fixed point z= alfa",
  ToPoint(alfa)
 
 )$