Euler's Method

A numerical technique for approximating solutions to differential equations by stepping along the slope field in small increments.

Definition

Euler's Method is a numerical technique for approximating the solution to a differential equation. Given an initial value problem dydx=f(x,y)\frac{dy}{dx} = f(x, y) with y(x0)=y0y(x_0) = y_0, we step forward in small increments of size hh:

yn+1=yn+hβ‹…f(xn,yn)y_{n+1} = y_n + h \cdot f(x_n, y_n)

At each step, we use the current slope f(xn,yn)f(x_n, y_n) to predict where the curve goes next. It is the simplest numerical ODE solver and the foundation for understanding more advanced methods.

Key terms:

  • hh β€” the step size (smaller hh gives more accuracy)
  • f(xn,yn)f(x_n, y_n) β€” the slope at the current point
  • (xn,yn)(x_n, y_n) β€” the current approximation point
One step of Euler's Method

Approximate y(0.1)y(0.1) given dydx=y\frac{dy}{dx} = y, y(0)=1y(0) = 1, using step size h=0.1h = 0.1.

y1=y0+hβ‹…f(x0,y0)=1+0.1β‹…(1)=1.1y_1 = y_0 + h \cdot f(x_0, y_0) = 1 + 0.1 \cdot (1) = 1.1

The exact answer is y(0.1)=e0.1β‰ˆ1.10517y(0.1) = e^{0.1} \approx 1.10517, so the error is about 0.0050.005.

Try it

Use Euler's Method with h=0.5h = 0.5 to approximate y(1)y(1) given dydx=x\frac{dy}{dx} = x, y(0)=0y(0) = 0.

Solution

Step 1: x0=0x_0 = 0, y0=0y_0 = 0. Slope =f(0,0)=0= f(0, 0) = 0. y1=0+0.5β‹…0=0y_1 = 0 + 0.5 \cdot 0 = 0

Step 2: x1=0.5x_1 = 0.5, y1=0y_1 = 0. Slope =f(0.5,0)=0.5= f(0.5, 0) = 0.5. y2=0+0.5β‹…0.5=0.25y_2 = 0 + 0.5 \cdot 0.5 = 0.25

So y(1)β‰ˆ0.25y(1) \approx 0.25. The exact answer is y=x22y = \frac{x^2}{2}, giving y(1)=0.5y(1) = 0.5. Error =0.25= 0.25 β€” large because hh is large.

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