We saw that, depending on the parameter, the Mandelbrot law can either "explode" when iterated or reach a steady state. It would be interesting to see for which parameters an "explosion" (a.k.a. a "divergence") happens. However, the parameter can be a complex number, so we need two dimensions to describe all possible parameters (e.g. x axis for the real component of the number and y axis for the imaginary component of the number). Then in color we can mark whether (or when) the iterations of the Mandelbrot law diverge. Such a type of 2D maps is called "heat maps", similar to the ones used in weather forecasts.


Side note: The colors attributed to different values are arbitrary. Above we picked blue for cold and red for hot, but we could have picked anything we want. The particular choice is called "the color map" or "the color scheme" of the plot (it has nothing to do with geographical maps). Later today we will see how to use custom color maps.

Below we give you the code that can be used to plot the heat map of "divergence of the Mandelbrot law versus parameter used in its evaluation". Let us play with it before trying to explain exactly how it works.

In [1]:
using Plots

function mandelbrot_law(current_state, parameter)
    next_state = current_state^2+parameter 
    return next_state

function mandelbrot_when_explodes(parameter, length) 
    current_state = 0
    for i in 2:length
        new_state = mandelbrot_law(current_state, parameter)
        if abs(new_state) > 20
            return i
            current_state = new_state
    return length

function get_grid_of_coordinates(rmin,rmax,imin,imax,step)
    real_coordinate = rmin:step:rmax
    imag_coordinate = imin:step:imax
    grid_of_complex_coordinates = zeros(ComplexF64, length(imag_coordinate), length(real_coordinate))

    for i_real in 1:length(real_coordinate)
        for i_imag in 1:length(imag_coordinate)
            grid_of_complex_coordinates[i_imag,i_real] = real_coordinate[i_real] + 1im * imag_coordinate[i_imag]
    return grid_of_complex_coordinates

function plot_mandelbrot(rmin,rmax,imin,imax,step,length)
    grid_of_complex_coordinates = get_grid_of_coordinates(rmin,rmax,imin,imax,step)
    heatmap(rmin:step:rmax,imin:step:imax, mandelbrot_when_explodes.(grid_of_complex_coordinates, length), ratio=1)