UPP/FGAMMA_8f_source.html

      REAL FUNCTION fGAMMA(X)

!D    DOUBLE PRECISION FUNCTION fGAMMA(X)

!----------------------------------------------------------------------

!

! This routine calculates the GAMMA function for a real argument X.

!   Computation is based on an algorithm outlined in reference 1.

!   The program uses rational functions that approximate the GAMMA

!   function to at least 20 significant decimal digits.  Coefficients

!   for the approximation over the interval (1,2) are unpublished.

!   Those for the approximation for X >= 12 are from reference 2.

!   The accuracy achieved depends on the arithmetic system, the

!   compiler, the intrinsic functions, and proper selection of the

!   machine-dependent constants.

!

!

!*******************************************************************

!*******************************************************************

!

! Explanation of machine-dependent constants

!

! beta   - radix for the floating-point representation

! maxexp - the smallest positive power of beta that overflows

! XBIG   - the largest argument for which GAMMA(X) is representable

!          in the machine, i.e., the solution to the equation

!                  GAMMA(XBIG) = beta**maxexp

! XINF   - the largest machine representable floating-point number;

!          approximately beta**maxexp

! EPS    - the smallest positive floating-point number such that

!          1.0+EPS > 1.0

! XMININ - the smallest positive floating-point number such that

!          1/XMININ is machine representable

!

!     Approximate values for some important machines are:

!

!                            beta       maxexp        XBIG

!

! CRAY-1         (S.P.)        2         8191        966.961

! Cyber 180/855

!   under NOS    (S.P.)        2         1070        177.803

! IEEE (IBM/XT,

!   SUN, etc.)   (S.P.)        2          128        35.040

! IEEE (IBM/XT,

!   SUN, etc.)   (D.P.)        2         1024        171.624

! IBM 3033       (D.P.)       16           63        57.574

! VAX D-Format   (D.P.)        2          127        34.844

! VAX G-Format   (D.P.)        2         1023        171.489

!

!                            XINF         EPS        XMININ

!

! CRAY-1         (S.P.)   5.45E+2465   7.11E-15    1.84E-2466

! Cyber 180/855

!   under NOS    (S.P.)   1.26E+322    3.55E-15    3.14E-294

! IEEE (IBM/XT,

!   SUN, etc.)   (S.P.)   3.40E+38     1.19E-7     1.18E-38

! IEEE (IBM/XT,

!   SUN, etc.)   (D.P.)   1.79D+308    2.22D-16    2.23D-308

! IBM 3033       (D.P.)   7.23D+75     2.22D-16    1.39D-76

! VAX D-Format   (D.P.)   1.70D+38     1.39D-17    5.88D-39

! VAX G-Format   (D.P.)   8.98D+307    1.11D-16    1.12D-308

!

!*******************************************************************

!*******************************************************************

!

! Error returns

!

!  The program returns the value XINF for singularities or

!     when overflow would occur.  The computation is believed

!     to be free of underflow and overflow.

!

!

!  Intrinsic functions required are:

!

!     INT, DBLE, EXP, LOG, REAL, SIN

!

!

! References: "An Overview of Software Development for Special

!              Functions", W. J. Cody, Lecture Notes in Mathematics,

!              506, Numerical Analysis Dundee, 1975, G. A. Watson

!              (ed.), Springer Verlag, Berlin, 1976.

!

!              Computer Approximations, Hart, Et. Al., Wiley and

!              sons, New York, 1968.

!

!  Latest modification: October 12, 1989

!

!  Authors: W. J. Cody and L. Stoltz

!           Applied Mathematics Division

!           Argonne National Laboratory

!           Argonne, IL 60439


!   10-30-19  Bo CUI - REMOVE "GOTO" STATEMENT

!

!----------------------------------------------------------------------

      implicit none


      INTEGER I,N

      LOGICAL PARITY

      REAL C,CONV,EPS,FACT,HALF,ONE,P,PI,Q,RES,SQRTPI,SUM,TWELVE,       &

          two,x,xbig,xden,xinf,xminin,xnum,y,y1,ysq,z,zero

      dimension c(7),p(8),q(8)

!----------------------------------------------------------------------

!  Mathematical constants

!----------------------------------------------------------------------

      DATA one,half,twelve,two,zero/1.0e0,0.5e0,12.0e0,2.0e0,0.0e0/,    &

           sqrtpi/0.9189385332046727417803297e0/,                       &

           pi/3.1415926535897932384626434e0/

!D    DATA ONE,HALF,TWELVE,TWO,ZERO/1.0D0,0.5D0,12.0D0,2.0D0,0.0D0/,

!D   1     SQRTPI/0.9189385332046727417803297D0/,

!D   2     PI/3.1415926535897932384626434D0/

!----------------------------------------------------------------------

!  Machine dependent parameters

!----------------------------------------------------------------------

      DATA xbig,xminin,eps/35.040e0,1.18e-38,1.19e-7/,                  &

           xinf/3.4e38/

!D    DATA XBIG,XMININ,EPS/171.624D0,2.23D-308,2.22D-16/,

!D   1     XINF/1.79D308/

!----------------------------------------------------------------------

!  Numerator and denominator coefficients for rational minimax

!     approximation over (1,2).

