function y = serie_Taylor(cell_df, x, x0)
  Nt = length(cell_df) ;
  a  = zeros(Nt, 1) ;
  for i = 0 : Nt-1
    ix_v = i+1 ;
    df_i = cell_df{ix_v} ;
    a(ix_v) = df_i(x0) / factorial(i) ;
  end
  y = calcola_polinomio(a, x-x0) ;
end