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I have faced with problem of not completely correct phase trajectories of ODE, which was produced by StreamPlot.

Next following code ends up with this plot:

 f[x_, y_] = x/(-x - 2 y)
 StreamPlot[{1, f[x, y]}, {x, -10, 10}, {y, -10, 10}, Frame -> False, 
 Axes -> True, AspectRatio -> 1/GoldenRatio, StreamStyle -> "PinDart"]

Phase trajectoies

Unfortunately, streamlines are interrupting on line $y = -\dfrac{x}{2}$, when they should go as they were. By this, I mean that there should be only clockwise directed streamlines.

Is there any straightforward way to deal with this issue? Thank you!

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    $\begingroup$ But are you sure this is not right? y=-x/2 is where the direction of tangent changes. Just plug in x=-1, y=1 above this line and x=-1 y=0 below this line, and you get the slope of +1 for the former case and -1 for the latter case. $\endgroup$ – MathX Mar 19 '16 at 0:38
  • $\begingroup$ What is the ODE you're trying to plot the trajectories of? $\endgroup$ – Rahul Mar 19 '16 at 1:34
  • $\begingroup$ @Rahul This one: $ y' = \dfrac{x}{-x-2y} $ $\endgroup$ – RuD_wow Mar 20 '16 at 13:44
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    $\begingroup$ @RuD_wow: The denominator of that ODE cannot be $y = -\dfrac{x}{2}$, else the ODE is undefined. This is a line as shown in the phase portrait, so the phase portrait you show above is is correct. $\endgroup$ – Moo Mar 20 '16 at 14:22
  • $\begingroup$ @Moo But, what if we will calculate the eigenvalues of system $\dfrac{dx}{dt}, \dfrac{dy}{dt} $ and, considering their Real values, make a conclusion that DE' s phase trajectories are logarithmic clockwise spiral? $\endgroup$ – RuD_wow Mar 21 '16 at 18:12
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Will this do the trick:

Clear[f]
f[x_, y_] := {1, x/(-x - 2 y)};
f[x_, y_] /; y > -x/2 := -{1, x/(-x - 2 y)};
StreamPlot[f[x, y], {x, -10, 10}, {y, -10, 10}, Frame -> False, 
 Axes -> True, AspectRatio -> 1/GoldenRatio, StreamStyle -> "PinDart"]
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  • $\begingroup$ Thank you a lot! I was thining about the same tricky way, but stucked on how to implement it. $\endgroup$ – RuD_wow Mar 20 '16 at 14:26

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