!----------------------------------------------------------------------

      DATA p/-1.71618513886549492533811e+0,2.47656508055759199108314e+1,&

             -3.79804256470945635097577e+2,6.29331155312818442661052e+2,&

             8.66966202790413211295064e+2,-3.14512729688483675254357e+4,&

             -3.61444134186911729807069e+4,6.64561438202405440627855e+4/

      DATA q/-3.08402300119738975254353e+1,3.15350626979604161529144e+2,&

            -1.01515636749021914166146e+3,-3.10777167157231109440444e+3,&

              2.25381184209801510330112e+4,4.75584627752788110767815e+3,&

            -1.34659959864969306392456e+5,-1.15132259675553483497211e+5/

!D    DATA P/-1.71618513886549492533811D+0,2.47656508055759199108314D+1,

!D   1       -3.79804256470945635097577D+2,6.29331155312818442661052D+2,

!D   2       8.66966202790413211295064D+2,-3.14512729688483675254357D+4,

!D   3       -3.61444134186911729807069D+4,6.64561438202405440627855D+4/

!D    DATA Q/-3.08402300119738975254353D+1,3.15350626979604161529144D+2,

!D   1      -1.01515636749021914166146D+3,-3.10777167157231109440444D+3,

!D   2        2.25381184209801510330112D+4,4.75584627752788110767815D+3,

!D   3      -1.34659959864969306392456D+5,-1.15132259675553483497211D+5/

!----------------------------------------------------------------------

!  Coefficients for minimax approximation over (12, INF).

!----------------------------------------------------------------------

      DATA c/-1.910444077728e-03,8.4171387781295e-04,                   &

           -5.952379913043012e-04,7.93650793500350248e-04,              &

           -2.777777777777681622553e-03,8.333333333333333331554247e-02, &

            5.7083835261e-03/

!D    DATA C/-1.910444077728D-03,8.4171387781295D-04,

!D   1     -5.952379913043012D-04,7.93650793500350248D-04,

!D   2     -2.777777777777681622553D-03,8.333333333333333331554247D-02,

!D   3      5.7083835261D-03/

!----------------------------------------------------------------------

!  Statement functions for conversion between integer and float

!----------------------------------------------------------------------

      conv(i) = real(i)

!D    CONV(I) = DBLE(I)

      parity = .false.

      fact = one

      n = 0

      y = x

      IF (y <= zero) THEN

!----------------------------------------------------------------------

!  Argument is negative

!----------------------------------------------------------------------

            y = -x

            y1 = aint(y)

            res = y - y1

            IF (res /= zero) THEN

                  IF (y1 /= aint(y1*half)*two) parity = .true.

                  fact = -pi / sin(pi*res)

                  y = y + one

               ELSE

                  res = xinf

                  fgamma = res

                  RETURN

            END IF

      END IF

!----------------------------------------------------------------------

!  Argument is positive

!----------------------------------------------------------------------

      IF (y < eps) THEN

!----------------------------------------------------------------------

!  Argument < EPS

!----------------------------------------------------------------------

            IF (y >= xminin) THEN

                  res = one / y

               ELSE

                  res = xinf

                  fgamma = res

                  RETURN

            END IF

         ELSE IF (y < twelve) THEN

            y1 = y

            IF (y < one) THEN

!----------------------------------------------------------------------

!  0.0 < argument < 1.0

!----------------------------------------------------------------------

                  z = y

                  y = y + one

               ELSE

!----------------------------------------------------------------------

!  1.0 < argument < 12.0, reduce argument if necessary

!----------------------------------------------------------------------

                  n = int(y) - 1

                  y = y - conv(n)

                  z = y - one

            END IF

!----------------------------------------------------------------------

!  Evaluate approximation for 1.0 < argument < 2.0

!----------------------------------------------------------------------

            xnum = zero

            xden = one

            DO 260 i = 1, 8

               xnum = (xnum + p(i)) * z

               xden = xden * z + q(i)

  260       CONTINUE

            res = xnum / xden + one

            IF (y1 < y) THEN

!----------------------------------------------------------------------

!  Adjust result for case  0.0 < argument < 1.0

!----------------------------------------------------------------------

                  res = res / y1

               ELSE IF (y1 > y) THEN

!----------------------------------------------------------------------

!  Adjust result for case  2.0 < argument < 12.0

!----------------------------------------------------------------------

                  DO 290 i = 1, n

                     res = res * y

                     y = y + one

  290             CONTINUE

            END IF

         ELSE

!----------------------------------------------------------------------

!  Evaluate for argument >= 12.0,

!----------------------------------------------------------------------

            IF (y <= xbig) THEN

                  ysq = y * y

                  sum = c(7)

                  DO 350 i = 1, 6

                     sum = sum / ysq + c(i)

  350             CONTINUE

                  sum = sum/y - y + sqrtpi

                  sum = sum + (y-half)*log(y)

                  res = exp(sum)

               ELSE

                  res = xinf

                  fgamma = res

                  RETURN

            END IF

      END IF

!----------------------------------------------------------------------

!  Final adjustments and return

!----------------------------------------------------------------------

      IF (parity) res = -res

      IF (fact /= one) res = fact / res

      fgamma = res

      RETURN

! ---------- Last line of fGAMMA ----------

      END