relibc_openlibm/slatec/rc.f

*DECK RC
      REAL FUNCTION RC (X, Y, IER)
C***BEGIN PROLOGUE  RC
C***PURPOSE  Calculate an approximation to
C             RC(X,Y) = Integral from zero to infinity of
C                              -1/2     -1
C                    (1/2)(t+X)    (t+Y)  dt,
C            where X is nonnegative and Y is positive.
C***LIBRARY   SLATEC
C***CATEGORY  C14
C***TYPE      SINGLE PRECISION (RC-S, DRC-D)
C***KEYWORDS  DUPLICATION THEOREM, ELEMENTARY FUNCTIONS,
C             ELLIPTIC INTEGRAL, TAYLOR SERIES
C***AUTHOR  Carlson, B. C.
C             Ames Laboratory-DOE
C             Iowa State University
C             Ames, IA  50011
C           Notis, E. M.
C             Ames Laboratory-DOE
C             Iowa State University
C             Ames, IA  50011
C           Pexton, R. L.
C             Lawrence Livermore National Laboratory
C             Livermore, CA  94550
C***DESCRIPTION
C
C   1.     RC
C          Standard FORTRAN function routine
C          Single precision version
C          The routine calculates an approximation to
C           RC(X,Y) = Integral from zero to infinity of
C
C                              -1/2     -1
C                    (1/2)(t+X)    (t+Y)  dt,
C
C          where X is nonnegative and Y is positive.  The duplication
C          theorem is iterated until the variables are nearly equal,
C          and the function is then expanded in Taylor series to fifth
C          order.  Logarithmic, inverse circular, and inverse hyper-
C          bolic functions can be expressed in terms of RC.
C
C
C   2.     Calling Sequence
C          RC( X, Y, IER )
C
C          Parameters on Entry
C          Values assigned by the calling routine
C
C          X      - Single precision, nonnegative variable
C
C          Y      - Single precision, positive variable
C
C
C
C          On Return  (values assigned by the RC routine)
C
C          RC     - Single precision approximation to the integral
C
C          IER    - Integer to indicate normal or abnormal termination.
C
C                     IER = 0 Normal and reliable termination of the
C                             routine.  It is assumed that the requested
C                             accuracy has been achieved.
C
C                     IER > 0 Abnormal termination of the routine
C
C          X and Y are unaltered.
C
C
C   3.    Error Messages
C
C         Value of IER assigned by the RC routine
C
C                  Value Assigned         Error Message Printed
C                  IER = 1                X.LT.0.0E0.OR.Y.LE.0.0E0
C                      = 2                X+Y.LT.LOLIM
C                      = 3                MAX(X,Y) .GT. UPLIM
C
C
C   4.     Control Parameters
C
C                  Values of LOLIM, UPLIM, and ERRTOL are set by the
C                  routine.
C
C          LOLIM and UPLIM determine the valid range of X and Y
C
C          LOLIM  - Lower limit of valid arguments
C
C                   Not less  than 5 * (machine minimum)  .
C
C          UPLIM  - Upper limit of valid arguments
C
C                   Not greater than (machine maximum) / 5 .
C
C
C                     Acceptable values for:   LOLIM       UPLIM
C                     IBM 360/370 SERIES   :   3.0E-78     1.0E+75
C                     CDC 6000/7000 SERIES :   1.0E-292    1.0E+321
C                     UNIVAC 1100 SERIES   :   1.0E-37     1.0E+37
C                     CRAY                 :   2.3E-2466   1.09E+2465
C                     VAX 11 SERIES        :   1.5E-38     3.0E+37
C
C          ERRTOL determines the accuracy of the answer
C
C                 The value assigned by the routine will result
C                 in solution precision within 1-2 decimals of
C                 "machine precision".
C
C
C          ERRTOL  - Relative error due to truncation is less than
C                    16 * ERRTOL ** 6 / (1 - 2 * ERRTOL).
