# Modelling & Simulations

**URL:** https://discourse.julialang.org/c/domain/models/21.md?page=5

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**Page:** 6

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## [Modelling collisions of agents with the environment in discrete time](https://discourse.julialang.org/t/modelling-collisions-of-agents-with-the-environment-in-discrete-time/133229)

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**Author:** [@simonsteiger](https://discourse.julialang.org/u/simonsteiger)\
**Replies:** 1\
**Last updated:** [November 2, 2025, 2:00pm UTC](https://discourse.julialang.org/t/modelling-collisions-of-agents-with-the-environment-in-discrete-time/133229 "2025-11-02T14:00:09Z")

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Hi everyone, I am new to agent-based modelling and have recently started using Agents.jl for a research project. Thanks a lot for this great package! The model should simulate how (for example) coconuts disperse throug…

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## [Examples of dynamical physical systems with exogenous inputs described by UDEs](https://discourse.julialang.org/t/examples-of-dynamical-physical-systems-with-exogenous-inputs-described-by-udes/133373)

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**Author:** [@Emil\_Martinsen](https://discourse.julialang.org/u/Emil_Martinsen)\
**Replies:** 4\
**Last updated:** [October 31, 2025, 12:55pm UTC](https://discourse.julialang.org/t/examples-of-dynamical-physical-systems-with-exogenous-inputs-described-by-udes/133373 "2025-10-31T12:55:21Z")

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Can anyone point me to some good examples (code, papers, tutorials) of a dynamical physical system that are modelled with UDEs, i.e., ODEs where parts are known and other parts are unknown and modelled by a neural networ…

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## [AD of UDEs - how to set it up?](https://discourse.julialang.org/t/ad-of-udes-how-to-set-it-up/133461)

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**Author:** [@Emil\_Martinsen](https://discourse.julialang.org/u/Emil_Martinsen)\
**Replies:** 13\
**Last updated:** [October 31, 2025, 7:13am UTC](https://discourse.julialang.org/t/ad-of-udes-how-to-set-it-up/133461 "2025-10-31T07:13:44Z")

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Hi! Im new to Julia and SciML but I am really curious to see how combining ODE’s with neural network could improve the predictive performance of the model we are developing for solvent-based CO2 capture. My current setu…

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## [Lumped beam model](https://discourse.julialang.org/t/lumped-beam-model/128512)

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**Author:** [@yjr](https://discourse.julialang.org/u/yjr)\
**Replies:** 8\
**Last updated:** [October 30, 2025, 10:52pm UTC](https://discourse.julialang.org/t/lumped-beam-model/128512 "2025-10-30T22:52:45Z")

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Hello, I’m trying to implement a 2D lumped beam model with modelingtoolkit. Using the small angle approximation, I managed to have something working with 5 elements. But increasing the number of elements to 6 make ODEPr…

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## [Memory issues for long SSA simulations with mass action jumps](https://discourse.julialang.org/t/memory-issues-for-long-ssa-simulations-with-mass-action-jumps/133530)

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**Author:** [@jworsfold](https://discourse.julialang.org/u/jworsfold)\
**Replies:** 1\
**Last updated:** [October 30, 2025, 10:53am UTC](https://discourse.julialang.org/t/memory-issues-for-long-ssa-simulations-with-mass-action-jumps/133530 "2025-10-30T10:53:39Z")

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I’m trying to find the stationary distribution of a Master Equation where the state space is bounded but can be too large to solve using FiniteStateProjection.jl so I’m left with running Gillespie simulations. Running l…

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## [ModelingToolkit Symbolics maxiters not enough](https://discourse.julialang.org/t/modelingtoolkit-symbolics-maxiters-not-enough/133463)

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**Author:** [@AlirezaMiraliakbar](https://discourse.julialang.org/u/AlirezaMiraliakbar)\
**Replies:** 2\
**Last updated:** [October 28, 2025, 7:08pm UTC](https://discourse.julialang.org/t/modelingtoolkit-symbolics-maxiters-not-enough/133463 "2025-10-28T19:08:48Z")

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Hi, I am trying to model a chemical process with modeling toolkit and I have set 4 of my variables to irreducible = true. However, with doing this, I get the warning message from Symbolics that: Warning: Did not converg…

