% p22.m - 5th eigenvector of Airy equation u_xx = lambda*x*u

  clf
  for N = 12:12:48
    [D,x] = cheb(N); D2 = D^2; D2 = D2(2:N,2:N);
    [V,Lam] = eig(D2,diag(x(2:N)));      % generalized ev problem
    Lam = diag(Lam); ii = find(Lam>0);
    V = V(:,ii); Lam = Lam(ii);
    [foo,ii] = sort(Lam); ii = ii(5); lambda = Lam(ii);
    v = [0;V(:,ii);0]; v = v/v(N/2+1)*airy(0);
    xx = -1:.01:1; vv = polyval(polyfit(x,v,N),xx);
    subplot(2,2,N/12), plot(xx,vv), grid on
    title(sprintf('N = %d     eig = %15.10f',N,lambda))
  end