function yi = SplineC2natural(x, y, xi)
  j=1;
    for i = 2: length(x)
        if xi <= x(i)
            j=i-1;
            k=i;
            break;
        endif
    endfor
    i=j;

    h(j)=x(j+1)-x(j);
    t=(xi-x(j))/(h(j));
    a=y(j);
    d=y(j+1);
    c=y(j+1)-(h(j)/3)* y(j);
    b=y(j)+ (h(j)/3)*y(j+1);
    yi= a*((1-t)^3)+ b*t*((1-t)^2)+c*(t^2)*(1-t)+ d*t^3;
 endfunction