function L = digraph_arc_sixty ( )

%*****************************************************************************80
%
%% digraph_arc_sixty() sets the arc list for the sixty digraph.
%
%  Discussion:
%
%    The nodes of the digraph are divisors of 60.  There is a link from I to
%    J if divisor I can be divided by divisor J.
%
%  Licensing:
%
%    This code is distributed under the MIT license. 
%
%  Modified:
%
%    17 April 2026
%
%  Author:
%
%    John Burkardt
%
%  Output:
%
%    integer L(54,2), the arc list.
%
  node_num = 12;
  arc_num = 54;
  L = zeros ( arc_num, 2 );

  d = [ 60, 30, 20, 15, 12, 10, 6, 5, 4, 3, 2, 1 ];

  arc_id = 0;
  for i = 1 : node_num
    for j = 1 : node_num
      if ( mod ( d(i), d(j) ) == 0 )
        arc_id = arc_id + 1;
        L(arc_id,1) = i;
        L(arc_id,2) = j;
      end
    end
  end

  return
end

