function [e1,e2,e3] = deri(x,h)

  ex_deri = 0.70710678118655;
  e1 = abs( (my_fun(x+h) - my_fun(x))/h - ex_deri );
  e2 = abs(  (my_fun(x+h) - my_fun(x-h))/(2*h) - ex_deri);
  e3 = abs( (my_fun(x) - my_fun(x-h))/h - ex_deri) ;