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## [Automatic symbolic function barrier generation](https://discourse.julialang.org/t/automatic-symbolic-function-barrier-generation/133487)

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**Author:** [@sablonl](https://discourse.julialang.org/u/sablonl)\
**Replies:** 0\
**Last updated:** [October 28, 2025, 4:00pm UTC](https://discourse.julialang.org/t/automatic-symbolic-function-barrier-generation/133487 "2025-10-28T16:00:10Z")

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Hi, I’m working with ModelingToolkit.jl and I have an issue with complex symbolic expressions, on which mtkcompile takes a lot of time. I discovered that I can hide expressions behind a function barrier using @register…

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## [Molly.jl - Shear Viscosity Notebook - Questions](https://discourse.julialang.org/t/molly-jl-shear-viscosity-notebook-questions/133466)

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**Author:** [@ziolai](https://discourse.julialang.org/u/ziolai)\
**Replies:** 2\
**Last updated:** [October 28, 2025, 11:35am UTC](https://discourse.julialang.org/t/molly-jl-shear-viscosity-notebook-questions/133466 "2025-10-28T11:35:09Z")

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This post concerns the tutorial at SINEQ Shear Viscosity. Q1: To what extend is the result obtained for the shear viscosity independent of the number of particles in the simulation? Q2: What happens in case that the nu…

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## [How to couple different wind farm models?](https://discourse.julialang.org/t/how-to-couple-different-wind-farm-models/133464)

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**Author:** [@ufechner7](https://discourse.julialang.org/u/ufechner7)\
**Replies:** 0\
**Last updated:** [October 27, 2025, 5:08pm UTC](https://discourse.julialang.org/t/how-to-couple-different-wind-farm-models/133464 "2025-10-27T17:08:10Z")

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We are brainstorming how to best couple different scale models, e.g., fast small-scale models with meso-scale models, for example, Floridyn.jl with a Julia version of TFSO-software / WAYVE · GitLab . I guess both models…

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## [Robust resampling/interpolation method](https://discourse.julialang.org/t/robust-resampling-interpolation-method/132665)

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**Author:** [@langestefan](https://discourse.julialang.org/u/langestefan)\
**Replies:** 25\
**Last updated:** [October 27, 2025, 1:15pm UTC](https://discourse.julialang.org/t/robust-resampling-interpolation-method/132665 "2025-10-27T13:15:54Z")

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I am working on a system identification problem with poor quality input data. The data describes a heating system, with thermostat temperatures, thermal power inputs, supply/return water temperatures etc. Some issues w…

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## [Solving SemilinearODE without using automatic differentiation](https://discourse.julialang.org/t/solving-semilinearode-without-using-automatic-differentiation/133434)

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**Author:** [@sanjeev](https://discourse.julialang.org/u/sanjeev)\
**Replies:** 4\
**Last updated:** [October 27, 2025, 2:10am UTC](https://discourse.julialang.org/t/solving-semilinearode-without-using-automatic-differentiation/133434 "2025-10-27T02:10:13Z")

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I have semilinear ODE with the time-dependent linear implicit part as function f1! So, instead of solving (I - γΔt \* J₁) \* u\_{n+1} = rhs, I want to use Krylov method to solve (I - γΔt \* J1) \* u\_{n+1} = rhs, where J1 \* u…

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## [ModelingToolKit and the number pi](https://discourse.julialang.org/t/modelingtoolkit-and-the-number-pi/132543)

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**Author:** [@ziolai](https://discourse.julialang.org/u/ziolai)\
**Replies:** 5\
**Last updated:** [October 26, 2025, 9:44am UTC](https://discourse.julialang.org/t/modelingtoolkit-and-the-number-pi/132543 "2025-10-26T09:44:37Z")

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I am likely misunderstanding the issue below. The issues, however, of ModelingToolkit not liking pi reported previously at Discretization of semi-coupled wave and diffusion equations fails for step size of pi/(25n) usin…

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## [Modeling Toolkit with ∇c · ∇Φ\_2 Term](https://discourse.julialang.org/t/modeling-toolkit-with-c-2-term/133180)

