function ext=derzeros3(n)
  tau  = cos( (0:n)*pi/n ); % nodes
  t    = linspace( -1, 1, 20*n+1 ); 
  rho  = [ones(1,n+1); zeros(1,n+1); zeros(1,n+1)]; % data
  r    = rho(:);
  r(4) = 1.5; % bump in curve
  [w,D]   = genbarywts( tau, 3 ); % get barycentric weights
  % Now differentiate at nodes and set up companion pencil for p'
  [C0,C1] = gcmp( D*r/norm(D*r,inf), tau, 3, w/norm(w,inf) );
  ext  = eig( C0, C1 ); % locate extrema
  yext = hermiteval( r, ext, tau, 3, w, D ); % extremal heights
  [y,yp]  = hermiteval( r, t, tau, 3, w, D ); % evaluate interpolant
  plot( tau,r(1:3:3*n+1),'bo', t,y, ext,yext,'r+' );
  axis( [-1,1,0,2] );