# Modelling & Simulations

**URL:** https://discourse.julialang.org/c/domain/models/21.md?page=31

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**Page:** 32

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## [RigidBodySim.jl pkg issues](https://discourse.julialang.org/t/rigidbodysim-jl-pkg-issues/104073)

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**Author:** [@jondavis847](https://discourse.julialang.org/u/jondavis847)\
**Replies:** 0\
**Last updated:** [September 20, 2023, 6:16pm UTC](https://discourse.julialang.org/t/rigidbodysim-jl-pkg-issues/104073 "2023-09-20T18:16:10Z")

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I’m starting out with RigidBodyDynamics.jl and can successfully create and sim models in its environment. I’d like to try RigidBodySim.jl, mostly for the interface to DifferentialEquations.jl for its problem types and so…

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## [Instabilities on a simple dataset when training a NODE on it](https://discourse.julialang.org/t/instabilities-on-a-simple-dataset-when-training-a-node-on-it/103916)

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**Author:** [@jarroyoe](https://discourse.julialang.org/u/jarroyoe)\
**Replies:** 5\
**Last updated:** [September 20, 2023, 4:50pm UTC](https://discourse.julialang.org/t/instabilities-on-a-simple-dataset-when-training-a-node-on-it/103916 "2023-09-20T16:50:28Z")

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I have an extremely simple dataset that for some reason keeps outputing instabilities when trying to train a NODE algorithm on it, no matter how low the tolerance is set or what solver I choose. I train it twice, once wi…

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## [Programmatically creating parameters in ModelingToolkit.jl leads to unexpected outcome](https://discourse.julialang.org/t/programmatically-creating-parameters-in-modelingtoolkit-jl-leads-to-unexpected-outcome/104063)

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**Author:** [@Datseris](https://discourse.julialang.org/u/Datseris)\
**Replies:** 5\
**Last updated:** [September 20, 2023, 2:03pm UTC](https://discourse.julialang.org/t/programmatically-creating-parameters-in-modelingtoolkit-jl-leads-to-unexpected-outcome/104063 "2023-09-20T14:03:06Z")

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I am trying to create programmatically a parameter (with @parameter) with ModelingToolkit.jl. While I am succeeding, the outcome is not exactly what I wish for. Here are the two examples: @variables t # indep…

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## [Angular Momentum Commutators In Symbolics.jl](https://discourse.julialang.org/t/angular-momentum-commutators-in-symbolics-jl/103493)

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**Author:** [@wes648](https://discourse.julialang.org/u/wes648)\
**Replies:** 2\
**Last updated:** [September 20, 2023, 1:21pm UTC](https://discourse.julialang.org/t/angular-momentum-commutators-in-symbolics-jl/103493 "2023-09-20T13:21:59Z")

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Could I get some help defining the angular momentum commutators symbolicaly in Julia? I’ve tried using SymbolicUtils @acrules to define Nx\*Ny - Ny\*Nx =\> im\*Nz but then when I run Nx\*Ny - Ny\*Nx it returns zero. I also tr…

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## [\`tstop\` error with Piecewise Deterministic Markov Processes](https://discourse.julialang.org/t/tstop-error-with-piecewise-deterministic-markov-processes/104031)

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**Author:** [@cdawg](https://discourse.julialang.org/u/cdawg)\
**Replies:** 7\
**Last updated:** [September 20, 2023, 11:10am UTC](https://discourse.julialang.org/t/tstop-error-with-piecewise-deterministic-markov-processes/104031 "2023-09-20T11:10:53Z")

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I’m asking for help for an annoying, (ironically) non-deterministic error when I solve Jumpy-ODE’s (i.e. “Piecewise Deterministic Markov Processes”). It tends to start happening as I increase the number of processes and …

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## [MTK & efficient formulation](https://discourse.julialang.org/t/mtk-efficient-formulation/103971)

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**Author:** [@BLI](https://discourse.julialang.org/u/BLI)\
**Replies:** 1\
**Last updated:** [September 18, 2023, 12:41pm UTC](https://discourse.julialang.org/t/mtk-efficient-formulation/103971 "2023-09-18T12:41:41Z")

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I have a file with a number of models (ModelingToolkit-MTK, functions, etc.) where, say, temperature, is a parameter. I want to explore what happens for different parameter values (e.g., temperature). To make sure that I…

