$ xmaple &
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> ?DEplot
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> with(DEtools):
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> DEplot([diff(x0(t),t) - x1 = 0, diff(x1(t),t) + x0 = 0],
       [t,x0,x1], 0..30, {[0,0,0.4]}, stepsize=0.07);
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> DEplot([diff(x0(t),t) - x1 = 0, diff(x1(t),t) + sin(x0) = 0],
       [t,x0,x1], 0..30, {[0,0,1.9]}, stepsize=0.07);
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> DEplot([diff(theta(t), t) - omega = 0, diff(omega(t), t) + sin(theta) = 0],
       [t,theta,omega], 0..12, {[0,0,1.0], [0,0,0.5], [0,2*Pi,1.0], 
       [0,2*Pi,0.5], [0,0,1.7], [0,2*Pi,1.7],[0,-Pi,0.01], [0,Pi,0.01], 
       [0, 3*Pi, -0.01],[0,Pi, -0.01], [0,-Pi, 2.3], [0, 3*Pi, -2.3], 
       [0, -Pi, 1.07], [0, 3*Pi, -1.07]}, stepsize=0.1, theta=-Pi..3*Pi, 
       scene=[t, theta,omega],title=`Phase portrait,plane pendulum, vs t`);
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> DEplot([diff(theta(t), t) - omega = 0, diff(omega(t), t) + sin(theta) = 0],
       [t,theta,omega], 0..12, {[0,0,1.0]}, scene=[t,omega]);.
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> DEplot([diff(theta(t), t) - omega = 0, diff(omega(t), t) + sin(theta) = 0], 
       [t,theta,omega], 0..12, {[0,0,1.0], [0,0,0.5], [0,2*Pi,1.0], 
       [0,2*Pi,0.5], [0,0,1.7], [0,2*Pi,1.7],[0,-Pi,0.01], [0,Pi,0.01], 
       [0, 3*Pi, -0.01],[0,Pi, -0.01], [0,-Pi, 2.3], [0, 3*Pi, -2.3], 
       [0, -Pi, 1.07], [0, 3*Pi, -1.07]}, stepsize=0.1, theta=-Pi..3*Pi, 
       scene=[theta,omega],title=`Phase portrait for the plane pendulum`);
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> DEplot([diff(x0(t), t)-x1=0, diff(x1(t),  t) + sin(x0)=0], [t, x0, x1], 
       0..30,{[0,0,2.0]}, stepsize=0.07);
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