ADAS Subroutine r8f21

C
      function r8f21(a,b,c,d,eps)
C-----------------------------------------------------------------------
C
C  ****************** fortran77 function: r8f21.for ********************
C
C  Purpose:  Evaluates series expansion of hypergeometric function
C            F(a,b;c;d).
C
C  Function:
C
C  Input : (r*8)  a        = 1st numerator parameter a
C  Input : (r*8)  b        = 2nd numerator parameter b
C  Input : (r*8)  d        = 3rd denominator parameter c
C  Input : (r*8)  d        = independent variable (equiv x)
C  Input : (r*8)  eps      = accuracy parameter
C
C  Output: (r*8)  r8f21    = F(a,b;c;d)
C
C  Routines:
C          Routine    Source    Brief description
C          ------------------------------------------------------------
C          i4unit     ADAS     Specifies unit for warning message ouput
C
C  Author:  H. P. summers, University of Strathclyde
C           ja7.08
C           tel. 0141-548-4196
C
C  Date:   24/02/03
C
C  Update: HP Summers    24/05/04
C          restructured and added standard warning
C
C  Update: AD Whiteford  16/03/05
C          renamed to r8f21
C
C  Update: HP Summers    17/04/07
C          Corrected description comments
C
C-----------------------------------------------------------------------
      REAL*8              A,           B,           C,           D
      REAL*8              EPS