function [ n_data, n, volume ] = sphere_unit_volume_values ( n_data )

%*****************************************************************************80
%
%% SPHERE_UNIT_VOLUME_VALUES returns some volumes of the unit sphere in ND.
%
%  Discussion:
%
%    The formula for the volume of the unit sphere in N dimensions is
%
%      Volume(N) = 2 * PI**(N/2) / ( N * Gamma ( N / 2 ) )
%
%    This function satisfies the relationships:
%
%      Volume(N) = 2 * PI * Volume(N-2) / N
%      Volume(N) = Area(N) / N
%
%    Some values of the function include:
%
%       N  Volume
%
%       1    1
%       2    1        * PI
%       3  ( 4 /   3) * PI
%       4  ( 1 /   2) * PI**2
%       5  ( 8 /  15) * PI**2
%       6  ( 1 /   6) * PI**3
%       7  (16 / 105) * PI**3
%       8  ( 1 /  24) * PI**4
%       9  (32 / 945) * PI**4
%      10  ( 1 / 120) * PI**5
%
%    In Mathematica, the function can be evaluated by:
%
%      2 * Pi^(n/2) / ( n * Gamma[n/2] )
%
%  Modified:
%
%    19 September 2004
%
%  Author:
%
%    John Burkardt
%
%  Reference:
%
%    Stephen Wolfram,
%    The Mathematica Book,
%    Fourth Edition,
%    Wolfram Media / Cambridge University Press, 1999.
%
%  Parameters:
%
%    Input/output, integer N_DATA.
%    On input, if N_DATA is 0, the first test data is returned, and
%    N_DATA is set to the index of the test data.  On each subsequent
%    call, N_DATA is incremented and that test data is returned.  When
%    there is no more test data, N_DATA is set to 0.
%
%    Output, integer N, the spatial dimension.
%
%    Output, real VOLUME, the volume of the unit 
%    sphere in that dimension.
%
  n_max = 20;

  n_vec = [ ...
     1,  2, ...
     3,  4, ...
     5,  6, ...
     7,  8, ...
     9, 10, ...
    11, 12, ...
    13, 14, ...
    15, 16, ...
    17, 18, ...
    19, 20 ];

  volume_vec = [ ...
     0.2000000000000000E+01, ... 
     0.3141592653589793E+01, ...
     0.4188790204786391E+01, ...
     0.4934802200544679E+01, ...
     0.5263789013914325E+01, ...
     0.5167712780049970E+01, ...
     0.4724765970331401E+01, ...
     0.4058712126416768E+01, ...
     0.3298508902738707E+01, ...
     0.2550164039877345E+01, ...
     0.1884103879389900E+01, ... 
     0.1335262768854589E+01, ...
     0.9106287547832831E+00, ...
     0.5992645293207921E+00, ...
     0.3814432808233045E+00, ...
     0.2353306303588932E+00, ...
     0.1409811069171390E+00, ...
     0.8214588661112823E-01, ...
     0.4662160103008855E-01, ...
     0.2580689139001406E-01  ];

  if ( n_data < 0 )
    n_data = 0;
  end

  n_data = n_data + 1;

  if ( n_max < n_data )
    n_data = 0;
    n = 0;
    volume = 0.0;
  else
    n = n_vec(n_data);
    volume = volume_vec(n_data);
  end

  return
end