recursive subroutine collatz_path ( n )

!*****************************************************************************80
!
!! collatz_path() uses recursion to print the path of a Collatz sequence.
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    09 March 2012
!
!  Author:
!
!    John Burkardt
!
!  Input:
!
!    integer N, the current member of the path.
!
  implicit none

  integer n

  write ( *, '(2x,i8)' ) n

  if ( n < 1 ) then

    write ( *, '(a)' ) ' '
    write ( *, '(a)' ) 'collatz_path(): Fatal error!'
    write ( *, '(a,i8,a)' ) '  Path member N = ', n, ' is not positive.'
    stop

  else if ( n == 1 ) then

  else if ( mod ( n, 2 ) == 0 ) then
    call collatz_path ( n / 2 )
  else
    call collatz_path ( 3 * n + 1 )
  end if

  return
end