Error analysis 1D FEM

I am trying to write matlab code on Posteriori Error analysis 1D FEM for -u" =f(x) for x [0,1] u(0) =0 u'(1)=0.When f(x)= 1, u(x) = x(2-x^2)/2, g(x) =approx function .I figure out that || ||_infy will not applicable as u' is not continuos at nodes.(correct me if I am wrong). I am not able to implement L_(infi) or Energy norms to get below Error (image attached) How to write MATLAB function for the below

errn 0nian Ig(ih + 0.5h)-a(ih + 0.5h) , i.e. we are computing the error at the midpoints of . Based on your table. conjecture the numerical order of convergence. What vector norm are you using in errn errn 0nian Ig(ih + 0.5h)-a(ih + 0.5h) , i.e. we are computing the error at the midpoints of . Based on your table. conjecture the numerical order of convergence. What vector norm are you using in errn