Procedure

     UDDC ( Derivative of function less than zero, df(x)/dx < 0 )

     SUBROUTINE UDDC ( UDFUNC, X, DX, ISDECR )

Abstract

     Return .TRUE. if the derivative of the callback function UDFUNC
     at a given abscissa value is negative.

Required_Reading

     None.

Keywords

     DERIVATIVE
     MATH

Declarations

     IMPLICIT NONE

     EXTERNAL              UDFUNC

     DOUBLE PRECISION      X
     DOUBLE PRECISION      DX
     LOGICAL               ISDECR

Brief_I/O

     VARIABLE  I/O  DESCRIPTION
     --------  ---  --------------------------------------------------
     UDFUNC     I   The routine that computes the scalar value
                    of interest.
     X          I   Independent variable of UDFUNC.
     DX         I   Interval from X for derivative calculation.
     ISDECR     O   Boolean indicating if the derivative is negative.

Detailed_Input

     UDFUNC   is the routine that returns the value of the scalar
              quantity function of interest at X. The calling
              sequence for UDFUNC is:

                 CALL UDFUNC ( X, VALUE )

              where:

                 X       the double precision value of the
                         independent variable of the function
                         at which to determine the scalar value.

                 VALUE   the double precision value returned by
                         UDFUNC at X.

              Functionally:

                 VALUE = UDFUNC ( X )

     X        is a scalar double precision value at which to determine
              the derivative of UDFUNC.

              For many SPICE uses, X will represent ephemeris time,
              expressed as seconds past J2000 TDB.

     DX       is a scalar double precision value representing half the
              interval in units of X separating the evaluation
              values of UDFUNC; the evaluations occur at (X + DX)
              and (X - DX).

              DX may be negative but must be non-zero.

Detailed_Output

     ISDECR   is a scalar boolean indicating if the first derivative
              of UDFUNC with respect to the independent variable
              at X is less than zero.

              Functionally:

                            d UDFUNC(x) |
                 ISDECR =   ----------- |  <  0
                                 dx     |
                                         X

Parameters

     None.

Exceptions

     1)  If DX has a value of zero, an error is signaled by a routine
         in the call tree of this routine.

Files

     If the evaluation of UDFUNC requires SPICE kernel data, the
     appropriate kernels must be loaded before calling this routine.

     -  SPK data: the calling application must load ephemeris data
        for the targets, observer, and any intermediate objects in
        a chain connecting the targets and observer for the time
        used in the evaluation. If aberration corrections are
        used, the states of target and observer relative to the
        solar system barycenter must be calculable from the
        available ephemeris data.

     -  If non-inertial reference frames are used, then PCK
        files, frame kernels, C-kernels, and SCLK kernels may be
        needed.

     Such kernel data are normally loaded once per program run, NOT
     every time this routine is called.

Particulars

     This routine only wraps a UDDF call, examining the sign of the
     derivative value returned by UDDF. Please refer to this routine
     for further details.

Examples

     See GFUDS.

Restrictions

     None.

Literature_References

     None.

Author_and_Institution

     N.J. Bachman       (JPL)
     J. Diaz del Rio    (ODC Space)
     E.D. Wright        (JPL)

Version

    SPICELIB Version 1.0.1, 26-OCT-2021 (JDR)

        Edited the header to comply with NAIF standard.

    SPICELIB Version 1.0.0, 31-MAR-2010 (EDW) (NJB)