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## Implicit Central Difference Method

The implicit central difference method is an implicit second order method for approximating the solution of the second order differential equation y''(x) = f(x, y, y') with initial conditions y(x0) = y0, y'(x0) = y'0.

The algorithm for the implicit central difference method is derived by using the central difference approximations for y'(x) and y''(x):
y'(x) = (y(x+h) - y(x-h) ) / 2h,

and
y''(x) = (y(x+h) - 2y(x) + y(x-h)) / h2.

Let xn = x0 + nh, yn be the approximation to y(xn), and fn = f(xn, yn, (yn + 1 - yn - 1 ) / 2h ), the procedure proceeds recursively via the implicit equation for yn + 1 as follows:
yn + 1 = 2 yn - yn - 1 + h2 fn.

In order to begin the recursion, two successive starting values of y are required, one of which is y0 and the other starting value y1 is approximated by
y1 = h y'(x0) + h2 f(x0, y0, y'(x0) ) / 2.

Particular classes of problems may have a more accurate estimate for y1.

### Function List

• void Implicit_Central_Difference_Method( double f0, double (*g)(double,double,double,double), double y[], double x0, double c, double h, int number_of_steps )

This function uses the Implicit Central Difference method to estimate the solution of the initial value problem, y'' = f(x,y); y(x0) = y[0] and y'(x0) = c, at x0 + nh where for
n = 1, …, number_of_steps and h is the step size. The argument f0 is the value of the function f(x,y,y') evaluated at x0, y[0], c. The function g(x[n],y[n],y[n - 1],h) returns the implicit value of y[n + 1] so that y[n + 1] = 2 y[n] - y[n - 1] + h^2] f[n]. On input, y[0] is the value of y(x) at x = x0. On output y[n] is the value of y(x) at x = x0 + n h for
n = 0, …, number_of_steps.