function L = graph_arc_coxeter ( )

%*****************************************************************************80
%
%% graph_arc_coxeter() sets up the arc list for the Coxeter graph.
%
%  Discussion:
%
%    The graph has 28 nodes.
%    The graph has 42 edges or 84 arcs.
%
%  Licensing:
%
%    This code is distributed under the MIT license. 
%
%  Modified:
%
%    02 May 2026
%
%  Author:
%
%    John Burkardt
%
%  Output:
%
%    integer L(arc_num): the arc list.
%
  L = [        ...
        1,  2; ...
        1,  8; ...
        1, 10; ...
        1, 11; ...
        1, 28; ...
        2,  1; ...
        2,  3; ...
        2,  9; ...
        2, 20; ...
        2, 21; ...
        2, 23; ...
        3,  2; ...
        3,  4; ...
        3,  7; ...
        3, 12; ...
        4,  3; ...
        4,  5; ...
        4, 25; ...
        4, 28; ...
        5,  4; ...
        5,  6; ...
        5, 14; ...
        5, 26; ...
        6,  5; ...
        6,  7; ...
        6, 16; ...
        6, 27; ...
        7,  3; ...
        7,  6; ...
        7,  8; ...
        7, 16; ...
        8,  1; ...
        8,  7; ...
        8,  9; ...
        9,  2; ...
        9,  8; ...
        9, 10; ...
        9, 16; ...
       10,  1; ...
       10,  9; ...
       10, 11; ...
       11,  1; ...
       11, 10; ...
       11, 12; ...
       11, 18; ...
       12,  3; ...
       12, 11; ...
       12, 13; ...
       12, 19; ...
       13, 12; ...
       13, 14; ...
       13, 20; ...
       14,  5; ...
       14, 13; ...
       14, 15; ...
       14, 18; ...
       15, 14; ...
       15, 16; ...
       15, 22; ...
       15, 24; ...
       15, 25; ...
       16,  6; ...
       16,  7; ...
       16,  9; ...
       16, 15; ...
       16, 17; ...
       16, 23; ...
       17, 16; ...
       17, 18; ...
       17, 21; ...
       17, 26; ...
       18, 11; ...
       18, 14; ...
       18, 17; ...
       18, 19; ...
       19, 12; ...
       19, 18; ...
       19, 20; ...
       19, 28; ...
       20,  2; ...
       20, 13; ...
       20, 19; ...
       20, 21; ...
       21,  2; ...
       21, 17; ...
       21, 20; ...
       21, 22; ...
       22, 15; ...
       22, 21; ...
       22, 23; ...
       23,  2; ...
       23, 16; ...
       23, 22; ...
       23, 24; ...
       24, 15; ...
       24, 23; ...
       24, 25; ...
       25,  4; ...
       25, 15; ...
       25, 24; ...
       25, 26; ...
       26,  5; ...
       26, 17; ...
       26, 25; ...
       26, 27; ...
       27,  6; ...
       27, 26; ...
       27, 28; ...
       28,  1; ...
       28,  4; ...
       28, 19; ...
       28, 27 ];

  return
end
