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I have to solve the next problem: We know that a=b=c=d=1 and x0=2,y0=3,q=0.2,On the t=[0,20] interval there is no fishing but on the t=[20,40] interval there is.Show the adequate trajectory(the first one is blue,the second is red)the equilibrium points(blue and red),and the point coresponding to the transition(green).

And after solving this we should get the next graph: enter image description here

Here is my take on it,but for some reason the red trajectory is off and I don't know how to find the green point.

a = b = c = d = 1;
x0 = 2;
y0 = 3;
q = 0.2;
egyensulypont1 = 
  Graphics[{Blue, Thick, PointSize[0.02], Point[{c/d, a/b}]}];
egyensulypont2 = 
  Graphics[{Red, Thick, PointSize[0.02], 
    Point[{(c + q)/d, (a - q)/b}]}];
Soln = NDSolve[{x'[t] == a*x[t] - b*x[t]*y[t], 
   y'[t] == -c*y[t] + d*x[t]*y[t], x[0] == x0, y[0] == y0}, {x, 
   y}, {t, 0, 20}]

Solh = NDSolve[{x'[t] == (a - q)*x[t] - b*x[t]*y[t], 
   y'[t] == -(c + q)*y[t] + d*x[t]*y[t], x[0] == x0, y[0] == y0}, {x, 
   y}, {t, 20, 40}]
Show[ParametricPlot[{x[t], y[t]} /. Soln, {t, 0, 20}, 
  PlotStyle -> Blue], 
 ParametricPlot[{x[t], y[t]} /. Solh, {t, 20, 40}, 
  PlotStyle -> Red], egyensulypont1, egyensulypont2]

And this is the graph that I got: enter image description here

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    $\begingroup$ You need to use the final value of Soln as the initial condition of Solh, instead of using the same initial condition of {x0, y0}. That is also the green point. $\endgroup$
    – Chris K
    Nov 22 '21 at 19:29
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    $\begingroup$ Could you please write it down? $\endgroup$
    – Angry
    Nov 22 '21 at 19:37
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This is what Chris means:

a = b = c = d = 1;
x0 = 2;
y0 = 3;
q = 0.2;
egyensulypont1 = 
  Graphics[{Blue, Thick, PointSize[0.02], Point[{c/d, a/b}]}];
egyensulypont2 = 
  Graphics[{Red, Thick, PointSize[0.02], 
    Point[{(c + q)/d, (a - q)/b}]}];
egyensulypont3 = 
  Graphics[{Green, Thick, PointSize[0.02], 
    Point[({x[t], y[t]} /. Soln /. t -> 20)]}];
Soln = NDSolve[{x'[t] == a*x[t] - b*x[t]*y[t], 
   y'[t] == -c*y[t] + d*x[t]*y[t], x[0] == x0, y[0] == y0}, {x, 
   y}, {t, 0, 20}]

Solh = NDSolve[{x'[t] == (a - q)*x[t] - b*x[t]*y[t], 
   y'[t] == -(c + q)*y[t] + d*x[t]*y[t], 
   x[0] == ({x[t], y[t]} /. Soln /. t -> 20)[[1, 1]], 
   y[0] == ({x[t], y[t]} /. Soln /. t -> 20)[[1, 2]]}, {x, y}, {t, 20, 40}]
Show[ParametricPlot[{x[t], y[t]} /. Soln, {t, 0, 20}, 
  PlotStyle -> Blue], 
 ParametricPlot[{x[t], y[t]} /. Solh, {t, 20, 40}, 
  PlotStyle -> Red], egyensulypont1, egyensulypont2, egyensulypont3]

enter image description here

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    $\begingroup$ Thank you so much! $\endgroup$
    – Angry
    Nov 22 '21 at 20:49

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