C
C
C              The accuracy of the computed approximation to the inte-
C              gral can be controlled by choosing the value of ERRTOL.
C              Truncation of a Taylor series after terms of fifth order
C              introduces an error less than the amount shown in the
C              second column of the following table for each value of
C              ERRTOL in the first column.  In addition to the trunca-
C              tion error there will be round-off error, but in prac-
C              tice the total error from both sources is usually less
C              than the amount given in the table.
C
C
C
C          Sample Choices:  ERRTOL   Relative Truncation
C                                    error less than
C                           1.0E-3    2.0E-17
C                           3.0E-3    2.0E-14
C                           1.0E-2    2.0E-11
C                           3.0E-2    2.0E-8
C                           1.0E-1    2.0E-5
C
C
C                    Decreasing ERRTOL by a factor of 10 yields six more
C                    decimal digits of accuracy at the expense of one or
C                    two more iterations of the duplication theorem.
C
C *Long Description:
C
C   RC Special Comments
C
C
C
C
C                  Check: RC(X,X+Z) + RC(Y,Y+Z) = RC(0,Z)
C
C                  where X, Y, and Z are positive and X * Y = Z * Z
C
C
C          On Input:
C
C          X and Y are the variables in the integral RC(X,Y).
C
C          On Output:
C
C          X and Y are unaltered.
C
C
C
C                    RC(0,1/4)=RC(1/16,1/8)=PI=3.14159...
C
C                    RC(9/4,2)=LN(2)
C
C
C
C          ********************************************************
C
C          Warning: Changes in the program may improve speed at the
C                   expense of robustness.
C
C
C   --------------------------------------------------------------------
C
C   Special Functions via RC
C
C
C
C                  LN X                X .GT. 0
C
C                                            2
C                  LN(X) = (X-1) RC(((1+X)/2)  , X )
C
C
C   --------------------------------------------------------------------
C
C                  ARCSIN X            -1 .LE. X .LE. 1
C
C                                      2
C                  ARCSIN X = X RC (1-X  ,1 )
C
C   --------------------------------------------------------------------
C
C                  ARCCOS X            0 .LE. X .LE. 1
C
C
C                                     2      2
C                  ARCCOS X = SQRT(1-X ) RC(X  ,1 )
C
C   --------------------------------------------------------------------
C
C                  ARCTAN X            -INF .LT. X .LT. +INF
C
C                                       2
C                  ARCTAN X = X RC(1,1+X  )
C
C   --------------------------------------------------------------------
C
C                  ARCCOT X            0 .LE. X .LT. INF
C
C                                 2   2
C                  ARCCOT X = RC(X  ,X +1 )
C
C   --------------------------------------------------------------------
C
C                  ARCSINH X           -INF .LT. X .LT. +INF
C
C                                      2
C                  ARCSINH X = X RC(1+X  ,1 )
C
C   --------------------------------------------------------------------
C
C                  ARCCOSH X           X .GE. 1
C
C                                    2        2
C                  ARCCOSH X = SQRT(X -1) RC(X  ,1 )
C
C   --------------------------------------------------------------------
C
C                  ARCTANH X           -1 .LT. X .LT. 1
C
C                                        2
C                  ARCTANH X = X RC(1,1-X  )
C
C   --------------------------------------------------------------------
C
C                  ARCCOTH X           X .GT. 1
C
C                                  2   2
C                  ARCCOTH X = RC(X  ,X -1 )
C
C   --------------------------------------------------------------------
C
C***REFERENCES  B. C. Carlson and E. M. Notis, Algorithms for incomplete
C                 elliptic integrals, ACM Transactions on Mathematical
C                 Software 7, 3 (September 1981), pp. 398-403.
C               B. C. Carlson, Computing elliptic integrals by
C                 duplication, Numerische Mathematik 33, (1979),
C                 pp. 1-16.
C               B. C. Carlson, Elliptic integrals of the first kind,
C                 SIAM Journal of Mathematical Analysis 8, (1977),
C                 pp. 231-242.
