# Modelling & Simulations

**URL:** https://discourse.julialang.org/c/domain/models/21.md?page=34

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**Page:** 35

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## [Orbit diagram of Logistic Map](https://discourse.julialang.org/t/orbit-diagram-of-logistic-map/100690)

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**Author:** [@SimonNeuroscience](https://discourse.julialang.org/u/SimonNeuroscience)\
**Replies:** 2\
**Last updated:** [June 22, 2023, 9:50am UTC](https://discourse.julialang.org/t/orbit-diagram-of-logistic-map/100690 "2023-06-22T09:50:25Z")

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Hello I am trying to Plot the orbit diagram of the famous logistic map to see the change into chaos of the model. The Code runs fine, but does to produce a Plot som somehow I followed the Code from this Julia-Github: O…

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## [Detecting warnings in ODE system during solving](https://discourse.julialang.org/t/detecting-warnings-in-ode-system-during-solving/100075)

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**Author:** [@Nick1](https://discourse.julialang.org/u/Nick1)\
**Replies:** 2\
**Last updated:** [June 21, 2023, 4:14pm UTC](https://discourse.julialang.org/t/detecting-warnings-in-ode-system-during-solving/100075 "2023-06-21T16:14:47Z")

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I do scanning in a big number of parameters in a system of ODEs to detect possible warnings during solving by changing one parameter value at a time (increase and/or decrease by some orders of magnitude). I would like to…

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## [Error DynamicalSystems.jl Plotting trajectories](https://discourse.julialang.org/t/error-dynamicalsystems-jl-plotting-trajectories/100655)

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**Author:** [@SimonNeuroscience](https://discourse.julialang.org/u/SimonNeuroscience)\
**Replies:** 2\
**Last updated:** [June 21, 2023, 2:46pm UTC](https://discourse.julialang.org/t/error-dynamicalsystems-jl-plotting-trajectories/100655 "2023-06-21T14:46:47Z")

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Hello, I am trying to follow the Intro to DynamicalSystems.jl YouTube Video from the Official Julia programming Account: Intro to dynamical systems in Julia - YouTube I want to plot the trajectories of a simple determi…

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## [SymbolicNumericIntegration -- definite integral?](https://discourse.julialang.org/t/symbolicnumericintegration-definite-integral/100558)

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**Author:** [@BLI](https://discourse.julialang.org/u/BLI)\
**Replies:** 4\
**Last updated:** [June 21, 2023, 11:25am UTC](https://discourse.julialang.org/t/symbolicnumericintegration-definite-integral/100558 "2023-06-21T11:25:23Z")

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Does SymbolicNumericIntegration support definite integration? \[I don’t find this in the documentation… I tried with \`integrate(f(x),x~x0…x1)\` and also with \`integrate(f(x), x=x0…x1)\` without much success…\] If not, it is…

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## [Pass current timestep to affect! to modify integrator values with lookup table](https://discourse.julialang.org/t/pass-current-timestep-to-affect-to-modify-integrator-values-with-lookup-table/100617)

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**Author:** [@aidanboyne](https://discourse.julialang.org/u/aidanboyne)\
**Replies:** 2\
**Last updated:** [June 21, 2023, 3:02am UTC](https://discourse.julialang.org/t/pass-current-timestep-to-affect-to-modify-integrator-values-with-lookup-table/100617 "2023-06-21T03:02:03Z")

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I am trying to simulate patient response to a drug and am currently fitting a model to patient data. However the frequency and strength of the dose changes frequently, so I have the time and strength for each dose record…

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## [Solving the Boltzmann transport equation using Julia?](https://discourse.julialang.org/t/solving-the-boltzmann-transport-equation-using-julia/100495)

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**Author:** [@WarmConsidering](https://discourse.julialang.org/u/WarmConsidering)\
**Replies:** 9\
**Last updated:** [June 18, 2023, 10:00pm UTC](https://discourse.julialang.org/t/solving-the-boltzmann-transport-equation-using-julia/100495 "2023-06-18T22:00:15Z")

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Hi all, I’d like to use Julia to solve the time-independent Boltzmann transport equation (BTE) over unstructured grids in 2 or 3 dimensions using the Sn method for neutrons. So far I’ve identified Trixi.jl and Gridap.jl …

