In Python: How to make a bifurcation diagram of the Lorenz system under a varying parameter value?

Question

So, I've seen the coded solution to my question in Mathematica, but with very little understanding of mathematica, I havn't been able to reproduce it yet.

This is what I'm trying to do with Python: https://mathematica.stackexchange.com/questions/159211/how-to-make-a-bifurcation-diagram-of-the-lorenz-system-under-a-varying-parameter

I'm thinking my errors are in understanding how to calculate what I'm looking for and how to adjust my visualization to make it look just like that in the link, but any ideas are welcome.

The code I have so far looks like this:

def lorenz_system(x,y,z,r,s=10,b=6):
    x_dot = s*(y-x)
    y_dot = r*x-y-x*z
    z_dot = x*z-b*z
    return x_dot, y_dot, z_dot

dr = 0.1              # parameter step size
r=np.arange(40,200,dr)  # parameter range
dt = 0.001             # time step
t = np.arange(0,10,dt) # time range

#initialize solution arrays
xs = np.empty(len(t) + 1)
ys = np.empty(len(t) + 1)
zs = np.empty(len(t) + 1)

#initial values x0,y0,z0 for the system
xs[0], ys[0], zs[0] = (1, 1, 1)

for R in r:
    for i in range(len(t)):
        #approximate numerical solutions to system
        x_dot, y_dot, z_dot = lorenz_system(xs[i], ys[i], zs[i],R)
        xs[i + 1] = xs[i] + (x_dot * dt)
        ys[i + 1] = ys[i] + (y_dot * dt)
        zs[i + 1] = zs[i] + (z_dot * dt)
    #calculate and plot the peak values of the z solution
    for i in range(0,len(zs)-1):
        #using only the positive values in the z solution
        if zs[i]>0:
            #find the local maxima
            if (zs[i-1] < zs[i] and zs[i] > zs[i+1]):
                if (zs[i]<=1000):
                    #plot the local maxima point of the z solution that used the parameter R in r
                    plt.scatter(R,zs[i], color='black')
plt.xlim(0,200)        
plt.ylim(0,400)

Recursing · Accepted Answer · 2020-07-11T12:32:36.347

There is a bug in the lorenz_system function, it should be z_dot = x * y - b * z.

The linked answer also "Uses final values from one run as initial conditions for the next as an easy way to stay near the attractor.", and plots both local minima and local maxima.

Here is a way to get a similar plot using your code

import numpy as np
import matplotlib.pyplot as plt


def lorenz_system(x, y, z, r, b=10, s=6):
    x_dot = b * (y - x)
    y_dot = r * x - y - x * z
    z_dot = x * y - s * z
    return x_dot, y_dot, z_dot


dr = 0.1  # parameter step size
r = np.arange(40, 200, dr)  # parameter range
dt = 0.001  # time step
t = np.arange(0, 10, dt)  # time range

# initialize solution arrays
xs = np.empty(len(t) + 1)
ys = np.empty(len(t) + 1)
zs = np.empty(len(t) + 1)

# initial values x0,y0,z0 for the system
xs[0], ys[0], zs[0] = (1, 1, 1)


# Save the plot points coordinates and plot the with a single call to plt.plot
# instead of plotting them one at a time, as it's much more efficient
r_maxes = []
z_maxes = []
r_mins = []
z_mins = []


for R in r:
    # Print something to show everything is running
    print(f"{R=:.2f}")
    for i in range(len(t)):
        # approximate numerical solutions to system
        x_dot, y_dot, z_dot = lorenz_system(xs[i], ys[i], zs[i], R)
        xs[i + 1] = xs[i] + (x_dot * dt)
        ys[i + 1] = ys[i] + (y_dot * dt)
        zs[i + 1] = zs[i] + (z_dot * dt)
    # calculate and save the peak values of the z solution
    for i in range(1, len(zs) - 1):
        # save the local maxima
        if zs[i - 1] < zs[i] and zs[i] > zs[i + 1]:
            r_maxes.append(R)
            z_maxes.append(zs[i])
        # save the local minima
        elif zs[i - 1] > zs[i] and zs[i] < zs[i + 1]:
            r_mins.append(R)
            z_mins.append(zs[i])

    # "use final values from one run as initial conditions for the next to stay near the attractor"
    xs[0], ys[0], zs[0] = xs[i], ys[i], zs[i]


plt.scatter(r_maxes, z_maxes, color="black", s=0.5, alpha=0.2)
plt.scatter(r_mins, z_mins, color="red", s=0.5, alpha=0.2)

plt.xlim(0, 200)
plt.ylim(0, 400)
plt.show()

Result:

In Python: How to make a bifurcation diagram of the Lorenz system under a varying parameter value?

1 Answers1