function [m]=mandelbrot(edge,x1,x2,y1,y2)
%[m]=mandelbrot(edge,x1,x2,y1,y2) is a function to check membership in the Mandelbrot set
%[m] is a 2 dimensional output array, m(edge,edge).  
%     Each element is 0 if the point is in the Mandelbrot set or n if it is not.
%     n=1 to 32 is the number of terms before element was proven not to belong.
% [edge] is the dimension of square array m
% [x1,x2] are min and max x values which are checked,(between -4 and 1)
% [y1,y2] are min and max y values which are checked, (between -2.25 and 2.25)
%    Note that the Student Edition of MATLAB is limited to a 127x127 array
%    Try calling '[m]=mandelbrot(100,-2,-1.9,.5,.6) or use mandelplot(edge,x1,x2,y1,y2)
%    MD Checkel   990103
maxloop=64;            %number of iterations to test whether sum of terms "blows up"
maxsize=9e5;           %threshold value at which sum is stopped.
if edge<1               
   edge=10;            %if array is dimensioned at 0, make it a 10x10 array 
end
m=zeros(edge);         %create array with zero's in it to confirm there is room
ystep=(y2-y1)/edge;    %calculate the step in x and y values to generate x and y arrays
xstep=(x2-x1)/edge;
iy=1;
for y=y1:ystep:y2       %loop at each y point to evaluate Mandelbrot sum for all x points.
   ix=1;
   for x=x1:xstep:x2    %loop to evaluate Mandelbrot sum for each x point 
      mx=x;             
      my=y;
      for i=1:maxloop   %loop to build up Mandelbrot sum 
         realnum=mx*mx;
         imagnum=my*my;
         if(realnum+imagnum)>maxsize       %check if Mandelbrot sum is over threshold
            break
         end
         my=(mx*my)+y;                  %increase value of each term by original x,y values
         mx=(realnum-imagnum)*.5 +x;
      end
      m(iy,ix)=maxloop-i;
      ix=ix+1;
   end
   iy=iy+1;
end