SUBROUTINE MFFTB7(C,FAC)**   PURPOSE:*       ELEMENTARY COOLEY-TUKEY RADIX 3 STEP APPLIED TO A VECTOR-OF*       2-VECTORS-OF-COMPLEX C[IMS,NM [IVS,NV [IES,NE]]].*       SEE REF.[1] FOR NOTATIONS.*       THIS ROUTINE CAN BE USED ONLY BY ROUTINE MFFTIM, WHICH CONTROLS*       ITS OPERATION THROUGH THE MFFTPA COMMON**   DUMMY ARGUMENTS :**   C   ARRAY BEING FOURIER  TRANSFORMED*   FAC PHASE FACTORS, PREPARED BY MFFTP; NOT MODIFIED IN OUTPUT*      COMMON /MFFTPA/  IMS,IVS,IES,NM,NV,NE,MX,LX,MLIM,MSTEP,LLIM,LSTEP,     $ NUSTEP,IVLIM,ILIM,MD2LIM,LD2LIM      INTEGER NUSTEP      COMPLEX C(0:NUSTEP-1,0:2),FAC(0:*)      COMPLEX T0,T1,T2,F1,F2      REAL SIN60      PARAMETER ( SIN60 =  8.6602540378443864E-1)*..  LAM>0          LAMF=MX          DO 100 LAM=LSTEP,LLIM,LSTEP            F1=FAC(LAMF)            F2=FAC(LAMF*2)            DO 90 MU=LAM,LAM+MLIM,MSTEP              DO 80 IV=MU,MU+IVLIM,IVS              DO 80 I=IV,IV+ILIM,IES                T0=C(I,1)*F1+C(I,2)*F2                T2=(C(I,1)*F1-C(I,2)*F2)*SIN60                T1=C(I,0)-0.5*T0                C(I,0)=C(I,0)+T0                C(I,1)=(T1+CMPLX(-AIMAG(T2),REAL(T2)))                C(I,2)=(T1-CMPLX(-AIMAG(T2),REAL(T2)))80            CONTINUE90          CONTINUE            LAMF=LAMF+MX100       CONTINUE          DO 92 MU=0,MLIM,MSTEP            DO 82 IV=MU,MU+IVLIM,IVS            DO 82 I=IV,IV+ILIM,IES                T0=C(I,1)+C(I,2)                T2=(C(I,1)-C(I,2))*SIN60                T1=C(I,0)-0.5*T0                C(I,0)=C(I,0)+T0                C(I,1)=(T1+CMPLX(-AIMAG(T2),REAL(T2)))                C(I,2)=(T1-CMPLX(-AIMAG(T2),REAL(T2)))82          CONTINUE92        CONTINUE      END