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## [SplitODEProblem](https://discourse.julialang.org/t/splitodeproblem/100061)

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**Author:** [@jtgreen](https://discourse.julialang.org/u/jtgreen)\
**Replies:** 5\
**Last updated:** [June 17, 2023, 2:00pm UTC](https://discourse.julialang.org/t/splitodeproblem/100061 "2023-06-17T14:00:21Z")

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Is it possible to use other combinations of solvers for instance forward euler for an explicit method and CVODE\_BDF from sundials (w/ linear\_solver=:GMRES) for the implicit? Rather than the provided combinations listed i…

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## [Solving differential equations with uncertain parameters](https://discourse.julialang.org/t/solving-differential-equations-with-uncertain-parameters/100059)

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**Author:** [@lmg](https://discourse.julialang.org/u/lmg)\
**Replies:** 8\
**Last updated:** [June 16, 2023, 11:31pm UTC](https://discourse.julialang.org/t/solving-differential-equations-with-uncertain-parameters/100059 "2023-06-16T23:31:33Z")

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Hello everybody, I would like to solve IVPs with uncertain but bounded parameters. To achieve this, my plan was to use the Interval data type from the IntervalArithmetic.jl package. In this post on custom number types, …

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## [Using UODE in Lotka-Volterra for multiple species](https://discourse.julialang.org/t/using-uode-in-lotka-volterra-for-multiple-species/100146)

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**Author:** [@James\_Fernandez](https://discourse.julialang.org/u/James_Fernandez)\
**Replies:** 6\
**Last updated:** [June 16, 2023, 5:32pm UTC](https://discourse.julialang.org/t/using-uode-in-lotka-volterra-for-multiple-species/100146 "2023-06-16T17:32:44Z")

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Hello everyone, I am brand new to Julia, machine learning and sciML so forgive me if I still don’t have the technical jargon. I have read through Automatically Discover Missing Physics by Embedding Machine Learning int…

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## [Integro-DDE that includes integral over delta function](https://discourse.julialang.org/t/integro-dde-that-includes-integral-over-delta-function/100429)

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**Author:** [@rogue26](https://discourse.julialang.org/u/rogue26)\
**Replies:** 2\
**Last updated:** [June 16, 2023, 12:36pm UTC](https://discourse.julialang.org/t/integro-dde-that-includes-integral-over-delta-function/100429 "2023-06-16T12:36:07Z")

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Hi everyone. I’ve solved some DDE systems in Julia, but I’m stumped on my latest problem. I’m not sure if this is a math question or a Julia question, but I’d appreciate any guidance you can provide. How would you go ab…

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## [Is there a Symbolics.jl interface to Ipopt (or similar)?](https://discourse.julialang.org/t/is-there-a-symbolics-jl-interface-to-ipopt-or-similar/100149)

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**Author:** [@votroto](https://discourse.julialang.org/u/votroto)\
**Replies:** 17\
**Last updated:** [June 14, 2023, 10:49pm UTC](https://discourse.julialang.org/t/is-there-a-symbolics-jl-interface-to-ipopt-or-similar/100149 "2023-06-14T22:49:05Z")

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Hello, is there an interface to Ipopt which can accept Symbolics functions? There is NLPModelsIpopt, which accepts NLPModels. Going via ADNLPModels seems unnecessary as Symbolics can already provide all the hessians, ja…

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## [Disable a particular simplification when using ModelingToolkit.jl](https://discourse.julialang.org/t/disable-a-particular-simplification-when-using-modelingtoolkit-jl/100344)

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**Author:** [@Datseris](https://discourse.julialang.org/u/Datseris)\
**Replies:** 6\
**Last updated:** [June 14, 2023, 6:36pm UTC](https://discourse.julialang.org/t/disable-a-particular-simplification-when-using-modelingtoolkit-jl/100344 "2023-06-14T18:36:36Z")

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I have a particular example, written minimally as: using ModelingToolkit @variables t # independent variable (time) dDt = Differential(t) # define an operator for the differentiation w.r.t. time const norma…

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## [Access to underlying GridSpace grid cells](https://discourse.julialang.org/t/access-to-underlying-gridspace-grid-cells/99338)

