function L = graph_arc_diamond ( )

%*****************************************************************************80
%
%% graph_arc_diamond() returns the arc list of the diamond graph.
%
%  Discussion:
%
%    The graph has 10 nodes.
%    The graph has 20 edges or 40 arcs.
%
%        1
%      /| |\
%     / | | \
%    2--3-4--5--(2)
%    |  | |  |
%    6--7-8--9--(6)
%     \ | | /
%      \| |/
%       10
%
%  Licensing:
%
%    This code is distributed under the MIT license. 
%
%  Modified:
%
%    30 April 2026
%
%  Author:
%
%    John Burkardt
%
%  Output:
%
%    integer L(arc_num,2): the arc list.
%
  L = [ ...
    1,  2; ...
    1,  3; ...
    1,  4; ...
    1,  5; ...
    2,  1; ...
    2,  3; ...
    2,  5; ...
    2,  6; ...
    3,  1; ...
    3,  2; ...
    3,  4; ...
    3,  7; ...
    4,  1; ...
    4,  3; ...
    4,  5; ...
    4,  8; ...
    5,  1; ...
    5,  2; ...
    5,  4; ...
    5,  9; ...
    6,  2; ...
    6,  7; ...
    6,  9; ...
    6, 10; ...
    7,  3; ...
    7,  6; ...
    7,  8; ...
    7, 10; ...
    8,  4; ...
    8,  7; ...
    8,  9; ...
    8, 10; ...
    9,  5; ...
    9,  6; ...
    9,  8; ...
    9, 10; ...
   10,  6; ...
   10,  7; ...
   10,  8; ...
   10,  9 ];

  return
end
