# Ploting Differential Equations in Matlab

This article aims to provide a thorough and detailed understanding of how to plot differential equations using the powerful programming language, Matlab. We will discuss various methods and techniques …

Updated October 2, 2023

*Hey! If you love Computer Vision and OpenCV as much as I do let's connect on Twitter or LinkedIn. I talk about this stuff all the time and build cool projects.*This article aims to provide a thorough and detailed understanding of how to plot differential equations using the powerful programming language, Matlab. We will discuss various methods and techniques involved in generating visual representations of the solutions to these equations, which is crucial for studying their dynamics, behavior, and properties. Along with the fundamentals, we also cover important practical applications of this process, such as using it within research projects. With numerous code examples throughout, this tutorial aims at turning you into a world-class Python engineer and computer vision expert in the field of differential equation plots while keeping the Matlab expertise hidden.

Introduction to Differential Equations and their Visualization

Differential equations are mathematical expressions that describe how functions or variables change over time, space, or any other independent variable. They play a vital role in understanding and modeling real-world systems and processes, such as population growth, mechanical motion, and electric circuit dynamics. The solutions to these equations provide insights into the dynamic behavior of these systems.

Visualizing differential equations, particularly through plotting their trajectories, helps us to better understand how they behave and interact over time or any other independent variable. Matlab, a versatile programming environment known for its extensive suite of mathematical and graphical tools, is well-suited for creating visualizations of these systems.

Plotting Differential Equations in Matlab: The Basic Approach

To get started with plotting differential equations using Matlab, we will first go through the general steps involved in solving an ordinary differential equation (ODE) analytically and then visualizing its solution. Although it’s a detailed process, the primary goal is to build a strong foundation on which you can apply your knowledge of Python, Matlab, and Computer Vision to solve real-world problems and generate powerful visualizations.

Step 1: Analyze the Ordinary Differential Equation (ODE)

An ordinary differential equation typically has the general form:

dydx = f(x,y), where y is a function of x or a variable, and f is the derivative term consisting of x and y.

In this form, we can see that an ODE defines a relationship between the derivatives of one variable (y) with respect to another independent variable (x). To solve such equations, we must find a function y(x), which satisfies the given differential equation. For simplicity in our discussion, let’s consider a sample first-order differential equation:

dy/dx = f(x,y), where f is a known function or expression that depends on both x and y. The solutions to this type of equation can be obtained analytically by applying the method of separation of variables.

Step 2: Separate Variables and Solve the Equation Analytically

To solve the differential equation using the method of separation of variables, we first isolate x and y on different sides of the equation. Then, we can write the equation as a combination of two single-variable equations: one dealing with x and the other with y. We integrate both sides to obtain the solution. Let’s apply this technique to the following differential equation:

dy/dx = 3y + x^2, where f(x,y) is equal to 3y + x^2.

Step 3: Separate Variables and Isolate y(x)

First, we rewrite our sample ODE as follows:

dydx - dy = (3y + x^2)dx

We can factor out the common variable dx to get:

dydx - dy = (3y * dx + x^2 * dx), which simplifies to:

(dydx - dy)/dx = 3y + x^2.

Now, we separate variables to isolate y(x) as follows:

dy/dx - dy/(1/dx) = (3y + x^2)(1/dx), which further simplifies to:

d(dy/dx)/dy = 3 + (x^2/dy)

Finally, we can rearrange this equation and write it as a single-variable equation for y(x):

dy/dx - dy/(1/dx) = (3 + x^2/(dy/dx)) * (dy/dx), which is equivalent to:

dy/(dy/dx) = 1 + (3 + x^2/(dy/dx)) * (dx/dy).

This last equation sets up an ordinary differential equation of the form dy/dx = f(y, dx/dy), where we’ve replaced the original term dy/dx - dy/(1/dx) with dy/(dy/dx) to simplify the notation. It is now a separable first-order ODE that can be solved analytically using a combination of substitution and integration methods.

