!*********************************************
!*              PRIME NUMBERS                *
!* ----------------------------------------- *
!* This small program tests if a given inte- *
!* ger number is a prime number or not.      *
!* ----------------------------------------- *
!* Ref.: "Mathématiques par l'informatique   *
!*        individuelle (1) par H. Lehning    *
!*        et D. Jakubowicz, Masson, Paris,   *
!*        1982".                             *
!*                                           *
!*              F90 version by J-P Moreau.   *
!*                  (www.jpmoreau.fr)        *
!*********************************************
!See also program factors.f90
PROGRAM Prime_numbers

!Labels: 10,50,100,200
IMPLICIT NONE
REAL*8 d,eps,i,m,n

  eps=1d-6
  print *,''
  write(*,10,advance='no'); read *, n
  print *,''
  !Test if multiple of 2
  d=n-2*int(n/2)
  if (dabs(d)<eps) goto 100
  !Test if multiple of 3
  d=n-3*int(n/3)
  if (dabs(d)<eps) goto 100
  !Test if multiple of 6i-1 and 6i+1 from i=6 to sqrt(N)+1
  i=6; m=int(dsqrt(n))+1
50 d=n-(i-1)*int(n/(i-1));
    if (dabs(d)<eps) goto 100
    d=n-(i+1)*int(n/(i+1));
    if (dabs(d)<eps) goto 100
    i=i+6
  if (i<=m) goto 50
  print *,' Prime number.'; goto 200
100 print *,' Not a prime number.'
200 print *,''
stop
10 format(' ? ')
end

! end of file prime.f90