!DECK DDOT
      DOUBLE PRECISION FUNCTION DDOT (N, DX, INCX, DY, INCY)
!***BEGIN PROLOGUE  DDOT
!***PURPOSE  Compute the inner product of two vectors.
!***LIBRARY   SLATEC (BLAS)
!***CATEGORY  D1A4
!***TYPE      DOUBLE PRECISION (SDOT-S, DDOT-D, CDOTU-C)
!***KEYWORDS  BLAS, INNER PRODUCT, LINEAR ALGEBRA, VECTOR
!***AUTHOR  Lawson, C. L., (JPL)
!           Hanson, R. J., (SNLA)
!           Kincaid, D. R., (U. of Texas)
!           Krogh, F. T., (JPL)
!***DESCRIPTION
!
!                B L A S  Subprogram
!    Description of Parameters
!
!     --Input--
!        N  number of elements in input vector(s)
!       DX  double precision vector with N elements
!     INCX  storage spacing between elements of DX
!       DY  double precision vector with N elements
!     INCY  storage spacing between elements of DY
!
!     --Output--
!     DDOT  double precision dot product (zero if N .LE. 0)
!
!     Returns the dot product of double precision DX and DY.
!     DDOT = sum for I = 0 to N-1 of  DX(LX+I*INCX) * DY(LY+I*INCY),
!     where LX = 1 if INCX .GE. 0, else LX = 1+(1-N)*INCX, and LY is
!     defined in a similar way using INCY.
!
!***REFERENCES  C. L. Lawson, R. J. Hanson, D. R. Kincaid and F. T.
!                 Krogh, Basic linear algebra subprograms for Fortran
!                 usage, Algorithm No. 539, Transactions on Mathematical
!                 Software 5, 3 (September 1979), pp. 308-323.
!***ROUTINES CALLED  (NONE)
!***REVISION HISTORY  (YYMMDD)
!   791001  DATE WRITTEN
!   890831  Modified array declarations.  (WRB)
!   890831  REVISION DATE from Version 3.2
!   891214  Prologue converted to Version 4.0 format.  (BAB)
!   920310  Corrected definition of LX in DESCRIPTION.  (WRB)
!   920501  Reformatted the REFERENCES section.  (WRB)
!***END PROLOGUE  DDOT
      DOUBLE PRECISION DX(*), DY(*)
!***FIRST EXECUTABLE STATEMENT  DDOT
      DDOT = 0.0D0
      IF (N .LE. 0) RETURN
      IF (INCX .EQ. INCY) IF (INCX-1) 5,20,60
!
!     Code for unequal or nonpositive increments.
!
    5 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
        DDOT = DDOT + DX(IX)*DY(IY)
        IX = IX + INCX
        IY = IY + INCY
   10 CONTINUE
      RETURN
!
!     Code for both increments equal to 1.
!
!     Clean-up loop so remaining vector length is a multiple of 5.
!
   20 M = MOD(N,5)
      IF (M .EQ. 0) GO TO 40
      DO 30 I = 1,M
         DDOT = DDOT + DX(I)*DY(I)
   30 CONTINUE
      IF (N .LT. 5) RETURN
   40 MP1 = M + 1
      DO 50 I = MP1,N,5
      DDOT = DDOT + DX(I)*DY(I) + DX(I+1)*DY(I+1) + DX(I+2)*DY(I+2) + &
                    DX(I+3)*DY(I+3) + DX(I+4)*DY(I+4)
   50 CONTINUE
      RETURN
!
!     Code for equal, positive, non-unit increments.
!
   60 NS = N*INCX
      DO 70 I = 1,NS,INCX
        DDOT = DDOT + DX(I)*DY(I)
   70 CONTINUE
      RETURN
      END