Question

Two steps of Euler's method For the following initial value problems, compute the first two approximations \(u_{1}\)and\(u_{2}\) given by Euler's method using the given time step. $$y^{\prime}(t)=2 y, y(0)=2 ; \Delta t=0.5$$

Short answer

Answer: The first two approximations are \(u_1 = 4\) and \(u_2 = 8\).

Step-by-step solution

Identify the differential equation and the initial condition

The given initial value problem is \(y'(t) = 2y\) with the initial condition \(y(0) = 2\). We have \(\Delta t = 0.5\). Now, we need to apply the Euler's method formula.

Use the Euler's method formula for the first approximation \(u_1\)

The formula for Euler's method is: $$u_{n+1} = u_n + \Delta t \cdot f(t_n, u_n)$$ For the initial condition, we have \(u_0 = y(0) = 2\) and \(t_0 = 0\). To find the first approximation \(u_1\), we need to use the function \(f(t,y) = 2y\) and plug in \(t_0\) and \(u_0\): $$u_1 = u_0 + \Delta t \cdot f(t_0, u_0) = 2 + 0.5 \cdot 2(2) = 2 + 0.5 \cdot 4 = 2 + 2 = 4$$

Use the Euler's method formula for the second approximation \(u_2\)

Now, we need to find the second approximation \(u_2\). Our \(t_1 = t_0 + \Delta t = 0 + 0.5 = 0.5\). Using the Euler's method formula again with \(t_1\) and \(u_1\): $$u_2 = u_1 + \Delta t \cdot f(t_1, u_1) = 4 + 0.5 \cdot 2(4) = 4 + 0.5 \cdot 8 = 4 + 4 = 8$$

Conclusion

The first two approximations using Euler's method for the initial value problem \(y'(t) = 2y\), \(y(0) = 2\), and \(\Delta t = 0.5\) are \(u_1 = 4\) and \(u_2 = 8\).

Key concepts

Initial Value Problem

An initial value problem is a type of differential equation along with a specified value at a starting point. Simply put, it involves finding a function that not only satisfies a specified differential equation but also meets a given condition at a specific point. This initial condition is crucial because it determines the specific solution from the family of possible solutions of the differential equation.

- **Example**: In our problem, the initial value problem is given by the differential equation \(y'(t) = 2y\) with the initial condition \(y(0) = 2\). This means that at time \(t = 0\), the value of the function \(y\) is 2.

Understanding initial value problems is essential because many real-world problems model phenomena with known starting conditions, such as population growth, decay rates, or even motion trajectories.

Differential Equations

Differential equations are mathematical equations that relate some function with its derivatives. In the context of initial value problems, they often describe how a quantity changes over time or space. These equations are fundamental for modeling a variety of complex systems in science and engineering.

- **Types**:

• **Ordinary Differential Equations (ODEs):** An ODE involves functions of one independent variable and their derivatives.
• **Partial Differential Equations (PDEs):** These involve functions of multiple independent variables and their partial derivatives.

- **Example**: Our example, \(y'(t) = 2y\), is an ODE. It indicates that the change in \(y\) with respect to \(t\) is proportionally constant to \(2y\).

Understanding differential equations is crucial for grasping how dynamic systems operate over time, for example, predicting the future behavior of a system given its current state.

Numerical Approximation

Numerical approximation methods are techniques used to find solutions for mathematical problems that cannot be solved with analytical methods, or when an exact solution is difficult to determine. Euler's Method is one such technique specifically used for approximating solutions to differential equations.

- **What is Euler's Method?**:

• This is a simple, first-order numerical procedure used to approximate solutions of ordinary differential equations (ODEs).
• By using a step-by-step approach, it approximates the solution over small intervals, gradually building up a solution curve.
• In our example, Euler's method was used to find \(u_{1}\) and \(u_{2}\) as progressive approximations of \(y(t)\) at each time step using a formula.

This method is especially useful when an equation cannot be easily integrated using standard calculus techniques.

First Order ODEs

First order ordinary differential equations (ODEs) are the simplest form of differential equations and involve only the first derivative of the unknown function. They have the general form \(y' = f(t, y)\), where \(y'\) represents the rate of change of \(y\).

- **Characteristics**:

• The equations describe how a process evolves over time based on its current state.
• Solutions often describe exponential growth or decay processes.

- **Example**: The equation \(y'(t) = 2y\) from the exercise is an example of a first order ODE, indicating exponential growth, since the rate of change is proportional to its current state \(2y\).

Understanding first order ODEs is fundamental in predicting future values in various fields such as physics, finance, and biology where systems depend on initial conditions.