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**Author:** [@nickbrady](https://discourse.julialang.org/u/nickbrady)\
**Replies:** 4\
**Last updated:** [October 24, 2025, 10:44pm UTC](https://discourse.julialang.org/t/modeling-toolkit-with-c-2-term/133180 "2025-10-24T22:44:38Z")

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I am trying to solve a system of PDE in the ModelingToolkit. I can solve the unsteady heat equation for multiple species and for Neumann and Dirichlet boundary conditions, see MWE. However, I am looking to solve more co…

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## [Issue using ModelingToolkit to simulate electrical circuits](https://discourse.julialang.org/t/issue-using-modelingtoolkit-to-simulate-electrical-circuits/133415)

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**Author:** [@JaviJR](https://discourse.julialang.org/u/JaviJR)\
**Replies:** 7\
**Last updated:** [October 24, 2025, 5:40pm UTC](https://discourse.julialang.org/t/issue-using-modelingtoolkit-to-simulate-electrical-circuits/133415 "2025-10-24T17:40:28Z")

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Hello everyone, Lately, I have been working quite extensively with ModelingToolkit, particularly migrating some dynamic control models that I had previously implemented in various closed-source programs (often achieving…

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## [Is it possible to use structural\_parameters as an argument of \`System\` in ModelingToolkit?](https://discourse.julialang.org/t/is-it-possible-to-use-structural-parameters-as-an-argument-of-system-in-modelingtoolkit/133396)

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**Author:** [@pmc4](https://discourse.julialang.org/u/pmc4)\
**Replies:** 6\
**Last updated:** [October 24, 2025, 4:15pm UTC](https://discourse.julialang.org/t/is-it-possible-to-use-structural-parameters-as-an-argument-of-system-in-modelingtoolkit/133396 "2025-10-24T16:15:21Z")

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Hello everyone. When building a model in ModelingToolkit, there is the option of doing it with the macro @mtkmodel. Inside the macro, we can define structural parameters for defining numerical variables. For example, co…

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## [Method Error when using Method of Lines to solve PDEs](https://discourse.julialang.org/t/method-error-when-using-method-of-lines-to-solve-pdes/132951)

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**Author:** [@gst1017](https://discourse.julialang.org/u/gst1017)\
**Replies:** 3\
**Last updated:** [October 24, 2025, 3:52pm UTC](https://discourse.julialang.org/t/method-error-when-using-method-of-lines-to-solve-pdes/132951 "2025-10-24T15:52:53Z")

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Hello all! I’m new to Julia and am trying to use the Method of Lines and MTK packages to solve a system of PDEs. I am running into repeated method errors, and the stacktrace keeps pointing to a line from Macro Expanded c…

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## [Beginning with PEtab, only one entry per measurement?](https://discourse.julialang.org/t/beginning-with-petab-only-one-entry-per-measurement/133388)

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**Author:** [@kraqbi](https://discourse.julialang.org/u/kraqbi)\
**Replies:** 0\
**Last updated:** [October 23, 2025, 2:11pm UTC](https://discourse.julialang.org/t/beginning-with-petab-only-one-entry-per-measurement/133388 "2025-10-23T14:11:20Z")

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Hello, I try PEtab for the first time and I am studying the documentation and tutorial of PEtab. I was wondering, the DataFrame I create, is it absolutely necessary to have just one entry per measurement? There is no b…

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## [Interpolated parameters into the PDESystems](https://discourse.julialang.org/t/interpolated-parameters-into-the-pdesystems/132736)

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**Author:** [@sraghuram](https://discourse.julialang.org/u/sraghuram)\
**Replies:** 6\
**Last updated:** [October 22, 2025, 12:47pm UTC](https://discourse.julialang.org/t/interpolated-parameters-into-the-pdesystems/132736 "2025-10-22T12:47:42Z")

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Hey I am trying to solve a system of equations where my input parameters are altitude dependent values. These values should be supplied into system of equations after discretizing the equation. I have created an Interpol…

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## [Catalyst.jl - model reaction network with changing parameters during drug dosing](https://discourse.julialang.org/t/catalyst-jl-model-reaction-network-with-changing-parameters-during-drug-dosing/133340)

