- #1

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## Main Question or Discussion Point

Hello everyone.

I have a second order ODE, which I reduced to two first order ODEs, and solved with ode45. Now I want to evaluate the solution and the derivative at certain points (specifically, I want to plot the Poincare section).

So, what I did is this:

Where:

But this is clearly wrong. I know the solution should be periodic, but I get something random. I think it's the plotting I'm doing incorrectly. Any help appreciated.

I have a second order ODE, which I reduced to two first order ODEs, and solved with ode45. Now I want to evaluate the solution and the derivative at certain points (specifically, I want to plot the Poincare section).

So, what I did is this:

Code:

```
>> sol=ode45(@...);
>> xint=a:b;
>> [Sxint, Spxint]=deval(sol, xint);
>> A=[Sxint, Spxint];
>> plot(Sxint, Spxint,'o')
```

*xint*is the points at which I want to evaluate the solution.*Sxint*is the value of the solution at the points xint.*Spxint*is the value of the derivative at those points.But this is clearly wrong. I know the solution should be periodic, but I get something random. I think it's the plotting I'm doing incorrectly. Any help appreciated.