function [ xhat ] = aitken( x )
%AITKEN aitken(x) applies Aitken's ?2 process to x, returning a vector with two fewer entries than x
    dx = circshift(x,-1)-x;
    ddx = circshift(dx,-1)-dx;
    xhat = x - dx.^2./ddx;
    xhat = xhat(1:end-2);
end