program duffing

      double precision x(2),t,dt,tfinish
      double precision f1,f2
      integer n,steps

      dt = 0.0001
      tfinish = 50
      steps = tfinish/dt+1
      print *, steps
      t = 0
      x(1) = 1
      x(2) = 1

      open(unit=13,file="data.d")

      do 10 n=1,steps
         print *, t
         write(13,*) t,x(1),x(2)
         f1 = x(2)
         f2 = cos(t) - 0.1*x(2)+x(1)-x(1)**3
         x(1) = x(1) + f1*dt
         x(2) = x(2) + f2*dt
         t = t + dt

 10   continue

      end