  h = 0.25;
  alpha = 0.5;
  n = 50;
  p = 0 : n - 1;
  h_list = h * alpha .^ p;
  fd_list = fd ( @(x)tan(x), pi/4, h_list );
  e_list = abs ( 2.0 - fd_list );
  plot ( p, e_list );
  print ( '-dpng', 'fd_plot2.png' );
  title ( 'Error versus index' )
  grid ( 'on' )