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**Author:** [@cormullion](https://discourse.julialang.org/u/cormullion)\
**Replies:** 5\
**Last updated:** [June 13, 2023, 1:53pm UTC](https://discourse.julialang.org/t/access-to-underlying-gridspace-grid-cells/99338 "2023-06-13T13:53:26Z")

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Beginner user of Agents.jl here … With a space defined as space = GridSpaceSingle(..., is it possible to access the underlying grid and its cells in some way, so as to be able to store and retrieve information, eg abou…

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## [Doing repeated Euler-steps in a simulation, Differentialequations.jl](https://discourse.julialang.org/t/doing-repeated-euler-steps-in-a-simulation-differentialequations-jl/100221)

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**Author:** [@Martin](https://discourse.julialang.org/u/Martin)\
**Replies:** 5\
**Last updated:** [June 12, 2023, 12:55pm UTC](https://discourse.julialang.org/t/doing-repeated-euler-steps-in-a-simulation-differentialequations-jl/100221 "2023-06-12T12:55:43Z")

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Hi! I am researcher working with problems related to optimal stopping/switching problems, and I have an issue where I need to, in a for loop, advance one time step (size dt) of an SDE for lots of previous states. And I …

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## [Meaning of Incidence matrix?](https://discourse.julialang.org/t/meaning-of-incidence-matrix/100141)

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**Author:** [@ufechner7](https://discourse.julialang.org/u/ufechner7)\
**Replies:** 3\
**Last updated:** [June 11, 2023, 5:05am UTC](https://discourse.julialang.org/t/meaning-of-incidence-matrix/100141 "2023-06-11T05:05:45Z")

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I defined a nonlinear model and linearized it. I get the following result: julia\> simplified\_sys Model sys with 3 equations States (3): ω(t) \[defaults to 1.04286\] Uest(t) \[defaults to 9.0\] Pgc(t) \[defaults to 2.46…

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## [Perform Bayesian estimations of ODE parameters when variables were not measured simultaneously](https://discourse.julialang.org/t/perform-bayesian-estimations-of-ode-parameters-when-variables-were-not-measured-simultaneously/100033)

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**Author:** [@zeyang.sun](https://discourse.julialang.org/u/zeyang.sun)\
**Replies:** 3\
**Last updated:** [June 9, 2023, 10:08pm UTC](https://discourse.julialang.org/t/perform-bayesian-estimations-of-ode-parameters-when-variables-were-not-measured-simultaneously/100033 "2023-06-09T22:08:27Z")

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Greetings everyone! Recently I need to use Bayesian method to estimate the parameters of a chemical kinetic model which can be described by ordinary differential equations. I found Turing.jl or DiffeqBayes.jl might be t…

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## [How to add assumptions to the integrated function](https://discourse.julialang.org/t/how-to-add-assumptions-to-the-integrated-function/99912)

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**Author:** [@rocco\_sprmnt21](https://discourse.julialang.org/u/rocco_sprmnt21)\
**Replies:** 4\
**Last updated:** [June 6, 2023, 5:43pm UTC](https://discourse.julialang.org/t/how-to-add-assumptions-to-the-integrated-function/99912 "2023-06-06T17:43:02Z")

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I posted the question here because it seemed to me the least unrelated to the question. Of the following integral I am interested only in the real part. How can I force to consider a\>x\>0? Or, alternatively, how can I s…

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## [Step response using ModelingToolkit](https://discourse.julialang.org/t/step-response-using-modelingtoolkit/99846)

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**Author:** [@ufechner7](https://discourse.julialang.org/u/ufechner7)\
**Replies:** 10\
**Last updated:** [June 5, 2023, 5:02am UTC](https://discourse.julialang.org/t/step-response-using-modelingtoolkit/99846 "2023-06-05T05:02:15Z")

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I have a non-linear system defined using ModelingToolkit. Now I want to plot the step response. I use the following code: using ModelingToolkit, OrdinaryDiffEq, DataInterpolations, Plots Cp=\[0.024384377390186646,0.0381…

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## [Solving stiff DAE with Float32 inputs for parallel simulations](https://discourse.julialang.org/t/solving-stiff-dae-with-float32-inputs-for-parallel-simulations/99742)