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## [Simulink-like block diagram editor for Julia?](https://discourse.julialang.org/t/simulink-like-block-diagram-editor-for-julia/78391)

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**Author:** [@pitsianis](https://discourse.julialang.org/u/pitsianis)\
**Replies:** 8\
**Last updated:** [September 17, 2023, 7:25pm UTC](https://discourse.julialang.org/t/simulink-like-block-diagram-editor-for-julia/78391 "2023-09-17T19:25:06Z")

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Is there any effort to build a block diagram editor for Julia to use with systems like ModelingToolkit.jl and Causal.jl?

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## [Error with remake function in ODE Problems](https://discourse.julialang.org/t/error-with-remake-function-in-ode-problems/103902)

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**Author:** [@Nick1](https://discourse.julialang.org/u/Nick1)\
**Replies:** 0\
**Last updated:** [September 15, 2023, 3:19pm UTC](https://discourse.julialang.org/t/error-with-remake-function-in-ode-problems/103902 "2023-09-15T15:19:20Z")

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Hi everyone, I am importing an SBML file and try to change the initial condition and remake the problem but I get the following error: ERROR: ArgumentError: This problem does not support symbolic maps with remake, i.e.…

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## [ODE problem error](https://discourse.julialang.org/t/ode-problem-error/103876)

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**Author:** [@River\_He](https://discourse.julialang.org/u/River_He)\
**Replies:** 2\
**Last updated:** [September 15, 2023, 2:41pm UTC](https://discourse.julialang.org/t/ode-problem-error/103876 "2023-09-15T14:41:01Z")

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Hello everyone, I’m currently working on solving an ODE problem using Julia, but I’ve run into some issues. Before diving into possible modifications to my model, I’d like to first rule out any potential problems arisin…

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## [PDE discretization error in MethodOfLines.jl](https://discourse.julialang.org/t/pde-discretization-error-in-methodoflines-jl/78566)

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**Author:** [@RickDW](https://discourse.julialang.org/u/RickDW)\
**Replies:** 23\
**Last updated:** [September 15, 2023, 11:27am UTC](https://discourse.julialang.org/t/pde-discretization-error-in-methodoflines-jl/78566 "2023-09-15T11:27:58Z")

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Hi all, I’m currently working on implementing a relatively simple reaction-diffusion-convection model of a 2D hydrogen flame. The PDESystem is created successfully, but the discretization step using methodoflines.jl fail…

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## [Examples for port-Hamiltonian modelling and control of physical systems?](https://discourse.julialang.org/t/examples-for-port-hamiltonian-modelling-and-control-of-physical-systems/103787)

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**Author:** [@ufechner7](https://discourse.julialang.org/u/ufechner7)\
**Replies:** 3\
**Last updated:** [September 14, 2023, 5:52pm UTC](https://discourse.julialang.org/t/examples-for-port-hamiltonian-modelling-and-control-of-physical-systems/103787 "2023-09-14T17:52:47Z")

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Are there any examples in Julia how to model and control port-Hamiltonian systems? See: https://www.math.rug.nl/arjan/DownloadLectures/Benelux08.pdf Can it be done with ModelingToolkit, or are there better ways?

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## [Optimization of parameters of a dynamic problem with ModelingToolkit](https://discourse.julialang.org/t/optimization-of-parameters-of-a-dynamic-problem-with-modelingtoolkit/103784)

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**Author:** [@BambOoxX](https://discourse.julialang.org/u/BambOoxX)\
**Replies:** 3\
**Last updated:** [September 14, 2023, 2:17pm UTC](https://discourse.julialang.org/t/optimization-of-parameters-of-a-dynamic-problem-with-modelingtoolkit/103784 "2023-09-14T14:17:42Z")

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Hello all ! I have a nonlinear ODE problem that I set up with ModelingToolkit (great job with this awesome package !), for which I can compute some steady-state (with SteadyStateProblem) or transient solutions (with ODE…

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## [@register\_symbolic with vector variables](https://discourse.julialang.org/t/register-symbolic-with-vector-variables/103419)