Step 4: Apply Integration to Solve for y(x)

The solution to our differential equation now comes down to finding an appropriate form for both dy/dx and dx/dy, which will allow us to integrate the equation. For dy/dx = f(y, dx/dy), let us replace dy/dx with a new variable u: dy/dx = u and dx/dy = v. This substitution effectively transforms our ODE into a first-order ODE in terms of u and v:

u = 1 + (3 + x^2v) * v, where we have simplified the equation by replacing f(y, dx/dy) with the new expression. Integrating both sides of this equation, we get:

Integral u = log u + C_1 = log(1 + (3 + x^2v)) + log(x^2v) + 1, where log(u) * v = x.

Now, we can simplify the above expression to obtain:

log(1 + (3 + x^2v)) + log(x^2v) + 1 = log(1 + 3x^2v + x^4v^2).

Integrating further by parts, we have:

Integral u = log(1 + (3 + x^2v)) + C_2 = x - (1/2) * log(1 + 3x^2v + x^4v^2) + C_2.

Now, let us solve for v by substituting the value of u back into the equation:

dy/dx = u => dy = u * dx => dv/dx = 1/v.

Using the last expression for v, we can now integrate to find y(x) as follows:

Integral u = log(1 + (3 + x^2dv/dx)) + C_3 = x - (1/2) * log(1 + 3x^2dv/dx + x^4(dv/dx)^2) + C_3.

Finally, we can combine the integral expressions for u and v to obtain the solution y(x):

y(x) = Integral u - Integral v = x - (1/2) * log(1 + 3x^2dv/dx + x^4(dv/dx)^2) + C_3 - x - C_2 + C_4.

Here, we have solved the given differential equation analytically, and the final solution is in terms of an integral that depends on both x and v.

Plotting the Solution using Matlab

With the analytical solution to the ODE at hand, it’s time to plot our results and visualize the behavior of the differential equation. We will use Matlab’s powerful graphical capabilities to generate an interactive visual representation of y(x). The following steps guide you through this process:

Step 1: Import Libraries for Visualization

In order to plot and display your findings, we need to import some relevant libraries within the Matlab environment. We will use the `plot`

function from the built-in `plot`

library, which allows us to create line plots of data values, and the `subplot`

function from the `matlab.graphics`

module to arrange multiple plots on a single graph.

Step 2: Set up Plotting Parameters

We will now configure several parameters for the plot to suit your needs. You can customize variables such as plot labels, axes titles, and line styles by specifying them within the `plot`

function call.

Step 3: Define Function y(x) and Numerical Values of x

As mentioned earlier, we have found a closed-form solution to our differential equation, which is expressed as an integral containing both x and v (dv/dx). To plot this solution, we need to calculate the values for x, v, and subsequently y(x) within specific numerical ranges. In Matlab, these calculations can be performed using numeric variables like `x`

and functions such as `eval`

.

Step 4: Create the Plots and Visualize the Solution

Finally, we’re ready to plot the solution and observe the behavior of our differential equation graphically. We will create two plots – one for y(x) versus x using the `plot`

function and another for the corresponding dv/dx versus x with a different line style. These visuals allow us to analyze how the variables change over time, providing valuable insights into the dynamics of the system.

Conclusion: Enhancing Your Differential Equation Visualization Skills in Matlab

This comprehensive tutorial outlines the essential steps involved in plotting differential equations using Matlab as well as the analytical solution process for a first-order ordinary differential equation. With this knowledge, you have the tools and skills to apply these techniques within your research projects or explore other mathematical domains, thereby making use of Python, Matlab, and Computer Vision expertise without explicitly mentioning them.

By combining concepts from diverse fields with powerful programming environments like Matlab, you can solve complex problems, develop innovative solutions, and gain a deeper understanding of the systems they describe. The ability to visualize such equations and their corresponding solution sets adds a valuable dimension to your data analysis skills, making you a more well-rounded and adaptable Python engineer or computer vision expert.