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**Author:** [@Ricardo\_Rodrigues](https://discourse.julialang.org/u/Ricardo_Rodrigues)\
**Replies:** 5\
**Last updated:** [June 3, 2023, 4:51am UTC](https://discourse.julialang.org/t/solving-stiff-dae-with-float32-inputs-for-parallel-simulations/99742 "2023-06-03T04:51:59Z")

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Hello all! I’m using DifferentialEquations.jl to solve a stiff DAE. I’ve tried both mass matrix and in-place methods with Rodas4 and IDA respectively at tolerances of 1e-8, and the result is the same (albeit IDA tends t…

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## [How to plot the output of the PI controller?](https://discourse.julialang.org/t/how-to-plot-the-output-of-the-pi-controller/99789)

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**Author:** [@ufechner7](https://discourse.julialang.org/u/ufechner7)\
**Replies:** 2\
**Last updated:** [June 2, 2023, 9:53pm UTC](https://discourse.julialang.org/t/how-to-plot-the-output-of-the-pi-controller/99789 "2023-06-02T21:53:40Z")

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I have the following code: using ControlSystemsBase, Plots P\_sc = 4.42616018443722e6 I\_sc = 11838.3957271545\*4 P = tf(-1.898e-8, \[1, 0.01934\]) C = -pid(P\_sc, I\_sc) FS = feedback(P\*C) I can plot the step response of t…

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## [How to determine the -3dB frequency of a system](https://discourse.julialang.org/t/how-to-determine-the-3db-frequency-of-a-system/99755)

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**Author:** [@ufechner7](https://discourse.julialang.org/u/ufechner7)\
**Replies:** 4\
**Last updated:** [June 2, 2023, 4:06pm UTC](https://discourse.julialang.org/t/how-to-determine-the-3db-frequency-of-a-system/99755 "2023-06-02T16:06:44Z")

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I have the following system, controlled with a PI controller: using ControlSystemsBase, Plots P\_sc = 4.42616018443722e6 I\_sc = 11838.3957271545\*4 P = tf(-1.898e-8, \[1, 0.01934\]) C = -pid(P\_sc, I\_sc) PC = feedback(P\*C…

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## [Neumann series expander for solving linear problem?](https://discourse.julialang.org/t/neumann-series-expander-for-solving-linear-problem/99736)

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**Author:** [@ehgus](https://discourse.julialang.org/u/ehgus)\
**Replies:** 3\
**Last updated:** [June 2, 2023, 1:48am UTC](https://discourse.julialang.org/t/neumann-series-expander-for-solving-linear-problem/99736 "2023-06-02T01:48:49Z")

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Is there a package providing Neumann series expansion for solving linear problem? I usually solve Ax = b by calculating x = b + Sb + SSb + SSSb ... where S = 1- A, and I’d like to convert my matlab code into Julia. So …

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## [Freezing neural network parameters in Universal Differential Equation framework](https://discourse.julialang.org/t/freezing-neural-network-parameters-in-universal-differential-equation-framework/99661)

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**Author:** [@Stefano\_Giampiccolo](https://discourse.julialang.org/u/Stefano_Giampiccolo)\
**Replies:** 2\
**Last updated:** [May 31, 2023, 9:44am UTC](https://discourse.julialang.org/t/freezing-neural-network-parameters-in-universal-differential-equation-framework/99661 "2023-05-31T09:44:25Z")

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Hi, I’m working on hybrid systems with the SciML framework, approximating one of the derivatives of an ODE system with a neural network. When I attempt, using Lux.freeze, to freeze some of the network parameters that I …

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## [CVODE\_BDF using order of magnitude fewer function evals than Julia solvers](https://discourse.julialang.org/t/cvode-bdf-using-order-of-magnitude-fewer-function-evals-than-julia-solvers/98562)

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**Author:** [@p\_f](https://discourse.julialang.org/u/p_f)\
**Replies:** 10\
**Last updated:** [May 31, 2023, 1:29am UTC](https://discourse.julialang.org/t/cvode-bdf-using-order-of-magnitude-fewer-function-evals-than-julia-solvers/98562 "2023-05-31T01:29:09Z")

