% p4.m - periodic spectral differentiation

% Set up grid and differentiation matrix:
  N = 24; h = 2*pi/N; x = h*(1:N)';
  column = [0 .5*(-1).^(1:N-1).*cot((1:N-1)*h/2)]';
  D = toeplitz(column,column([1 N:-1:2]));

% Differentiation of a hat function:
  v = max(0,1-abs(x-pi)/2); clf
  subplot(3,2,1), plot(x,v,'.-','markersize',13)
  axis([0 2*pi -.5 1.5]), grid on, title('function')
  subplot(3,2,2), plot(x,D*v,'.-','markersize',13)
  axis([0 2*pi -1 1]), grid on, title('spectral derivative')

% Differentiation of exp(sin(x)):
  v = exp(sin(x)); vprime = cos(x).*v;
  subplot(3,2,3), plot(x,v,'.-','markersize',13)
  axis([0 2*pi 0 3]), grid on
  subplot(3,2,4), plot(x,D*v,'.-','markersize',13)
  axis([0 2*pi -2 2]), grid on
  error = norm(D*v-vprime,inf);
  text(2.2,1.4,['max error = ' num2str(error)])