H1 seminorm of a nls.

Hi, i have nonlinear system and i find the results of a variable n. I don't know how write the code for calculate an H1 seminorm. Substantially i have a system were i can change a variable, h. The exact solution is h=1/4096. I have to change this value (ex. h=1/64) and evaluate the H1 seminorm. |n-n_h|_1,2. my prof told me that |f| = norm(f)+norm(diff(f)), but if i calculate at first the norm of h=1/64 and after the norm of n=1/4096, the difference between the norm, doesn't coincide with the expected value.. I expect a result of 10e-2 about, instead i find a result of 100 or more..