src/numerical/slatec/fortran/zs1s2.f

   1 *DECK ZS1S2
   2       SUBROUTINE ZS1S2 (ZRR, ZRI, S1R, S1I, S2R, S2I, NZ, ASCLE, ALIM,
   3      +   IUF)
   4 C***BEGIN PROLOGUE  ZS1S2
   5 C***SUBSIDIARY
   6 C***PURPOSE  Subsidiary to ZAIRY and ZBESK
   7 C***LIBRARY   SLATEC
   8 C***TYPE      ALL (CS1S2-A, ZS1S2-A)
   9 C***AUTHOR  Amos, D. E., (SNL)
  10 C***DESCRIPTION
  11 C
  12 C     ZS1S2 TESTS FOR A POSSIBLE UNDERFLOW RESULTING FROM THE
  13 C     ADDITION OF THE I AND K FUNCTIONS IN THE ANALYTIC CON-
  14 C     TINUATION FORMULA WHERE S1=K FUNCTION AND S2=I FUNCTION.
  15 C     ON KODE=1 THE I AND K FUNCTIONS ARE DIFFERENT ORDERS OF
  16 C     MAGNITUDE, BUT FOR KODE=2 THEY CAN BE OF THE SAME ORDER
  17 C     OF MAGNITUDE AND THE MAXIMUM MUST BE AT LEAST ONE
  18 C     PRECISION ABOVE THE UNDERFLOW LIMIT.
  19 C
  20 C***SEE ALSO  ZAIRY, ZBESK
  21 C***ROUTINES CALLED  ZABS, ZEXP, ZLOG
  22 C***REVISION HISTORY  (YYMMDD)
  23 C   830501  DATE WRITTEN
  24 C   910415  Prologue converted to Version 4.0 format.  (BAB)
  25 C   930122  Added ZEXP and ZLOG to EXTERNAL statement.  (RWC)
  26 C***END PROLOGUE  ZS1S2
  27 C     COMPLEX CZERO,C1,S1,S1D,S2,ZR
  28       DOUBLE PRECISION AA, ALIM, ALN, ASCLE, AS1, AS2, C1I, C1R, S1DI,
  29      * S1DR, S1I, S1R, S2I, S2R, ZEROI, ZEROR, ZRI, ZRR, ZABS
  30       INTEGER IUF, IDUM, NZ
  31       EXTERNAL ZABS, ZEXP, ZLOG
  32       DATA ZEROR,ZEROI  / 0.0D0 , 0.0D0 /
  33 C***FIRST EXECUTABLE STATEMENT  ZS1S2
  34       NZ = 0
  35       AS1 = ZABS(S1R,S1I)
  36       AS2 = ZABS(S2R,S2I)
  37       IF (S1R.EQ.0.0D0 .AND. S1I.EQ.0.0D0) GO TO 10
  38       IF (AS1.EQ.0.0D0) GO TO 10
  39       ALN = -ZRR - ZRR + LOG(AS1)
  40       S1DR = S1R
  41       S1DI = S1I
  42       S1R = ZEROR
  43       S1I = ZEROI
  44       AS1 = ZEROR
  45       IF (ALN.LT.(-ALIM)) GO TO 10
  46       CALL ZLOG(S1DR, S1DI, C1R, C1I, IDUM)
  47       C1R = C1R - ZRR - ZRR
  48       C1I = C1I - ZRI - ZRI
  49       CALL ZEXP(C1R, C1I, S1R, S1I)
  50       AS1 = ZABS(S1R,S1I)
  51       IUF = IUF + 1
  52    10 CONTINUE
  53       AA = MAX(AS1,AS2)
  54       IF (AA.GT.ASCLE) RETURN
  55       S1R = ZEROR
  56       S1I = ZEROI
  57       S2R = ZEROR
  58       S2I = ZEROI
  59       NZ = 1
  60       IUF = 0
  61       RETURN
  62       END