amos/zsqrt.f

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