c exponential growth //example population growthdP/dt = lambda*P
c  e.g.   lambda ~ 1 /years    lambda ~ 1 /time
      real lambda / .02 /
      kount=5
      kprint=kount
      p0=1.
      tscale=1./lambda
      dt=tscale/100.
      tf=2.*tscale
      nstep=int(tf/dt)
      print 100 , 0., p0
      do 10 i=1,nstep
      t=dt*float(i)
      p1=p0+dt*lambda*p0
      if(i.eq.kprint)then
      print 100 , t , p1
      kprint=kprint + kount
      endif
      p0=p1
10    continue
100   format(2x,'t,Poblacion(Billones)=',2(4x,e11.4))
      stop
      end