C***ROUTINES CALLED  R1MACH, XERMSG
C***REVISION HISTORY  (YYMMDD)
C   790801  DATE WRITTEN
C   890531  Changed all specific intrinsics to generic.  (WRB)
C   891009  Removed unreferenced statement labels.  (WRB)
C   891009  REVISION DATE from Version 3.2
C   891214  Prologue converted to Version 4.0 format.  (BAB)
C   900315  CALLs to XERROR changed to CALLs to XERMSG.  (THJ)
C   900326  Removed duplicate information from DESCRIPTION section.
C           (WRB)
C   900510  Changed calls to XERMSG to standard form, and some
C           editorial changes.  (RWC))
C   920501  Reformatted the REFERENCES section.  (WRB)
C***END PROLOGUE  RC
      CHARACTER*16 XERN3, XERN4, XERN5
      INTEGER IER
      REAL C1, C2, ERRTOL, LAMDA, LOLIM
      REAL MU, S, SN, UPLIM, X, XN, Y, YN
      LOGICAL FIRST
      SAVE ERRTOL,LOLIM,UPLIM,C1,C2,FIRST
      DATA FIRST /.TRUE./
C
C***FIRST EXECUTABLE STATEMENT  RC
      IF (FIRST) THEN
         ERRTOL = (R1MACH(3)/16.0E0)**(1.0E0/6.0E0)
         LOLIM  = 5.0E0 * R1MACH(1)
         UPLIM  = R1MACH(2) / 5.0E0
C
         C1 = 1.0E0/7.0E0
         C2 = 9.0E0/22.0E0
      ENDIF
      FIRST = .FALSE.
C
C         CALL ERROR HANDLER IF NECESSARY.
C
      RC = 0.0E0
      IF (X.LT.0.0E0.OR.Y.LE.0.0E0) THEN
         IER = 1
         WRITE (XERN3, '(1PE15.6)') X
         WRITE (XERN4, '(1PE15.6)') Y
         CALL XERMSG ('SLATEC', 'RC',
     *      'X.LT.0 .OR. Y.LE.0 WHERE X = ' // XERN3 // ' AND Y = ' //
     *      XERN4, 1, 1)
         RETURN
      ENDIF
C
      IF (MAX(X,Y).GT.UPLIM) THEN
         IER = 3
         WRITE (XERN3, '(1PE15.6)') X
         WRITE (XERN4, '(1PE15.6)') Y
         WRITE (XERN5, '(1PE15.6)') UPLIM
         CALL XERMSG ('SLATEC', 'RC',
     *      'MAX(X,Y).GT.UPLIM WHERE X = '  // XERN3 // ' Y = ' //
     *      XERN4 // ' AND UPLIM = ' // XERN5, 3, 1)
         RETURN
      ENDIF
C
      IF (X+Y.LT.LOLIM) THEN
         IER = 2
         WRITE (XERN3, '(1PE15.6)') X
         WRITE (XERN4, '(1PE15.6)') Y
         WRITE (XERN5, '(1PE15.6)') LOLIM
         CALL XERMSG ('SLATEC', 'RC',
     *      'X+Y.LT.LOLIM WHERE X = ' // XERN3 // ' Y = ' // XERN4 //
     *      ' AND LOLIM = ' // XERN5, 2, 1)
         RETURN
      ENDIF
C
      IER = 0
      XN = X
      YN = Y
C
   30 MU = (XN+YN+YN)/3.0E0
      SN = (YN+MU)/MU - 2.0E0
      IF (ABS(SN).LT.ERRTOL) GO TO 40
      LAMDA = 2.0E0*SQRT(XN)*SQRT(YN) + YN
      XN = (XN+LAMDA)*0.250E0
      YN = (YN+LAMDA)*0.250E0
      GO TO 30
C
   40 S = SN*SN*(0.30E0+SN*(C1+SN*(0.3750E0+SN*C2)))
      RC = (1.0E0+S)/SQRT(MU)
      RETURN
      END