FUNCTION rtflsp(func,x1,x2,xacc)
      INTEGER MAXIT
      REAL rtflsp,x1,x2,xacc,func
      EXTERNAL func
      PARAMETER (MAXIT=30)
      INTEGER j
      REAL del,dx,f,fh,fl,swap,xh,xl
      fl=func(x1)
      fh=func(x2)
      if(fl*fh.gt.0.) pause 'root must be bracketed in rtflsp'
      if(fl.lt.0.)then
        xl=x1
        xh=x2
      else
        xl=x2
        xh=x1
        swap=fl
        fl=fh
        fh=swap
      endif
      dx=xh-xl
      do 11 j=1,MAXIT
        rtflsp=xl+dx*fl/(fl-fh)
        f=func(rtflsp)
        if(f.lt.0.) then
          del=xl-rtflsp
          xl=rtflsp
          fl=f
        else
          del=xh-rtflsp
          xh=rtflsp
          fh=f
        endif
        dx=xh-xl
        if(abs(del).lt.xacc.or.f.eq.0.)return
11    continue
      pause 'rtflsp exceed maximum iterations'
      END
C  (C) Copr. 1986-92 Numerical Recipes Software .