kepler.f

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      SUBROUTINE kepler(I,DTI)
*
*
*       Step reduction of hierarchy.
*       ----------------------------
*
*
      include 'common6.h'
*
*
*       Only examine hierarchical configurations (NP < 5 OR STEP < DTMIN).
      i1 = 2*(i - n) - 1
      IF (list(1,i1).GT.4.AND.dti.GT.dtmin) go to 30
*
*       Set list membership & Kepler period.
      np1 = list(1,i1) + 1
      semi = -0.5d0*body(i)/h(i-n)
      tk = twopi*semi*sqrt(semi/body(i))
*
*       Consider the most dominant perturbers having comparable step to c.m.
      DO 20 l = 2,np1
          j = list(l,i1)
          IF (step(j).GT.4.0*step(i)) go to 20 
          rij2 = (x(1,i) - x(1,j))**2 + (x(2,i) - x(2,j))**2 +
     &                                  (x(3,i) - x(3,j))**2
          rij = sqrt(rij2)
          dt2 = 0.1*rij*sqrt(etai*rij/(body(i) + body(j)))
          dt = 0.25d0*sqrt(etai)*rij*tk/semi
*       Compare predicted c.m. step with conservative Kepler expressions.
          dt = min(dt,dt2)
          IF (dti.LT.dt) go to 20 
          dti = dt
*
*       Check whether to reduce step of dominant perturber.
          IF (step(j).LT.dt) go to 20
          IF (t0(j).EQ.time) THEN
              step(j) = 0.5d0*step(j)
              tnew(j) = t0(j) + step(j)
              go to 20
          END IF
          jclose = j
   20 CONTINUE
*
   30 RETURN
*
      END