amos/zs1s2.f

   1       SUBROUTINE ZS1S2(ZRR, ZRI, S1R, S1I, S2R, S2I, NZ, ASCLE, ALIM,
   2      * IUF)
   3 C***BEGIN PROLOGUE  ZS1S2
   4 C***REFER TO  ZBESK,ZAIRY
   5 C
   6 C     ZS1S2 TESTS FOR A POSSIBLE UNDERFLOW RESULTING FROM THE
   7 C     ADDITION OF THE I AND K FUNCTIONS IN THE ANALYTIC CON-
   8 C     TINUATION FORMULA WHERE S1=K FUNCTION AND S2=I FUNCTION.
   9 C     ON KODE=1 THE I AND K FUNCTIONS ARE DIFFERENT ORDERS OF
  10 C     MAGNITUDE, BUT FOR KODE=2 THEY CAN BE OF THE SAME ORDER
  11 C     OF MAGNITUDE AND THE MAXIMUM MUST BE AT LEAST ONE
  12 C     PRECISION ABOVE THE UNDERFLOW LIMIT.
  13 C
  14 C***ROUTINES CALLED  AZABS,AZEXP,AZLOG
  15 C***END PROLOGUE  ZS1S2
  16 C     COMPLEX CZERO,C1,S1,S1D,S2,ZR
  17       DOUBLE PRECISION AA, ALIM, ALN, ASCLE, AS1, AS2, C1I, C1R, S1DI,
  18      * S1DR, S1I, S1R, S2I, S2R, ZEROI, ZEROR, ZRI, ZRR, AZABS
  19       INTEGER IUF, IDUM, NZ
  20       DATA ZEROR,ZEROI  / 0.0D0 , 0.0D0 /
  21       NZ = 0
  22       AS1 = AZABS(S1R,S1I)
  23       AS2 = AZABS(S2R,S2I)
  24       IF (S1R.EQ.0.0D0 .AND. S1I.EQ.0.0D0) GO TO 10
  25       IF (AS1.EQ.0.0D0) GO TO 10
  26       ALN = -ZRR - ZRR + DLOG(AS1)
  27       S1DR = S1R
  28       S1DI = S1I
  29       S1R = ZEROR
  30       S1I = ZEROI
  31       AS1 = ZEROR
  32       IF (ALN.LT.(-ALIM)) GO TO 10
  33       CALL AZLOG(S1DR, S1DI, C1R, C1I, IDUM)
  34       C1R = C1R - ZRR - ZRR
  35       C1I = C1I - ZRI - ZRI
  36       CALL AZEXP(C1R, C1I, S1R, S1I)
  37       AS1 = AZABS(S1R,S1I)
  38       IUF = IUF + 1
  39    10 CONTINUE
  40       AA = DMAX1(AS1,AS2)
  41       IF (AA.GT.ASCLE) RETURN
  42       S1R = ZEROR
  43       S1I = ZEROI
  44       S2R = ZEROR
  45       S2I = ZEROI
  46       NZ = 1
  47       IUF = 0
  48       RETURN
  49       END