```      SUBROUTINE DSWAP(N,DX,INCX,DY,INCY)
*     .. Scalar Arguments ..
INTEGER INCX,INCY,N
*     ..
*     .. Array Arguments ..
DOUBLE PRECISION DX(*),DY(*)
*     ..
*
*  Purpose
*  =======
*
*     interchanges two vectors.
*     uses unrolled loops for increments equal one.
*     jack dongarra, linpack, 3/11/78.
*     modified 12/3/93, array(1) declarations changed to array(*)
*
*
*     .. Local Scalars ..
DOUBLE PRECISION DTEMP
INTEGER I,IX,IY,M,MP1
*     ..
*     .. Intrinsic Functions ..
INTRINSIC MOD
*     ..
IF (N.LE.0) RETURN
IF (INCX.EQ.1 .AND. INCY.EQ.1) GO TO 20
*
*       code for unequal increments or equal increments not equal
*         to 1
*
IX = 1
IY = 1
IF (INCX.LT.0) IX = (-N+1)*INCX + 1
IF (INCY.LT.0) IY = (-N+1)*INCY + 1
DO 10 I = 1,N
DTEMP = DX(IX)
DX(IX) = DY(IY)
DY(IY) = DTEMP
IX = IX + INCX
IY = IY + INCY
10 CONTINUE
RETURN
*
*       code for both increments equal to 1
*
*
*       clean-up loop
*
20 M = MOD(N,3)
IF (M.EQ.0) GO TO 40
DO 30 I = 1,M
DTEMP = DX(I)
DX(I) = DY(I)
DY(I) = DTEMP
30 CONTINUE
IF (N.LT.3) RETURN
40 MP1 = M + 1
DO 50 I = MP1,N,3
DTEMP = DX(I)
DX(I) = DY(I)
DY(I) = DTEMP
DTEMP = DX(I+1)
DX(I+1) = DY(I+1)
DY(I+1) = DTEMP
DTEMP = DX(I+2)
DX(I+2) = DY(I+2)
DY(I+2) = DTEMP
50 CONTINUE
RETURN
END

```