today, i stuty a differential equation, firsy, i conver the equation from cartesian to polar as follow:
restart:
sys := diff(x(t), t) = -y(t)+x(t)*(1-2*x(t)^2-3*y(t)^2), diff(y(t), t) = x(t)+y(t)*(1-2*x(t)^2-3*y(t)^2):
then using transformation x=r*cos(theta),y=r*sin(theta) convert it to polar coordinate,and get:
sys1:=diff(r(t), t) = r(t)*(1-2*r(t)^2-r(t)^2*sin(theta(t))^2), diff(theta(t), t) = 1
then plot...