% Usage: r = d2cd3p_varh(f,x,h0,s,n)
% 3-point centered-difference formula for 2-nd derivative, for varying
% stepsize h
%
% Input:
% f - Matlab inline function 
% x - point where the derivative is computed
% h0 - initial value of stepsize
% s - factor by which the stepsize is multiplied at each iteration
% n - number of total iteration
%
% Output:
% r - matrix containing computed values of the derivative
%
% Examples:
% r=d2cd3p(@sin,0,1,0.1,20);
% r=d2cd3p(@myfunc,0,1,0.1,50);          here 'myfunc' is a user-defined function in M-file
% r=d2cd3p(inline('sin(x)'),0,.1,.5,50);
% r=d2cd3p(inline('sin(x)-cos(x)'),0,1,0.1,20);

function [r,hh] = d2cd3p_varh(f,x,h0,s,n)
r=[];
h=h0;
for i = 1:n
    r(i) = d2cd3p(f,x,h);
    hh(i) = h;
    h = h * s;
end;