1      SUBROUTINE ZSQRT(AR, AI, BR, BI)
2C***BEGIN PROLOGUE  ZSQRT
3C***REFER TO  ZBESH,ZBESI,ZBESJ,ZBESK,ZBESY,ZAIRY,ZBIRY
4C
5C     DOUBLE PRECISION COMPLEX SQUARE ROOT, B=CSQRT(A)
6C
7C***ROUTINES CALLED  ZABS
8C***END PROLOGUE  ZSQRT
9      DOUBLE PRECISION AR, AI, BR, BI, ZM, DTHETA, DPI, DRT
10      DOUBLE PRECISION ZABS
11
12      DATA DRT , DPI / 7.071067811865475244008443621D-1,
13     1                 3.141592653589793238462643383D+0/
14      ZM = ZABS(CMPLX(AR,AI,kind=KIND(1.0D0)))
15      ZM = DSQRT(ZM)
16      IF (AR.EQ.0.0D+0) GO TO 10
17      IF (AI.EQ.0.0D+0) GO TO 20
18      DTHETA = DATAN(AI/AR)
19      IF (DTHETA.LE.0.0D+0) GO TO 40
20      IF (AR.LT.0.0D+0) DTHETA = DTHETA - DPI
21      GO TO 50
22   10 IF (AI.GT.0.0D+0) GO TO 60
23      IF (AI.LT.0.0D+0) GO TO 70
24      BR = 0.0D+0
25      BI = 0.0D+0
26      RETURN
27   20 IF (AR.GT.0.0D+0) GO TO 30
28      BR = 0.0D+0
29      BI = DSQRT(DABS(AR))
30      RETURN
31   30 BR = DSQRT(AR)
32      BI = 0.0D+0
33      RETURN
34   40 IF (AR.LT.0.0D+0) DTHETA = DTHETA + DPI
35   50 DTHETA = DTHETA*0.5D+0
36      BR = ZM*DCOS(DTHETA)
37      BI = ZM*DSIN(DTHETA)
38      RETURN
39   60 BR = ZM*DRT
40      BI = ZM*DRT
41      RETURN
42   70 BR = ZM*DRT
43      BI = -ZM*DRT
44      RETURN
45      END
46