C ++********************************************************************
C                                                                      *
C                                                                      *
C                                                                      *
C **********************************************************************
C=* FROM: SPIDER - MODULAR IMAGE PROCESSING SYSTEM.   AUTHOR: J.FRANK  *
C=* Copyright (C) 1985-2005  Health Research Inc.                      *
C=*                                                                    *
C=* HEALTH RESEARCH INCORPORATED (HRI),                                *   
C=* ONE UNIVERSITY PLACE, RENSSELAER, NY 12144-3455.                   *
C=*                                                                    *
C=* Email:  spider@wadsworth.org                                       *
C=*                                                                    *
C=* This program is free software; you can redistribute it and/or      *
C=* modify it under the terms of the GNU General Public License as     *
C=* published by the Free Software Foundation; either version 2 of the *
C=* License, or (at your option) any later version.                    *
C=*                                                                    *
C=* This program is distributed in the hope that it will be useful,    *
C=* but WITHOUT ANY WARRANTY; without even the implied warranty of     *
C=* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU  *
C=* General Public License for more details.                           *
C=*                                                                    *
C=* You should have received a copy of the GNU General Public License  *
C=* along with this program; if not, write to the                      *
C=* Free Software Foundation, Inc.,                                    *
C=* 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.      *
C=*                                                                    *
C **********************************************************************
C                                                                      *
C                                                                      *
C                                                                      *
C  PURPOSE:                                                            *
C                                                                      *
C  PARAMETERS:                                                         *
C                                                                      *
C        0         2         3         4         5         6         7 *
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C***********************************************************************

      SUBROUTINE INVBT(X,P,Q,IFAULT,BETAIN)

      IMPLICIT  REAL*8  (A-H,O-Z)
      LOGICAL*1 INDEX
      DATA  ACU/0.1D0/

      BETAIN=X
C
C
      IFAULT=1
      IF(P.LE.0.0.OR.Q.LE.0.0)RETURN
      IFAULT=2
      IF(X.LT.0.0.OR.X.GT.1.0)RETURN
      IFAULT=0
C            IF(X.LT.ACU.OR.X.GT.(1.0-ACU))  RETURN
C
C
      PSQ=P+Q
      CX=1.0-X
      IF(P.GE.PSQ*X)GO TO 1
      XX=CX
      CX=X
      PP=Q
      QQ=P
      INDEX=.TRUE.
      GO TO 2
    1 XX=X
      PP=P
      QQ=Q
      INDEX=.FALSE.
    2 TERM=1.0
      AI=1.0
      BETAIN=1.0
      WNS=QQ+CX+PSQ
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      RX=XX/CX
    3 TEMP=QQ-AI
      IF(ABS(WNS).LT.ACU)   RX=XX
    4 TERM=TERM*TEMP*RX/(PP+AI)
      BETAIN=BETAIN+TERM
      TEMP=ABS(TERM)
      IF(TEMP.LE.ACU.AND.TEMP.LE.ACU*BETAIN)   GO TO 5
      AI=AI+1.0
      WNS=WNS-1.
      IF(WNS.GE.0.)GO TO 3
      TEMP=PSQ
      PSQ=PSQ+1.0
      GO TO 4
C
C
    5 BET1=GAM(P)+GAM(Q)-GAM(P+Q)
      BET1=PP*DLOG(XX)+(QQ-1.0)*DLOG(CX)-BET1
      BET1=DSIGN(1.0D0,BET1)*DMIN1(709.0D0,DABS(BET1))
      BETAIN=BETAIN*DEXP(BET1)/PP
      IF(INDEX)   BETAIN=1.0-BETAIN
      END