Lots of tiny bubbles.

Contents

Domain

The basic domain is the quadruple unit square,

$$\max{(|x|,|y|)} \le 1, \ z = x+iy $$

Colors

I could use the HSV colormap.

   hsv_bubbles(@(z) z)

But I prefer "periodic parula", the parula colormap concatenated with its reverse, [parula; flipud(parula)].

   bubbles(@(z) z)

Powers

z^2

   bubbles(@(z) z.^2)

z^3

   bubbles(@(z) z.^3)

z^9

   bubbles(@(z) z.^9)

1/z

   bubbles(@(z) 1./z)

sqrt(z)

A complex number has two square roots. One is in the right half plane.

   bubbles(@sqrt)

-sqrt(z)

And the other is in the left half plane.

   bubbles(@(z) -sqrt(z))

Trig functions

sin(z)

   bubbles(@sin)

cos(z)

   bubbles(@cos)

tan(z)

   bubbles(@tan)

cot(z)

   bubbles(@cot)

Exponentials

exp(z)

These polar angles are between -1 and +1 radian.

   bubbles(@exp)

exp(pi*z)/exp(pi)

These fill out the entire complex plane.

   bubbles(@(z) exp(pi*z)/exp(pi))

log(z)

   bubbles(@log)

Polynomials and rationals

z^3 - z

   bubbles(@(z) z.^3-z)

.5/(z^5-z/5)

   bubbles(@(z) .5./(z.^5-z/5))

Essential singularity.

exp(-1/(8 z^2)

   bubbles(@(z) exp(-1./(8*z.^2)))

Quiz

What is happening in the animation at the top of this post? If you think you know, or even if you just know part of the answer, submit a comment. I'll have some sort of prize for the first, or best, solution.

Code

Also available at https://blogs.mathworks.com/cleve/files/bubbles.m.

   type bubbles

function bubbles(F)
    % bubbles. Color portrait of complex-valued function F(z),
    % ex. bubbles(@sin)
    %     bubbles(@(z) .5./(z.^5-z/5) )
    
    if nargin < 1
       F = @(z)z;
    end
    
    axis(1.5*[-1 1 -1 1])
    axis square
    box on
    cla
    
    m = 256;
    colormap = [parula(m);flipud(parula(m))]; 
        
    n = 25;
    s = -1:2/(n-1):1;
    [x,y] = meshgrid(s);
    z = x + y*1i;
    circle = exp((0:32)/16*pi*1i)/n;
    
    w = F(z);
    r = abs(w);
    theta = angle(w)+pi;
    
    scale = 20;
    for k = 1:n
        for j = 1:n
            t = m*theta(k,j)/pi/scale;
            idx = ceil(scale*t+realmin);
            color = colormap(idx,:);
            p = w(k,j) + r(k,j)*circle;
            patch(real(p),imag(p),color)
        end
    end
 
    titleF = char(F);
    if (titleF(1) == '@')
        titleF(1:4) = [];
    end
    title(titleF)
    snapnow
end

Get the MATLAB code

Published with MATLAB® R2019b