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Consider solving an initial value problemy′(t) = f (t, y), y(0) = y0.(i) Derive the equations of the general step of the generalised Heun’s method, whichevaluates the right–hand side f (t, y) at the points t = tn and t = tn + 14 h in eachinterval [tn, tn + h].

Question

Consider solving an initial value problemy′(t) = f (t, y), y(0) = y0.(i) Derive the equations of the general step of the generalised Heun’s method, whichevaluates the right–hand side f (t, y) at the points t = tn and t = tn + 14 h in eachinterval [tn, tn + h].

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Solution

The generalised Heun's method is a numerical method used to solve ordinary differential equations (ODEs). It is a two-step method that uses the information at the beginning of the interval and an estimate at the end of the interval to find a better estimate of the solution at the end of the interval.

Here is how you derive the equations of the general step of the generalised Heun’s method:

  1. Start with the initial value problem: y′(t) = f(t, y), y(0) = y0.

  2. The generalised Heun's method is a predictor-corrector method. The predictor step is given by Euler's method: y* = yn + h*f(tn, yn), where h is the step size, tn is the current time, and yn is the current estimate of y.

  3. The corrector step is where the method evaluates the right-hand side f(t, y) at the points t = tn and t = tn + 1/4 h. This is done by taking the average of the function evaluated at these two points: f_avg = 1/2 * (f(tn, yn) + f(tn + 1/4 h, y*)).

  4. The corrected estimate for y at the end of the interval is then given by: yn+1 = yn + h*f_avg.

  5. Repeat steps 2-4 for each interval [tn, tn + h].

This method is more accurate than Euler's method because it uses an estimate at the end of the interval to correct the initial estimate. This makes it a second-order method, meaning the error decreases quadratically as the step size h decreases.

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