# LORENZ_SIMULATION Display a solution of the Lorenz Equations

LORENZ_SIMULATION, a MATLAB program which computes and displays solutions to the Lorenz equations for various initial conditions.

The Lorenz equations are an extremely simplified model of the weather, in which three differential equations describe the evolution of three variables. There are also three parameters whose values play the role of physical constants.

It was while studying these equations that Edward Lorenz discovered what he called "deterministic chaos". That is, at least for certain values of the parameters, solutions were extremely sensitive to the initial conditions.

The Lorenz equations are often written as

```        x' = sigma * ( y - x )
y' = x * ( rho - z ) - y
z' = x * y - beta * z
```
where the parameters are positive. Typical values for the parameters are
```        sigma = 10
beta = 8 / 3 = 2.666...
rho = 28
```
The value of rho has a strong influence on the behavior of solutions. The value rho=28 results in chaotic solutions, whereas rho=99.96 yields a regular pattern.

The LORENZ_SIMULATION program shows a 3D plot of the behavior of a solution to the Lorenz equations. The initial condition is varied randomly each time the user runs the program.

There is actually already a Lorenz demonstration program, called lorenzdemo(), which is part of the MATLAB demo set.

We modified the original program because we wanted to be able to vary the initial condition for the integration. This turned out to be far more difficult than we realized, since the lorenzdemo program is full of code that looks important, but actually is not used.

The key is that the lorenzdemo displays some buttons, and as soon as you push a button, it calls "lorenz", which is a built in MATLAB function. This function is probably the same as lorenzdemo. However, if you change lorenzdemo, you have not changed "lorenz"! In other words, you can make all the changes you want to lorenzdemo, but most of them will have no effect whatsoever, since lorenzdemo actually calls "lorenz".

It is inadvertently cryptic coding like this that fills the wards of our hospitals with bitter, disillusioned people who have lost all faith in their ability to understand programming.

This version, with the slightly altered name "lorenz_simulation", calls itself. Thus, if you change this code, you will see a change when you run it. For our purposes, we were able to change the initial condition Y0, and have that effect the calculation as we wanted.

Note, also, that the ordering of the variables is somewhat different than is to be seen in other contexts. For LORENZ_SIMULATION, the equations are

```        x' = y * z - beta * x
y' = sigma * ( z - y )
z' = y * ( rho - x ) - z
```

### Usage:

lorenz_simulation
runs the program, picking an initial condition at random.

### Languages:

LORENZ_SIMULATION is available in a MATLAB version.

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### Reference:

1. Kathleen Alligood, Tim Sauer, James Yorke,
Chaos: An Introduction to Dynamical Systems,
Springer, 1997,
ISBN: 0-387-94677-2,
LC: QA614.8.A44.
2. James Gleick,
Chaos: The Making of a New Science,
Viking, 1987,
ISBN13: 978-0140092509,
LC: Q172.5.C45.G54.
3. Edward Lorenz,
Deterministic Nonperiodic Flow,
Journal of Atmospheric Science,
Volume 20, 1963, pages 130-141.

### Source Code:

• lorenz_simulation.m, the Lorenz equation simulation program, modified to vary the initial condition.

Last revised on 16 February 2019