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**Author:** [@jondavis847](https://discourse.julialang.org/u/jondavis847)\
**Replies:** 10\
**Last updated:** [September 14, 2023, 4:00pm UTC](https://discourse.julialang.org/t/register-symbolic-with-vector-variables/103419 "2023-09-14T16:00:31Z")

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It seems that @register\_symbolic will successfully stop tracing on scalar variables, but continues to trace with vector variables. Anyway currently to get register\_symbolic to stop tracing on vector variables? julia\> f(…

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## [Adding and updating a scalar to the Integerator Cache in DifferentialEquations.jl](https://discourse.julialang.org/t/adding-and-updating-a-scalar-to-the-integerator-cache-in-differentialequations-jl/103805)

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**Author:** [@cdawg](https://discourse.julialang.org/u/cdawg)\
**Replies:** 12\
**Last updated:** [September 14, 2023, 3:28pm UTC](https://discourse.julialang.org/t/adding-and-updating-a-scalar-to-the-integerator-cache-in-differentialequations-jl/103805 "2023-09-14T15:28:02Z")

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Background. I have a nice stochastic jump-piecewise-ODE behaving as it should. Thanks @ChrisRackauckas and team! Basically this would have been completely impossible in any other language/package! Now I want to calculat…

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## [2D wave equation with open boundary conditions](https://discourse.julialang.org/t/2d-wave-equation-with-open-boundary-conditions/103290)

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**Author:** [@thoth291](https://discourse.julialang.org/u/thoth291)\
**Replies:** 32\
**Last updated:** [September 14, 2023, 1:57pm UTC](https://discourse.julialang.org/t/2d-wave-equation-with-open-boundary-conditions/103290 "2023-09-14T13:57:38Z")

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Here is the equation I’m trying to solve and I’m surely doing something wrong - but I don’t know what exactly. Could you please help me to understand where I’m thinking wrong about it? using NeuralPDE, Lux, LuxCUDA, Ra…

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## [Symbolic ODE/PDE in Vector form with Symbolics/ModelingToolkit](https://discourse.julialang.org/t/symbolic-ode-pde-in-vector-form-with-symbolics-modelingtoolkit/103793)

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**Author:** [@Bizzi](https://discourse.julialang.org/u/Bizzi)\
**Replies:** 6\
**Last updated:** [September 14, 2023, 1:52pm UTC](https://discourse.julialang.org/t/symbolic-ode-pde-in-vector-form-with-symbolics-modelingtoolkit/103793 "2023-09-14T13:52:06Z")

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I’ve been having a hard time trying to use vector-valued equations (as opposed to vectors of equations) in Symbolics/ModelingToolkit. I remember reading somewhere in either docs that his feature was somewhat experimental…

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## [Implementing Maple water hammer example (PDE's)](https://discourse.julialang.org/t/implementing-maple-water-hammer-example-pdes/103501)

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**Author:** [@victor](https://discourse.julialang.org/u/victor)\
**Replies:** 3\
**Last updated:** [September 14, 2023, 10:00am UTC](https://discourse.julialang.org/t/implementing-maple-water-hammer-example-pdes/103501 "2023-09-14T10:00:08Z")

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Hi I’m trying to recreate this Maple water hammer example in Julia. Link to Maple example below. I’m not able to get anything close to stable results. Some help here would be greatly appreciated. (I know I’ve made some …

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## [Tips for improving performance of NODEs with sparsely sampled time series?](https://discourse.julialang.org/t/tips-for-improving-performance-of-nodes-with-sparsely-sampled-time-series/103828)

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**Author:** [@jarroyoe](https://discourse.julialang.org/u/jarroyoe)\
**Replies:** 1\
**Last updated:** [September 13, 2023, 8:46pm UTC](https://discourse.julialang.org/t/tips-for-improving-performance-of-nodes-with-sparsely-sampled-time-series/103828 "2023-09-13T20:46:41Z")

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I have a synthetic time series coming from a model with a daily time scale. The real dynamics closely follow a sine function, but I’m trying to sample at a monthly time scale (every 30 time steps). A sample time series l…

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## [Comparing time series with lead/lag tolerances](https://discourse.julialang.org/t/comparing-time-series-with-lead-lag-tolerances/99227)