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**Author:** [@Mindy](https://discourse.julialang.org/u/Mindy)\
**Replies:** 2\
**Last updated:** [October 21, 2025, 6:14pm UTC](https://discourse.julialang.org/t/catalyst-jl-model-reaction-network-with-changing-parameters-during-drug-dosing/133340 "2025-10-21T18:14:25Z")

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Hi, I am trying to implement a infusion dosing with my reaction network. The infusion rate will be changing depending on the time. If during the infusion duration, there’s a value for k\_inf, otherwise it will be 0. and t…

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## [MethodofLines - Staggered Grids - MethodError by function discretize()](https://discourse.julialang.org/t/methodoflines-staggered-grids-methoderror-by-function-discretize/133312)

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**Author:** [@ziolai](https://discourse.julialang.org/u/ziolai)\
**Replies:** 0\
**Last updated:** [October 20, 2025, 4:57pm UTC](https://discourse.julialang.org/t/methodoflines-staggered-grids-methoderror-by-function-discretize/133312 "2025-10-20T16:57:20Z")

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Greetings, The tutorial example on staggered grids at staggered results in the method error MethodError: no method matching view(::Nothing, ::UnitRange{Int64}) The function \`view\` exists, but no method is defined for t…

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## [Computing Steady State With Known Constraints](https://discourse.julialang.org/t/computing-steady-state-with-known-constraints/132489)

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**Author:** [@Aviv\_Barnea](https://discourse.julialang.org/u/Aviv_Barnea)\
**Replies:** 5\
**Last updated:** [October 19, 2025, 6:51pm UTC](https://discourse.julialang.org/t/computing-steady-state-with-known-constraints/132489 "2025-10-19T18:51:10Z")

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Hi, new to ModelingToolkit.jl, and trying to port some models in Nuclear Reactor computations. So here’s the simplest model (point reactor) I wrote: using ModelingToolkit, Plots, OrdinaryDiffEq using ModelingToolkit: t\_…

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## [Adjoint sensitivities for non-numeric types in ModelingToolkit.jl](https://discourse.julialang.org/t/adjoint-sensitivities-for-non-numeric-types-in-modelingtoolkit-jl/132082)

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**Author:** [@mxp](https://discourse.julialang.org/u/mxp)\
**Replies:** 2\
**Last updated:** [October 17, 2025, 4:51pm UTC](https://discourse.julialang.org/t/adjoint-sensitivities-for-non-numeric-types-in-modelingtoolkit-jl/132082 "2025-10-17T16:51:26Z")

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I’d like to use non-numeric types in ModelingToolkit.jl, but with adjoint sensitivities. When using DifferentialEquations.jl there is support for this through SciMLStructures.jl as described here. So I assumed this would…

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## [Identification using LowLevelParticleFilters.jl](https://discourse.julialang.org/t/identification-using-lowlevelparticlefilters-jl/132393)

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**Author:** [@langestefan](https://discourse.julialang.org/u/langestefan)\
**Replies:** 19\
**Last updated:** [October 17, 2025, 1:04pm UTC](https://discourse.julialang.org/t/identification-using-lowlevelparticlefilters-jl/132393 "2025-10-17T13:04:10Z")

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I am looking for an example (or some guidance) on how to fit a Kalman smoother to data using log-likelihood maximization using LowLevelParticleFilters.jl. I already have a model structure and have collected a dataset. I…

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## [ModelingToolkit problem generation errors for array-valued parameters with Measurements](https://discourse.julialang.org/t/modelingtoolkit-problem-generation-errors-for-array-valued-parameters-with-measurements/133210)

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**Author:** [@bspanoghe](https://discourse.julialang.org/u/bspanoghe)\
**Replies:** 1\
**Last updated:** [October 17, 2025, 9:27am UTC](https://discourse.julialang.org/t/modelingtoolkit-problem-generation-errors-for-array-valued-parameters-with-measurements/133210 "2025-10-17T09:27:02Z")

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I’ve been running into errors trying to use Measurements.jl combined with ModelingToolkit.jl when there’s array-valued parameters involved. Consider the following toy problem: using ModelingToolkit, Measurements import …

