Question: Phase diagrams with a system of more then two differential equations

Hi, I have a system of four first order, nonlinear, differential equations with four dependent variables.  The variable F(t) can be set equal to zero or to some known function. 


A*(diff(xx(t), t)) + epsilon*xx(t) + B*(diff(phi(t), t))*cos(theta(t)) + C*phi(t)^2*sin(theta(t)) = F(t),

D*(diff(xx(t), t))*cos(theta(t)) + E*diff(phi(t), t)) + F*sin(theta(t)) = 0,

xx(t) = diff(x(t), t),

phi(t) = diff(theta(t), t)


I want to plot two dependent variables against each other in a phase plot.  Plotting three dependent variables in a 3D plot would also be useful. I have looked at the various phase plot methods including dfield, phaseportrait and DEplot.  However, every example I have seen involves only two system equations and two dependent variables.  Is there a way to plot two dependent varaibles in a system of four differential equations?


Please Wait...