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# Incomplete ellipse due to StreamPlot command

2018-01-24 07:03:43

I wrote the following in Mathematica code but the result was not the expected. As you can see it seems like just half of an elipse is there, what can I do to see the complete elipse?

StreamPlot[{2 A (1 - .0001 A) - .01 A L, -.5 L + .0001 A L}, {A, 0,

1000}, {L, 0, 1000}]

Well, as David already pointed out you haven't explained why you think it should resemble an ellipse. You can take an ellipse equation and calculate the partial derivatives and rotate them by 90 degrees:

ellipse = x^2/8 + y^2/2 - 1;

RotationMatrix[Pi/2].(D[ellipse, #] & /@ {x, y})

StreamPlot[%, {x, -10, 10}, {y, -10, 10}]

Since your working with DE anyway, you might know that this is equivalent to taking the Curl

Curl[ellipse, {x, y}]

(* {-y, x/4} *)

Now, do the same for the generalized ellipse I gave in this answer in expr and calculate the curl. If you expand everything, you see that there is no term that contains x^2 or x*y. Your example however contains such terms. Again, why do

• Well, as David already pointed out you haven't explained why you think it should resemble an ellipse. You can take an ellipse equation and calculate the partial derivatives and rotate them by 90 degrees:

ellipse = x^2/8 + y^2/2 - 1;

RotationMatrix[Pi/2].(D[ellipse, #] & /@ {x, y})

StreamPlot[%, {x, -10, 10}, {y, -10, 10}]

Since your working with DE anyway, you might know that this is equivalent to taking the Curl

Curl[ellipse, {x, y}]

(* {-y, x/4} *)

Now, do the same for the generalized ellipse I gave in this answer in expr and calculate the curl. If you expand everything, you see that there is no term that contains x^2 or x*y. Your example however contains such terms. Again, why do you think this should be an ellipse?

2018-01-24 08:25:02