 that the convergence quickly settles into a linear rate. 
% We could estimate this rate by doing a least-squares fit to a straight line 
% using MATLAB's built-in |polyfit|. Keep in mind that the values for small $k$ 
% should be left out of the computation, as they don't represent the linear trend. 
%%
p = polyfit(5:12,log(err(5:12)),1)
%% 
% The first value is the slope, which must be negative. We can exponentiate 
% it to get the convergence constant $\sigma$.
%%
sigma = exp(p(1))
%% 
% The numerical va