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**Author:** [@airpmb](https://discourse.julialang.org/u/airpmb)\
**Replies:** 2\
**Last updated:** [September 13, 2023, 2:55pm UTC](https://discourse.julialang.org/t/comparing-time-series-with-lead-lag-tolerances/99227 "2023-09-13T14:55:41Z")

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Is anyone aware of a Julia package that implements testing of time series output against baselines? By this I mean that I’d like to be able to compare simulation outputs over time against some “truth model” equivalent o…

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## [How can I training one function at the time with NeuralPDE?](https://discourse.julialang.org/t/how-can-i-training-one-function-at-the-time-with-neuralpde/103398)

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**Author:** [@Knud\_Sorensen](https://discourse.julialang.org/u/Knud_Sorensen)\
**Replies:** 3\
**Last updated:** [September 12, 2023, 6:32pm UTC](https://discourse.julialang.org/t/how-can-i-training-one-function-at-the-time-with-neuralpde/103398 "2023-09-12T18:32:25Z")

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If you have 2 or more functions as Neural nets, say f and g. And they are coupled by a differential equation. And we already idea how f should look like say we have a mock function f\_m for f How would I? Train f to …

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## [Error when using Unitful and MonteCarloMeasurements with DifferentialEquations](https://discourse.julialang.org/t/error-when-using-unitful-and-montecarlomeasurements-with-differentialequations/103464)

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**Author:** [@rwalters31](https://discourse.julialang.org/u/rwalters31)\
**Replies:** 4\
**Last updated:** [September 12, 2023, 6:26pm UTC](https://discourse.julialang.org/t/error-when-using-unitful-and-montecarlomeasurements-with-differentialequations/103464 "2023-09-12T18:26:42Z")

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I am trying to use both Unitful and MonteCarloMeasurements with DifferentialEquations to better understand the impact of uncertainty on a second order ODE. Using either Untiful or MonteCarloMeasurements alone with Diffe…

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## [Scaling Independent Variables in Composed \`ODESystem\`s](https://discourse.julialang.org/t/scaling-independent-variables-in-composed-odesystem-s/103431)

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**Author:** [@cadojo](https://discourse.julialang.org/u/cadojo)\
**Replies:** 1\
**Last updated:** [September 12, 2023, 5:34pm UTC](https://discourse.julialang.org/t/scaling-independent-variables-in-composed-odesystem-s/103431 "2023-09-12T17:34:39Z")

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I have code that implements the nondimensional form of a simplified three-body problem. The length units are scaled by the distance between the two celestial bodies, a, and the time units are scaled by the orbital period…

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## [Is periodic boundary conditions posible in NeuralPDE.jl?](https://discourse.julialang.org/t/is-periodic-boundary-conditions-posible-in-neuralpde-jl/103674)

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**Author:** [@Knud\_Sorensen](https://discourse.julialang.org/u/Knud_Sorensen)\
**Replies:** 1\
**Last updated:** [September 8, 2023, 3:24pm UTC](https://discourse.julialang.org/t/is-periodic-boundary-conditions-posible-in-neuralpde-jl/103674 "2023-09-08T15:24:29Z")

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Hi. With NeuralPDE i have this conditions. L=10 bcs0 = \[G(x,y,0)~0, G(-L,y,z)~G(L,y,z), G(x,-L,z)~G(x,L,z)\] I get loss like 0 loss: 67.44680398772921 pde: \[0.8533529399741632, 1.3552322555600897, 20.41989735134126\] b…

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## [How to use ParentScope with arrays](https://discourse.julialang.org/t/how-to-use-parentscope-with-arrays/103579)

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**Author:** [@contradict](https://discourse.julialang.org/u/contradict)\
**Replies:** 2\
**Last updated:** [September 6, 2023, 5:49pm UTC](https://discourse.julialang.org/t/how-to-use-parentscope-with-arrays/103579 "2023-09-06T17:49:10Z")

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This used to work in ModelingToolkit 8.65.0 but stopped in 8.66.0. Was it ever supposed to work? Is there a way to move array variables up the hierarchy? @parameters a\[1:2\]=\[1, 2\] b=4 c=1 @variables t x(t)=ParentScope(a…

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## [What are the advantages of using ModelingToolkit to make and solve ODEProblems?](https://discourse.julialang.org/t/what-are-the-advantages-of-using-modelingtoolkit-to-make-and-solve-odeproblems/103563)

