C This file: http://ftp.aset.psu.edu/pub/ger/fortran/hdk/primes.for
C
C======FIND FIRST NPRIME PRIMES.
C CLASSROOM EXAMPLE - HDK, 1974.
      INTEGER NPRIME,COUNT,P
      LOGICAL ISPRIM
      NPRIME=3000
      COUNT=1
C--Print the first prime, the 501th, the 1001th, ..., the last prime.
      WRITE(*,*)'THE FIRST',NPRIME,'PRIMES ARE:'
      WRITE(*,*) '2 IS PRIME'
      IF(NPRIME.EQ. 1) STOP

      DO 1 P = 3,999999999,2
      IF(ISPRIM(P)) THEN
        COUNT = COUNT + 1
        IF (MOD(COUNT-1,500).EQ.0 .OR. COUNT.EQ.NPRIME)
     x      WRITE(*,*) P,'IS PRIME.'
        IF(COUNT .EQ. NPRIME) STOP
      END IF
1     CONTINUE
      WRITE(*,*) 'NEXT PRIME IS LARGER THAN 10**9'
      STOP
      END
      LOGICAL FUNCTION ISPRIM(P)
      INTEGER P,I
      IF(P .LT. 2) THEN
        ISPRIM=.FALSE.
        RETURN
      END IF
      DO 1 I = 2,P
         IF(I * I .GT. P) THEN
           ISPRIM=.TRUE.
           RETURN
         ELSE IF(MOD(P,I) .EQ. 0) THEN
                ISPRIM=.FALSE.
                RETURN
         END IF
1     CONTINUE
      WRITE(*,*) 'THIS SHOULD NOT HAPPEN'
      STOP
      END