franzidi

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These are questions asked by franzidi

Hello,

I want to draw a 2d directionfield with arrows for a differential equation system with three equations. I have tried the following: 

mu := 1/(365*80);
alpha := 20;
psi := 100;
varphi := 2;
K := 100;
theta := 0.5;
sigma := 0.001;

with(DEtools);
with(PDEtools);
beta := 0.3;
des := diff(S(t), t) = (psi + mu)*K - beta*S(t)*In(t) - (mu + varphi + psi)*S(t) + (theta - psi)*V(t) - psi*In(t), diff(V(t), t) = varphi*S(t) - sigma*beta*In(t)*V(t) - (mu + theta)*V(t), diff(In(t), t) = beta*S(t)*In(t) + sigma*beta*In(t)*V(t) - (mu + alpha)*In(t);

phaseportrait([des], [S(t), V(t), In(t)], t = 0 .. 100, [[S(0) = 70, V(0) = 0, In(0) = 30]], V = 0 .. 100, In = 0 .. 100, stepsize = 0.05, arrows = medium, scene = [V(t), In(t)], linecolour = sin(t*Pi/2))

The problem is, Maple plots only the curve for the initial values and not the arrows of the direction field. How can I solve this problem?

Tanks

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