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**Author:** [@markjholmes](https://discourse.julialang.org/u/markjholmes)\
**Replies:** 7\
**Last updated:** [September 6, 2023, 5:07pm UTC](https://discourse.julialang.org/t/what-are-the-advantages-of-using-modelingtoolkit-to-make-and-solve-odeproblems/103563 "2023-09-06T17:07:42Z")

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For context, I come from an R background using the deSolve package. The first examples for ModelingToolkit make use of a specific structure of model definition, using the @parameters, @variables, @named macros. The gene…

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## [Handling prematurely terminated ODE integration in parameter estimation](https://discourse.julialang.org/t/handling-prematurely-terminated-ode-integration-in-parameter-estimation/103514)

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**Author:** [@Neodym](https://discourse.julialang.org/u/Neodym)\
**Replies:** 3\
**Last updated:** [September 5, 2023, 10:25pm UTC](https://discourse.julialang.org/t/handling-prematurely-terminated-ode-integration-in-parameter-estimation/103514 "2023-09-05T22:25:30Z")

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Hi, I’m trying to estimate parameters of an ODE model using DifferentialEquations, SciMLSensitivity, and Optimization packages. In particular, I’ve found that Ipopt works quite well for my problem. Sometimes, though, t…

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## [Mixed NN and functions fail with NeuralPDE](https://discourse.julialang.org/t/mixed-nn-and-functions-fail-with-neuralpde/103447)

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**Author:** [@Knud\_Sorensen](https://discourse.julialang.org/u/Knud_Sorensen)\
**Replies:** 7\
**Last updated:** [September 5, 2023, 3:13pm UTC](https://discourse.julialang.org/t/mixed-nn-and-functions-fail-with-neuralpde/103447 "2023-09-05T15:13:04Z")

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Hi. I’m trying to use NeuralPDE to approximate a solution to a PDE containing both Neural nets and functions. Something goes wrong, can you help ? I am new to NeuralPDE, so it might just be a newbie error. Here is the …

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## [Symbolics.jl expressions that don't reduce to single type](https://discourse.julialang.org/t/symbolics-jl-expressions-that-dont-reduce-to-single-type/102551)

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**Author:** [@Eemilp](https://discourse.julialang.org/u/Eemilp)\
**Replies:** 5\
**Last updated:** [September 3, 2023, 4:27pm UTC](https://discourse.julialang.org/t/symbolics-jl-expressions-that-dont-reduce-to-single-type/102551 "2023-09-03T16:27:33Z")

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Hi, I’m trying to use Symbolics.jl to simplify some expressions in the domain of quantum mechanics. The problem I’m facing is that I would like to simplify a partial expression that does not exactly reduce to a single t…

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## [Error with ODE Tableaus by DiffEqDevTools](https://discourse.julialang.org/t/error-with-ode-tableaus-by-diffeqdevtools/103448)

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**Author:** [@simon79](https://discourse.julialang.org/u/simon79)\
**Replies:** 5\
**Last updated:** [September 2, 2023, 10:46pm UTC](https://discourse.julialang.org/t/error-with-ode-tableaus-by-diffeqdevtools/103448 "2023-09-02T22:46:16Z")

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I am having errors with the tableau methods for explicitRK using DiffEqDevTools listed in tableau-methods-list The code using DiffEqBase using DiffEqDevTools using OrdinaryDiffEq using ODEInterfaceDiffEq using SciMLB…

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## [Does a Sequential Modular Approach (SMA) algorithm exist for solving component-based systems of non-linear equations](https://discourse.julialang.org/t/does-a-sequential-modular-approach-sma-algorithm-exist-for-solving-component-based-systems-of-non-linear-equations/103389)

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**Author:** [@chris-hampel-CA](https://discourse.julialang.org/u/chris-hampel-CA)\
**Replies:** 2\
**Last updated:** [September 1, 2023, 5:41pm UTC](https://discourse.julialang.org/t/does-a-sequential-modular-approach-sma-algorithm-exist-for-solving-component-based-systems-of-non-linear-equations/103389 "2023-09-01T17:41:17Z")

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Hello, I am polling this board to see if anyone knows if an algorithm exists in any Julia package to perform the Sequential Modular Approach (SMA) technique for solving systems of connected components where each compone…

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