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## [Problems installing Agents under v1.12.0](https://discourse.julialang.org/t/problems-installing-agents-under-v1-12-0/133215)

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**Author:** [@Niall](https://discourse.julialang.org/u/Niall)\
**Replies:** 4\
**Last updated:** [October 17, 2025, 4:42am UTC](https://discourse.julialang.org/t/problems-installing-agents-under-v1-12-0/133215 "2025-10-17T04:42:06Z")

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Hi. I’ve just returned to julia after absence and have installed version 1.12.0. I now have problems installing and loading a set of 8 packages relating to Agents. Here is the Pkg output: (@v1.12) pkg\> up Agents Upd…

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## [Overdetermined system when using modelingtoolkitize](https://discourse.julialang.org/t/overdetermined-system-when-using-modelingtoolkitize/132917)

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**Author:** [@jessehummel](https://discourse.julialang.org/u/jessehummel)\
**Replies:** 2\
**Last updated:** [October 15, 2025, 4:00pm UTC](https://discourse.julialang.org/t/overdetermined-system-when-using-modelingtoolkitize/132917 "2025-10-15T16:00:00Z")

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Hi, I would like to convert a numeric differential-algebraic equation to a symbolic model using modelingtoolkitize. However, when then trying to convert it back to an ODEProblem, I get an overdetermined warning and when …

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## [Collision checking for RigidBodyDynamics: AI coding agents experiment](https://discourse.julialang.org/t/collision-checking-for-rigidbodydynamics-ai-coding-agents-experiment/133136)

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**Author:** [@Lukasz\_Juszkiewicz](https://discourse.julialang.org/u/Lukasz_Juszkiewicz)\
**Replies:** 0\
**Last updated:** [October 14, 2025, 10:57am UTC](https://discourse.julialang.org/t/collision-checking-for-rigidbodydynamics-ai-coding-agents-experiment/133136 "2025-10-14T10:57:21Z")

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Recently, I read a lot how great AI coding agents are. I was little bit skeptical, but I thought it’s high time to give it a shot. Goal: develop something useful related to robotics without writing a single line of code…

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## [DAE solve asking for an unreasonable amount of memory](https://discourse.julialang.org/t/dae-solve-asking-for-an-unreasonable-amount-of-memory/133051)

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**Author:** [@sanjeev](https://discourse.julialang.org/u/sanjeev)\
**Replies:** 5\
**Last updated:** [October 12, 2025, 5:45am UTC](https://discourse.julialang.org/t/dae-solve-asking-for-an-unreasonable-amount-of-memory/133051 "2025-10-12T05:45:49Z")

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I have a system of differential equations (DAE) I am solving using DifferentialEquations.jl . It works well when I have larger discretization (fewer points), but when I use smaller discretization, it asks for a lot of me…

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## [Problem solving DDE: "no method matching \_initialize\_dae!"](https://discourse.julialang.org/t/problem-solving-dde-no-method-matching-initialize-dae/133049)

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**Author:** [@Tangjuan](https://discourse.julialang.org/u/Tangjuan)\
**Replies:** 2\
**Last updated:** [October 11, 2025, 3:32am UTC](https://discourse.julialang.org/t/problem-solving-dde-no-method-matching-initialize-dae/133049 "2025-10-11T03:32:53Z")

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I am trying to solve a DDEProblem in Julia, but I encounter the error no method matching \_initialize\_dae!. Could anyone explain why this happens and how to fix it?

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## [How to extract the free term of a polynomial with symbolic coefficients?](https://discourse.julialang.org/t/how-to-extract-the-free-term-of-a-polynomial-with-symbolic-coefficients/132326)

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**Author:** [@Gregstrq](https://discourse.julialang.org/u/Gregstrq)\
**Replies:** 2\
**Last updated:** [October 9, 2025, 7:37am UTC](https://discourse.julialang.org/t/how-to-extract-the-free-term-of-a-polynomial-with-symbolic-coefficients/132326 "2025-10-09T07:37:18Z")

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Let’s say, I use Symbolics.jl to define a polynomial in λ with symbolic coefficients. using Symbolics @variables λ p q f = λ^3 + p\*λ + q How do I get the free term of the polynomial with respect to λ? I can use Symbo…

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