I'm trying to plot solutions to two dimensional ordinary differential equations. I found that Sage makes it easy to plot a vector field and, using ode_solver(), I can plot solutions on top of the vector field by specifying an initial condition y_0 and some range of time to run (t_span).
However, this method I'm using seems to be quite ad hoc, as I have to choose the right initial conditions and time span / know a lot about my system in order to plot a nice picture. Let's make this more concrete:
Say I want to draw a nice phase portrait for
$\dot{x} = -y$
$\dot{y} = -x$
First I generate the vector field:
var('x y')
VF=plot_vector_field([-y,-x],[x,-2,2],[y,-2,2])
Then I use ode_solver() to plot solutions with initial conditions going around the unit circle:
T = ode_solver()
T.function=lambda t,y:[-y[1],-y[0]]
solutions = []
c = circle((0,0), 1, rgbcolor=(1,0,1))
for k in range(0,8):
T.ode_solve(y_0=[cos(k*pi/4),sin(k*pi,t_span=[0,1],num_points=100)
solutions.append(line([p[1] for p in T.solution]))
This generates the following picture:
But if I change run the system for one more unit of time (set t_span=[0,2]), the picture gets skewed:
This makes sense, of course, because the magnitude of the vectors along $y=-x$ get big as you get further away from the origin. Similarly, the trajectory along $y=x$ has trouble getting to the origin because the magnitude of those vectors get very small. Which all makes me think there's a better way to do this. Thoughts?
