Hi,

Thank you very much for your idea in previous discussion: this is the link.

http://www.mapleprimes.com/questions/202744-Calculs-Using-Maple

I asked how can I plot the phase portrait of this system

Sys1 := {diff(r(t),t) = r(t)^2*sin(theta(t)), diff(theta(t),t) = -r(t)^2*(-2*cos(theta(t))^2+1)};

I get this answer:

1) For me it's not necessary to give an initial condition to plot the pharse portrait.

2) What is the line in this phase portrait.

3) Here, I try a second method using Matlab code, I get:

and this is the code:

[r, theta] = meshgrid(0:pi/4:2*pi, 0:pi/4:2*pi);

rdot =r.^2.*sin(theta) ;

thetadot = r.^2.*(1-2*sin(theta).^2);

quiver(r,theta,rdot,thetadot)

xlabel('r')

ylabel('theta')

the arrow in the phase portrait are not the same.

Can someone give me more clear information about this problem.

Many thinks.