fdisk.f

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      SUBROUTINE fdisk(XI,XIDOT,FM,FD)
*
*
*       Miyamoto disk force.
*       --------------------
*
      IMPLICIT REAL*8  (a-h,o-z)
      common/galaxy/ gmg,rg(3),vg(3),fg(3),fgd(3),tg,
     &               omega,disk,a,b,v02,rl2,gmb,ar,gam,zdum(7)
      REAL*8  xi(3),xidot(3),fm(3),fd(3)
*
*
*       Obtain force & derivative for global variables XI & XIDOT.
      r2 = xi(1)**2 + xi(2)**2
      bz = sqrt(b**2 + xi(3)**2)
      az = sqrt(r2 + (a + bz)**2)
      az3 = disk/az**3
*       Note missing square root sign in (b^2 + z^2) of Book eq. (8.52).
      azdot = xi(1)*xidot(1) + xi(2)*xidot(2) +
     &                         (a + bz)*xi(3)*xidot(3)/bz
      fm(1) = -az3*xi(1)
      fm(2) = -az3*xi(2)
      fm(3) = -az3*xi(3)*(a + bz)/bz
*     RDOT = (XI(1)*XIDOT(1) + XI(2)*XIDOT(2))/SQRT(R2)
      fd(1) = -az3*(xidot(1) - 3.0*azdot*xi(1)/az**2)
      fd(2) = -az3*(xidot(2) - 3.0*azdot*xi(2)/az**2)
*       Note wrong sign in first term of eq. (8.52) (see printer errata).
      y1 = 3.0*(a + bz)*xi(3)*azdot/(az**2*bz)
*       Note printer mistake of dZ/dt in denominator (see errata).
      y2 = (a*b**2 + bz**3)*xidot(3)/bz**3
      fd(3) = az3*(y1 - y2)
*       Refer to Miyamoto & Nagai (PASJ 27, 533) and Book eq. (8.52).
*
      RETURN
*
      END