function cx = cheby_u ( n, x )

%% CHEBY_U evaluates the Chebyshev polynomials of the second kind.
%
%  Differential equation:
%
%    (1-X*X) Y'' - 3 X Y' + N (N+2) Y = 0
%
%  First terms:
%
%    U(0)(X) =   1
%    U(1)(X) =   2 X
%    U(2)(X) =   4 X**2 -   1
%    U(3)(X) =   8 X**3 -   4 X
%    U(4)(X) =  16 X**4 -  12 X**2 +  1
%    U(5)(X) =  32 X**5 -  32 X**3 +  6 X
%    U(6)(X) =  64 X**6 -  80 X**4 + 24 X**2 - 1
%    U(7)(X) = 128 X**7 - 192 X**5 + 80 X**3 - 8X
%
%  Recursion:
%
%    U(0)(X) = 1,
%    U(1)(X) = 2 * X,
%    U(N)(X) = 2 * X * U(N-1)(X) - U(N-2)(X)
%
%  Norm:
%
%    Integral ( -1 <= X <= 1 ) ( 1 - X**2 ) * U(N)(X)**2 dX = PI/2
%
%  Licensing:
%
%    This code is distributed under the GNU LGPL license.
%
%  Modified:
%
%    23 July 2004
%
%  Author:
%
%    John Burkardt
%
%  Parameters:
%
%    Input, integer N, the highest polynomial to compute.
%
%    Input, real X, the point at which the polynomials are to be computed.
%
%    Output, real CX(1:N+1), the values of the N+1 Chebyshev polynomials.
%
  if ( n < 0 )
    cx = [];
    return
  end

  cx(1) = 1.0;

  if ( n < 1 )
    return
  end

  cx(2) = 2.0 * x;

  for i = 2 : n
    cx(i+1) = 2.0 * x * cx(i) - cx(i-1);
  end

  return
end