subroutine digraph_arc_example_abc ( L )

!*****************************************************************************80
!
!! digraph_arc_example_abc() sets up the arc list associated with a network.
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    15 April 2026
!
!  Author:
!
!    John Burkardt
!
!  Output:
!
!    integer L(3,2): the arc list.
!
  implicit none

  integer, parameter :: arc_num = 3

  integer L(arc_num,2)

  L = reshape ( (/ &
     1, 2, 3, &
     2, 3, 1 /), (/ arc_num, 2 /) )

  return
end
subroutine digraph_arc_example_abcd ( L )

!*****************************************************************************80
!
!! digraph_arc_example_abcd() sets up the arc list associated with a network.
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    15 April 2026
!
!  Author:
!
!    John Burkardt
!
!  Output:
!
!    integer L(5,2): the arc list.
!
  implicit none

  integer, parameter :: arc_num = 5

  integer L(arc_num,2)

  L = reshape ( (/  &
     1, 2, 3, 3, 4, &
     2, 3, 1, 4, 1 /), (/ arc_num, 2 /) )

  return
end
subroutine digraph_arc_example_cycler ( L )

!*****************************************************************************80
!
!! digraph_arc_example_cycler() sets adjacency information for the cycler digraph.
!
!  Diagram:
!  
!           A
!           V
!    9--><--7---<--3--><---4
!    |            /|      /
!    V           A |     /
!    |          /  |    /
!    5----<----1   V   A
!    |        /    |  /
!    V       A     | /
!    |      /      |/
!    2-->---8---<--6
!     \------>----/
!
!  Licensing:
!
!    This code is distributed under the MIT license. 
!
!  Modified:
!
!    22 October 1999
!
!  Author:
!
!    John Burkardt
!
!  Output:
!
!    integer L(16,2): the arc list
!
  implicit none

  integer, parameter :: arc_num = 16

  integer L(arc_num,2) 

  L = reshape ( (/  &
    1, 1, 2, 2, 3, 3, 3, 4, 5, 6, 6, 7, 7, 8, 9, 9, &
    3, 5, 6, 8, 4, 6, 7, 3, 2, 4, 8, 7, 9, 1, 5, 7 /), (/ arc_num, 2 /) )
 
  return
end
subroutine digraph_arc_print ( L, arc_num, title )

!*****************************************************************************80
!
!! digraph_arc_print() prints out an arc list for a digraph.
!
!  Discussion:
!
!    This routine actually allows the entries to have ANY value.
!    Values between 0 and 9 will be printed as is.  Other values will
!    be printed as '*'.
!
!  Licensing:
!
!    This code is distributed under the MIT license. 
!
!  Modified:
!
!    15 April 2026
!
!  Author:
!
!    John Burkardt
!
!  Input:
!
!    integer L(arc_num,2): the arc list of a digraph.
!
!    integer arc_num: the number of arcs.  
!
!    character ( len = * ) TITLE, a title.
!
  implicit none

  integer arc_num

  integer arc_id
  integer L(arc_num,2)
  character ( len = * ) title

  if ( len_trim ( title ) /= 0 ) then
    write ( *, '(a)' ) ' '
    write ( *, '(a)' ) trim ( title )
  end if

  write ( *, '(a)' ) ' '

  do arc_id = 1, arc_num
    write ( *, '(i2,a,i2,a,i2,a)' ) &
      arc_id, ': (', L(arc_id,1), ',', L(arc_id,2), ')'
  end do

  return
end
subroutine digraph_arc_example_five ( L )

!*****************************************************************************80
!
!! digraph_arc_example_five() sets up the arc list associated with a network.
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    15 April 2026
!
!  Author:
!
!    John Burkardt
!
!  Output:
!
!    integer L(5,2): the arc list.
!
  implicit none

  integer, parameter :: arc_num = 5

  integer L(arc_num,2)

  L = reshape ( (/ &
    1, 2, 3, 4, 5, &
    2, 5, 1, 1, 1 /), (/ arc_num, 2 /) )

  return
end
subroutine digraph_arc_example_moler ( L )

!*****************************************************************************80
!
!! digraph_arc_example_moler() returns the arc list for Moler's example 3.
!
!  Discussion:
!
!    This matrix appears on page 5 of the reference.
!
!    We added a self-link for node 5, which otherwise has no outlinks.
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    10 April 2026
!
!  Author:
!
!    John Burkardt
!
!  Reference:
!
!    Cleve Moler,
!    Experiments with Matlab,
!    Chapter 7: Google PageRank,
!    https://www.mathworks.com/moler/exm/chapters/pagerank.pdf
!
!  Output:
!
!    integer L(10,2), the arc list.
!
  implicit none

  integer, parameter :: arc_num = 10

  integer L(arc_num,2)

  L = reshape ( (/ &
    1, 1, 2, 2, 3, 3, 3, 4, 5, 6, &
    2, 6, 3, 4, 4, 5, 6, 1, 5, 1 /), (/ arc_num, 2 /) )

  return
end
subroutine digraph_arc_example_sauer ( L )

!*****************************************************************************80
!
!! digraph_arc_example_sauer() sets up the arc list associated with a network.
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    15 April 2026
!
!  Author:
!
!    John Burkardt
!
!  Reference:
!
!    Timothy Sauer,
!    "How search engines rate page quality",
!    Numerical Analysis,
!    Pearson, 2006.
!    ISBN: 0-321-2698-9,
!    LC: QA297.S348
!
!  Output:
!
!    integer L(34,2): the arc list.
!
  implicit none

  integer, parameter :: arc_num = 34

  integer L(arc_num,2)

  L = reshape ( (/ &
     5,  1,  3,  2,  4,  8,  2,  9,  3,  9, &
     2, 12,  3, 12,  1, 13,  5,  6,  7,  9, &
    14,  6,  7,  8, 12, 14,  4, 15, 10, 14, &
    13, 15, 11, 14,                         &
     1,  2,  2,  3,  3,  4,  5,  5,  6,  6, &
     7,  7,  8,  8,  9,  9, 10, 10, 10, 10, &
    10, 11, 11, 11, 11, 11, 12, 12, 13, 13, &
    14, 14, 15, 15 /), (/ arc_num, 2 /) )

  return
end
subroutine digraph_arc_example_sixty ( L )

!*****************************************************************************80
!
!! digraph_arc_example_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:
!
!    15 April 2026
!
!  Author:
!
!    John Burkardt
!
!  Output:
!
!    integer L(54,2): the arc list.
!    There is an arc from I to J if D(I) is divisible by D(J).
!
  implicit none

  integer, parameter :: arc_num = 54
  integer, parameter :: node_num = 12

  integer arc_id
  integer L(arc_num,2)
  integer d(node_num)
  integer i
  integer j

  d(1:12) = (/ 60, 30, 20, 15, 12, 10, 6, 5, 4, 3, 2, 1 /)

  arc_id = 0

  do i = 1, node_num
    do j = 1, node_num
      if ( mod ( d(i), d(j) ) == 0 ) then
        arc_id = arc_id + 1
        L(arc_id,1) = i
        L(arc_id,2) = j
      end if
    end do
  end do

  return
end