var('pred prey t')
LV= desolve_system_rk4([pred*(1-prey),-prey*(1-pred)],[pred,prey],ics=[0,.5,2.],ivar=t,end_points=50)
# phase plot
line([e[1:] for e in LV])
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line(LV) #.show(viewer='tachyon')
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# plot how predator and prey population varies over time
predt = line([e[:-1] for e in LV],linestyle=':')
preyt = line([e[::2] for e in LV])
predt + preyt
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