% Rapporto incrementale fisso
h      = 1e-3 ;
d_dx_h = @(g,x) ( g(x+h) - g(x-h) ) / (2*h) ;

% Derivata come limite
d_dx_lim = @(g,x) limite( @(h) ( g(x+h)-g(x-h) ) / (2*h) , 0 , .1 , 1e-6 , true ) ;

f = @(x) x.^3 ;
n = 2 ;
fn_h   = derivata_n( d_dx_h   , f , n ) ;
fn_lim = derivata_n( d_dx_lim , f , n ) ;