Second derivative of solution from ode45

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e_frog
e_frog 2020-11-4
评论： e_frog 2020-11-5
I have a system of 2 second order ODEs, which i converted to a system of fist order ODEs, which ode45 (or ode15s) needs as an input
my system input looks like this:
as an output i get the vector , but I need the derivatives and . How can i get those?
I tried using the gradient function as suggested by Star Strider in this question: How to output derivative from ODE 45 but unfortunately that did not help.
Thanks in advance!

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Bjorn Gustavsson
Bjorn Gustavsson 2020-11-4
编辑：Bjorn Gustavsson 2020-11-4
After integrating your ODE-system you will have for some , and you have the function for your ODE-equations, f(t,X), one way to calculate the accelerations at times will be to simply call:
dxd2xdyd2ydt(i,:) = f(t_i,X(i,:));
Typically I use a regularly space array for the time in the call to ode45 and use the gradient-function, but the above should work - that is after all what we expect odeNN: to give us a solution X where that is true...
HTH
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James Tursa
James Tursa 2020-11-4
编辑：James Tursa 2020-11-4
If the ode_exo_ho( ) function is vectorized (meaning it will accept vectors and properly calculate a vector result), then yes it is just this easy. If ode_exo_ho( ) is not vectorized, then you would have to call it in a loop, but still pretty easy.
The ease of vectorizing your function depends on what it is doing. Sometimes it is just a simple matter of replacing the operators * and / and ^ with their element-wise counterparts .* and ./ and .^

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