Hi, I’m new to Julia. I have successfully coded a runge kutta implicit method in Julia on Windows using VS code. I have built everything from scratch. However, the running time is about 5 minutes. So I want to use NLsolve to find root faster for the implicit solver My system of ODEs to solve with…

I just don’t understand why we need to provide u to nlsolve, which is just the array of variables for any particular ODE function I think I misdirected you initially to provide u to nlsolve, because I misunderstood what the mathematical problem was. I think I understand correctly now that k is a …

Hi, thanks for replying! The m is the same as the n in the f function. The m is actually in another function. I extracted the parts with m and k =ones…The k should have 6 ones in vector form: 3 ones for each variable in the original function: function fu(u, t) p = 1000 du = [ u[2], p * …

[image] Vick: I have built everything from scratch. However, the running time is about 5 minutes. So I want to use NLsolve to find root faster for the implicit solver Note that the algorithm you’re using here is not efficient, so you could optimize the code a bit but it will still be slow.

I’d like to try to see for myself.

Yes, you need to modify your code. Either change your f to match the function signature that NLsolve expects, or write another function with the right signature that calls your f with some fixed integration parameters.

Have you tried your modifications? Because it’s not working for me!

[image] Vick: t = 0, dt = 0.001, u = [2, 0]; and k = [1, 1, 1, 1, 1,1] How about you start with something lkke this? function f(u = [2, 0], t = 0, dt = 0.001, k = [1, 1, 1, 1, 1,1]) ... You probably need to rearrange the arguments.

Here’s a complete working example. My f is like your ODE integrator, with a couple parameters a,b. I then define an f!(fx, x) function that calls f with fixed parameters and has the right signature for nlsolve. julia> function f(x, a, b) [(x[1]+3)*(x[2]^3-7) + a; sin(x[2]*exp(x[1]) - b)]…

[image] John_Gibson: solution.zero In nlsolve ( … ) You separated the numbers with a semi-colon; what is the difference between a semi-colon and a comma in Julia?

I cannot do that! This is an ODE solver, The t, dt, u and the length of k changes based on the ODE function we are trying to solve. In this particular example (van der Pol equation) there are 2 variables: u[2, 0] at t =0. Therefore k takes 2 x 3 ones (n*3) …3 is the number of unknown for the Radau …

Nlsolve for find root for a system of ODE for an implicit runge kutta method

New to Julia

John_Gibson July 17, 2022, 6:49pm 8

No, I haven’t. It’s too much work for me to figure out what you mean without a mathematical specification of the problem and code that doesn’t run. Maybe I can give you a complete working version of something similar for reference.

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Nlsolve for find root for a system of ODE for an implicit runge kutta method

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