!*********************************************
!*    Factorization of an integer number     *
!* ----------------------------------------- *
!* Sample run:                               *
!*                                           *
!* ? 394616                                  *
!*                                           *
!* 2 2 2 107 461                             *
!*                                           *
!* ----------------------------------------- *
!* Ref.: "Mathématiques par l'informatique   *
!*        individuelle (1) par H. Lehning    *
!*        et D. Jakubowicz, Masson, Paris,   *
!*        1982".                             *
!*                                           *
!*        F90 Version By J-P Moreau, Paris.  *
!*               (www.jpmoreau.fr)           *
!*********************************************
PROGRAM Factors
IMPLICIT NONE
REAL*4  D, N
INTEGER I, IN
!Enter integer number to be factorized
print *,''
write(*,10,advance='no')
read *, N
print *,''
!Test if multiple of 2
50 D=N-2*INT(N/2)
if (D.eq.0) then 
  write(*,300,advance='no')  2
  N=N/2
  goto 50
end if
!Test if multiple of 3
100 D=N-3*INT(N/3)
if (D.eq.0) then 
  write(*,300,advance='no')  3
  N=N/3
  goto 100
end if
!Test of divisors 6i-1 and 6i+1 up to sqr(N)
!Prime numbers are of the form 6i-1 or 6i+1
do i=6, INT(sqrt(N))+1, 6
150 D=N-(i-1)*INT(N/(i-1))
  if (D.eq.0) then 
    write(*,300,advance='no')  i-1
	N=N/(i-1)
	goto 150
  end if
200 D=N-(i+1)*INT(N/(i+1))
  if (D.eq.0) then 
    write(*,300,advance='no')  i+1
	N=N/(i+1)
	goto 200
  end if
end do
if (N>1) then
  IN=INT(N)
  write(*,300)  IN
else
  print *,''
end if

print *,''
stop
10  format(' ? ')
300 format(I4,' ')
end

!end of file factors.f90