      SUBROUTINE ZFFTI1 (N,WA,IFAC)
      IMPLICIT DOUBLE PRECISION (A-H,O-Z)
      DIMENSION       WA(*)      ,IFAC(*)    ,NTRYH(4)
      DATA NTRYH(1),NTRYH(2),NTRYH(3),NTRYH(4)/3,4,2,5/
      NL = N
      NF = 0
      J = 0
  101 J = J+1
      IF (J.le.4) GO TO 102
      GO TO 103
  102 NTRY = NTRYH(J)
      GO TO 104
  103 NTRY = NTRYH(4)+2*(J-4)
  104 NQ = NL/NTRY
      NR = NL-NTRY*NQ
      IF (NR.eq.0) GO TO 105
      GO TO 101
  105 NF = NF+1
      IFAC(NF+2) = NTRY
      NL = NQ
      IF (NTRY .NE. 2) GO TO 107
      IF (NF .EQ. 1) GO TO 107
      DO 106 I=2,NF
         IB = NF-I+2
         IFAC(IB+2) = IFAC(IB+1)
  106 CONTINUE
      IFAC(3) = 2
  107 IF (NL .NE. 1) GO TO 104
      IFAC(1) = N
      IFAC(2) = NF
      TPI =  6.28318530717958647692D0
      ARGH = TPI/FLOAT(N)
      I = 2
      L1 = 1
      DO 110 K1=1,NF
         IP = IFAC(K1+2)
         LD = 0
         L2 = L1*IP
         IDO = N/L2
         IDOT = IDO+IDO+2
         IPM = IP-1
         DO 109 J=1,IPM
            I1 = I
            WA(I-1) = 1.0D0
            WA(I) = 0.0D0
            LD = LD+L1
            FI = 0.0D0
            ARGLD = FLOAT(LD)*ARGH
            DO 108 II=4,IDOT,2
               I = I+2
               FI = FI+1.D0
               ARG = FI*ARGLD
               WA(I-1) = COS(ARG)
               WA(I) = SIN(ARG)
  108       CONTINUE
            IF (IP .LE. 5) GO TO 109
            WA(I1-1) = WA(I-1)
            WA(I1) = WA(I)
  109    CONTINUE
         L1 = L2
  110 CONTINUE
      RETURN
      END
