SUBROUTINE MFFTA6(C,FAC)**   PURPOSE:*       ELEMENTARY GENTLEMAN-SANDE RADIX 2 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 MFFTDM, 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-1,0:1),FAC(0:*)      COMPLEX T0,F          IF(MX.GT.2*LX)THEN          DO 100 LAM=0,LLIM,LSTEP            MUF=LX            DO 90 MU=LAM+MSTEP,LAM+MD2LIM,MSTEP              F=CONJG(FAC(MUF))              DO 80  IV=MU,MU+IVLIM,IVS              DO 80 I=IV,IV+ILIM,IES                T0=C(I,0)                C(I,0)=T0+C(I,1)                C(I,1)=(T0-C(I,1))*F80            CONTINUE              MUF=MUF+LX90          CONTINUE            MUF=MUF-LX            DO 91 MU=LAM+MD2LIM+2*MSTEP,LAM+MLIM,MSTEP              F=-FAC(MUF)              DO 81 IV=MU,MU+IVLIM,IVS              DO 81 I=IV,IV+ILIM,IES                T0=C(I,0)                C(I,0)=T0+C(I,1)                C(I,1)=(T0-C(I,1))*F81            CONTINUE              MUF=MUF-LX91          CONTINUE            DO 82 IV=MD2LIM+MSTEP+LAM,MD2LIM+MSTEP+LAM+IVLIM,IVS            DO 82 I=IV,IV+ILIM,IES               T0=C(I,0)              C(I,0)=T0+C(I,1)              C(I,1)=CMPLX(AIMAG(T0-C(I,1)),-REAL(T0-C(I,1)))82          CONTINUE            DO 83 IV=LAM,LAM+IVLIM,IVS            DO 83 I=IV,IV+ILIM,IES                T0=C(I,0)                C(I,0)=T0+C(I,1)                C(I,1)=T0-C(I,1)83          CONTINUE100       CONTINUE        ELSE          IF(MX.EQ.1)GOTO 1000* IF MX > 1 COME HERE            MUF=LX            DO 200 MU=MSTEP,MD2LIM,MSTEP              F=CONJG(FAC(MUF))              DO 190 LAM=MU,MU+LLIM,LSTEP                DO 180 IV=LAM,LAM+IVLIM,IVS                DO 180 I=IV,IV+ILIM,IES                  T0=C(I,0)                  C(I,0)=T0+C(I,1)                  C(I,1)=(T0-C(I,1))*F  180           CONTINUE  190         CONTINUE              MUF=MUF+LX200         CONTINUE            MUF=MUF-LX            DO 201 MU=MD2LIM+2*MSTEP,MLIM,MSTEP              F=-FAC(MUF)              DO 191 LAM=MU,MU+LLIM,LSTEP                DO 181 IV=LAM,LAM+IVLIM,IVS                DO 181 I=IV,IV+ILIM,IES                  T0=C(I,0)                  C(I,0)=T0+C(I,1)                  C(I,1)=(T0-C(I,1))*F  181           CONTINUE  191         CONTINUE              MUF=MUF-LX201         CONTINUE             DO 192 LAM=MD2LIM+MSTEP,MD2LIM+MSTEP+LLIM,LSTEP             DO 182 IV=LAM,LAM+IVLIM,IVS             DO 182 I=IV,IV+ILIM,IES               T0=C(I,0)               C(I,0)=T0+C(I,1)               C(I,1)=CMPLX(AIMAG(T0-C(I,1)),-REAL(T0-C(I,1)))182           CONTINUE192         CONTINUE1000        DO 193 LAM=0,LLIM,LSTEP              DO 183 IV=LAM,LAM+IVLIM,IVS              DO 183 I=IV,IV+ILIM,IES                T0=C(I,0)                C(I,0)=T0+C(I,1)                C(I,1)=T0-C(I,1)183           CONTINUE193         CONTINUE        ENDIF      END