SUBROUTINE DEXPDF(X,PDF)
C
C     PURPOSE--THIS SUBROUTINE COMPUTES THE PROBABILITY DENSITY
C              FUNCTION VALUE FOR THE DOUBLE EXPONENTIAL
C              (LAPLACE) DISTRIBUTION WITH MEAN = 0 AND
C              STANDARD DEVIAITON = SQRT(2).
C              THIS DISTRIBUTION IS DEFINED FOR ALL X AND HAS
C              THE PROBABILITY DENSITY FUNCTION
C              F(X) = 0.5*EXP(-ABS(X)). 
C     INPUT  ARGUMENTS--X      = THE SINGLE PRECISION VALUE AT
C                                WHICH THE PROBABILITY DENSITY
C                                FUNCTION IS TO BE EVALUATED.
C     OUTPUT ARGUMENTS--PDF    = THE SINGLE PRECISION PROBABILITY
C                                DENSITY FUNCTION VALUE.
C     OUTPUT--THE SINGLE PRECISION PROBABILITY DENSITY
C             FUNCTION VALUE PDF.
C     PRINTING--NONE.
C     RESTRICTIONS--NONE.
C     OTHER DATAPAC   SUBROUTINES NEEDED--NONE.
C     FORTRAN LIBRARY SUBROUTINES NEEDED--EXP.
C     MODE OF INTERNAL OPERATIONS--SINGLE PRECISION.
C     LANGUAGE--ANSI FORTRAN. 
C     REFERENCES--JOHNSON AND KOTZ, CONTINUOUS UNIVARIATE
C                 DISTRIBUTIONS--2, 1970, PAGES 22-36.
C     WRITTEN BY--JAMES J. FILLIBEN
C                 STATISTICAL ENGINEERING LABORATORY (205.03)
C                 NATIONAL BUREAU OF STANDARDS
C                 WASHINGTON, D. C. 20234
C                 PHONE:  301-921-2315
C     ORIGINAL VERSION--JUNE      1972. 
C     UPDATED         --SEPTEMBER 1975. 
C     UPDATED         --NOVEMBER  1975. 
C     UPDATED         --SEPTEMBER 1978. 
C
C---------------------------------------------------------------------
C
      IPR=6
C
C     CHECK THE INPUT ARGUMENTS FOR ERRORS.
C     NO INPUT ARGUMENT ERRORS POSSIBLE 
C     FOR THIS DISTRIBUTION.
C
C-----START POINT-----------------------------------------------------
C
      ARG=X
      IF(X.LT.0.0)ARG=-X
      PDF=0.5*EXP(-ARG)
C
      RETURN
      END