Numerical methods are essential for solving complex mathematical problems, especially when analytical solutions are challenging or impossible to obtain. One such method is **Heun's method**, also known as the improved Euler method. This calculator helps you approximate the solution of ordinary differential equations (ODEs) using Heun's method.

## Purpose and Functionality

Heun's method is used to estimate the solution of ODEs of the form:

dydx=f(x,y)\frac{dy}{dx} = f(x, y)dxdy=f(x,y)

where f(x,y)f(x, y)f(x,y) is a function describing the relationship between xxx and yyy. The calculator simplifies the process of solving such equations by performing the following tasks:

**Predicting**: Estimates the value of yyy at the next step using a simple Euler method.**Correcting**: Refines the estimate by averaging the slopes at the current and predicted points.

This approach provides a more accurate solution than the basic Euler method.

## Heun's Method Algorithm

**1. Inputs:**

**Initial x (x0x_0x0)**: The starting point of the interval.**Initial y (y0y_0y0)**: The value of yyy at x0x_0x0.**Step Size (hhh)**: The interval between points where the function is approximated.**End x (xendx_{\text{end}}xend)**: The point where you want to estimate yyy.

**2. Predicting Step:**ypredict=yn+h⋅f(xn,yn)y_{\text{predict}} = y_n + h \cdot f(x_n, y_n)ypredict=yn+h⋅f(xn,yn)

**3. Correcting Step:**yn+1=yn+h2[f(xn,yn)+f(xn+1,ypredict)]y_{n+1} = y_n + \frac{h}{2} \left[ f(x_n, y_n) + f(x_{n+1}, y_{\text{predict}}) \right]yn+1=yn+2h[f(xn,yn)+f(xn+1,ypredict)]

Here:

- xn+1=xn+hx_{n+1} = x_n + hxn+1=xn+h
- yn+1y_{n+1}yn+1 is the refined estimate of yyy at xn+1x_{n+1}xn+1.

## Step-by-Step Example

Let's work through an example to see how Heun's method works.

**Given:**

- Differential Equation: dydx=x+y\frac{dy}{dx} = x + ydxdy=x+y
- Initial Condition: y(0)=1y(0) = 1y(0)=1
- End x: x=1x = 1x=1
- Step Size: h=0.1h = 0.1h=0.1

**Steps:**

**Start with Initial Values:**- x0=0x_0 = 0x0=0
- y0=1y_0 = 1y0=1

**First Iteration:****Predictor Step:**ypredict=y0+h⋅(x0+y0)=1+0.1⋅(0+1)=1.1y_{\text{predict}} = y_0 + h \cdot (x_0 + y_0) = 1 + 0.1 \cdot (0 + 1) = 1.1ypredict=y0+h⋅(x0+y0)=1+0.1⋅(0+1)=1.1**Corrector Step:**y1=y0+h2[(x0+y0)+(x1+ypredict)]y_1 = y_0 + \frac{h}{2} \left[ (x_0 + y_0) + (x_1 + y_{\text{predict}}) \right]y1=y0+2h[(x0+y0)+(x1+ypredict)] y1=1+0.12[(0+1)+(0.1+1.1)]=1+0.05⋅2.2=1.11y_1 = 1 + \frac{0.1}{2} \left[ (0 + 1) + (0.1 + 1.1) \right] = 1 + 0.05 \cdot 2.2 = 1.11y1=1+20.1[(0+1)+(0.1+1.1)]=1+0.05⋅2.2=1.11- Update xxx and yyy:x1=x0+h=0.1x_1 = x_0 + h = 0.1x1=x0+h=0.1 y1=1.11y_1 = 1.11y1=1.11

**Repeat Steps:**Continue the process until xxx reaches the end value (1 in this case).

## Example Calculation Table

Step | xxx | yyy (Initial) | ypredicty_{\text{predict}}ypredict | yyy (Corrected) |
---|---|---|---|---|

1 | 0 | 1 | 1.1 | 1.11 |

2 | 0.1 | 1.11 | 1.221 | 1.2321 |

3 | 0.2 | 1.2321 | 1.36531 | 1.37713 |

## Conclusion

Heun's method is a valuable tool for approximating solutions to ordinary differential equations. The calculator simplifies this process by automating the steps involved in Heun's method, making it easier to handle complex equations. By inputting initial conditions, step size, and end point, you can quickly obtain accurate estimates for your differential equations. This method is particularly useful in various fields such as engineering, physics, and finance, where differential equations are common.