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SUBROUTINE ZS1S2(ZRR, ZRI, S1R, S1I, S2R, S2I, NZ, ASCLE, ALIM,
* IUF)
C***BEGIN PROLOGUE ZS1S2
C***REFER TO ZBESK,ZAIRY
C
C ZS1S2 TESTS FOR A POSSIBLE UNDERFLOW RESULTING FROM THE
C ADDITION OF THE I AND K FUNCTIONS IN THE ANALYTIC CON-
C TINUATION FORMULA WHERE S1=K FUNCTION AND S2=I FUNCTION.
C ON KODE=1 THE I AND K FUNCTIONS ARE DIFFERENT ORDERS OF
C MAGNITUDE, BUT FOR KODE=2 THEY CAN BE OF THE SAME ORDER
C OF MAGNITUDE AND THE MAXIMUM MUST BE AT LEAST ONE
C PRECISION ABOVE THE UNDERFLOW LIMIT.
C
C***ROUTINES CALLED AZABS,AZEXP,AZLOG
C***END PROLOGUE ZS1S2
C COMPLEX CZERO,C1,S1,S1D,S2,ZR
DOUBLE PRECISION AA, ALIM, ALN, ASCLE, AS1, AS2, C1I, C1R, S1DI,
* S1DR, S1I, S1R, S2I, S2R, ZEROI, ZEROR, ZRI, ZRR, AZABS
INTEGER IUF, IDUM, NZ
DATA ZEROR,ZEROI / 0.0D0 , 0.0D0 /
NZ = 0
AS1 = AZABS(S1R,S1I)
AS2 = AZABS(S2R,S2I)
IF (S1R.EQ.0.0D0 .AND. S1I.EQ.0.0D0) GO TO 10
IF (AS1.EQ.0.0D0) GO TO 10
ALN = -ZRR - ZRR + DLOG(AS1)
S1DR = S1R
S1DI = S1I
S1R = ZEROR
S1I = ZEROI
AS1 = ZEROR
IF (ALN.LT.(-ALIM)) GO TO 10
CALL AZLOG(S1DR, S1DI, C1R, C1I, IDUM)
C1R = C1R - ZRR - ZRR
C1I = C1I - ZRI - ZRI
CALL AZEXP(C1R, C1I, S1R, S1I)
AS1 = AZABS(S1R,S1I)
IUF = IUF + 1
10 CONTINUE
AA = DMAX1(AS1,AS2)
IF (AA.GT.ASCLE) RETURN
S1R = ZEROR
S1I = ZEROI
S2R = ZEROR
S2I = ZEROI
NZ = 1
IUF = 0
RETURN
END
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