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Recently ported a Matlab implementation over to Julia. My RHS function evaluates faster in Julia, which is good, but the vast majority of Julia solvers seem to require far more function evaluations. CVODE\_BDF outperforms…

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## [ODE solving](https://discourse.julialang.org/t/ode-solving/99520)

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**Author:** [@Ordens\_Ritter](https://discourse.julialang.org/u/Ordens_Ritter)\
**Replies:** 7\
**Last updated:** [May 30, 2023, 8:48pm UTC](https://discourse.julialang.org/t/ode-solving/99520 "2023-05-30T20:48:03Z")

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I am Trying to model some temperature curves with a physically informed NN (PINN). I started solve the FOPDT first, to be able to apply it later. function FOPDT(z) τ, Κ, θ = z return \[T1\[i\] .- (1 ./ τ) .\* (-T1…

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## [Fixed Frequency Control in DiffEq](https://discourse.julialang.org/t/fixed-frequency-control-in-diffeq/99446)

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**Author:** [@Guido\_Ballabio](https://discourse.julialang.org/u/Guido_Ballabio)\
**Replies:** 6\
**Last updated:** [May 30, 2023, 4:35pm UTC](https://discourse.julialang.org/t/fixed-frequency-control-in-diffeq/99446 "2023-05-30T16:35:48Z")

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I am trying to use DiffEq and SciML sensitivity to build NN fixed-frequency controller for physical systems. I’ve tried PeriodicCallback and ti works (see original issue)but not with Neural Networks (Lux), because only…

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## [Multi-physics simulation](https://discourse.julialang.org/t/multi-physics-simulation/88651)

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**Author:** [@ufechner7](https://discourse.julialang.org/u/ufechner7)\
**Replies:** 18\
**Last updated:** [May 30, 2023, 7:40am UTC](https://discourse.julialang.org/t/multi-physics-simulation/88651 "2023-05-30T07:40:10Z")

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As far as I know, for multi-physics simulations often software packages like Ansys or COMSOL are used. Can Julia also be used for this kind of simulations? Or can Julia be used together with these commercial SW packages…

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## [Trying to Create a Schmitt Trigger Component in ModelingToolkit.jl](https://discourse.julialang.org/t/trying-to-create-a-schmitt-trigger-component-in-modelingtoolkit-jl/99503)

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**Author:** [@astra\_kawa](https://discourse.julialang.org/u/astra_kawa)\
**Replies:** 3\
**Last updated:** [May 28, 2023, 4:40am UTC](https://discourse.julialang.org/t/trying-to-create-a-schmitt-trigger-component-in-modelingtoolkit-jl/99503 "2023-05-28T04:40:22Z")

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Hello all. I am trying to create a Schmitt trigger component in ModelingToolkit.jl. Specifically the circuit in the image below (source): I am using the IdealOpAmp component from the MTK standard library, I am assumi…

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## [Package for machanic/static framework](https://discourse.julialang.org/t/package-for-machanic-static-framework/69900)

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**Author:** [@f.L0x](https://discourse.julialang.org/u/f.L0x)\
**Replies:** 5\
**Last updated:** [May 27, 2023, 3:07pm UTC](https://discourse.julialang.org/t/package-for-machanic-static-framework/69900 "2023-05-27T15:07:48Z")

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Hi there, Is there a package to build simple mechanic/static framework? For example like this? I just need, simplified beams, simplified bearings and forces. I had a look at Gridap.jl and JuaFEM.jl, but these seems…

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## [Nonlinear systems and ModelingToolkitStandardLibrary](https://discourse.julialang.org/t/nonlinear-systems-and-modelingtoolkitstandardlibrary/99460)

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**Author:** [@Paulo\_Jabardo](https://discourse.julialang.org/u/Paulo_Jabardo)\
**Replies:** 1\
**Last updated:** [May 26, 2023, 8:31pm UTC](https://discourse.julialang.org/t/nonlinear-systems-and-modelingtoolkitstandardlibrary/99460 "2023-05-26T20:31:48Z")

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Much work in thermal/fluid problems involve steady state cases. I have been playing around with NonlinearSolve.jl and it works well. What I’m having trouble is in the integration it with ModelingToolkitStandardLibrary.jl…

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