# Modelling & Simulations

**URL:** https://discourse.julialang.org/c/domain/models/21.md?page=44

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**Page:** 45

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## [Looking for co-authors for animal movement and epidemiology ABM](https://discourse.julialang.org/t/looking-for-co-authors-for-animal-movement-and-epidemiology-abm/85614)

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**Author:** [@franfram](https://discourse.julialang.org/u/franfram)\
**Replies:** 1\
**Last updated:** [August 11, 2022, 1:13pm UTC](https://discourse.julialang.org/t/looking-for-co-authors-for-animal-movement-and-epidemiology-abm/85614 "2022-08-11T13:13:56Z")

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Hello everyone, I just started building an ABM for animal movement and epidemiology (PhD Project) Is anyone working on something similar and would like to team up? Kindly, Francis

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## [Solving system of ode with Modeling Toolkit](https://discourse.julialang.org/t/solving-system-of-ode-with-modeling-toolkit/85524)

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**Author:** [@qwerty](https://discourse.julialang.org/u/qwerty)\
**Replies:** 3\
**Last updated:** [August 10, 2022, 10:08am UTC](https://discourse.julialang.org/t/solving-system-of-ode-with-modeling-toolkit/85524 "2022-08-10T10:08:45Z")

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I solved this ode system with DifferentialEquations and got the correct result. I am trying with ModelingToolkit but I don’t get the correct result. What am I doing wrong? p₀, q₀ = 0.0, 3.0 @variables t @parameters P(…

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## [Integrator AutoVern9(Rodas5) take a lot of time DifferentialEquations.jl](https://discourse.julialang.org/t/integrator-autovern9-rodas5-take-a-lot-of-time-differentialequations-jl/85470)

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**Author:** [@Sergey\_Novak](https://discourse.julialang.org/u/Sergey_Novak)\
**Replies:** 8\
**Last updated:** [August 9, 2022, 2:34pm UTC](https://discourse.julialang.org/t/integrator-autovern9-rodas5-take-a-lot-of-time-differentialequations-jl/85470 "2022-08-09T14:34:50Z")

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I have systems: function HR!(du, u, p, t) function sigma(x) return 1.0 / ( 1.0 + exp( -10.0 \* ( x - ( - 0.25 ) ) ) ) end a, b, c, d, s, xr, r, I, vs, k1, k2, el\_link = p x1, y1, …

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## [Component based ModelingToolkit](https://discourse.julialang.org/t/component-based-modelingtoolkit/85499)

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**Author:** [@JOAO\_PEREIRA](https://discourse.julialang.org/u/JOAO_PEREIRA)\
**Replies:** 1\
**Last updated:** [August 8, 2022, 9:45pm UTC](https://discourse.julialang.org/t/component-based-modelingtoolkit/85499 "2022-08-08T21:45:34Z")

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Hi, I would like to solve the system below as a component based modeling. DifferentialEquations is a possibility to solve as steady states. ‘’’ using ModelingToolkit, NonlinearSolve function plant\_7\_1\_b() ModelingToo…

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## [Resources for Differentiable Programming](https://discourse.julialang.org/t/resources-for-differentiable-programming/85369)

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**Author:** [@franfram](https://discourse.julialang.org/u/franfram)\
**Replies:** 3\
**Last updated:** [August 8, 2022, 1:19pm UTC](https://discourse.julialang.org/t/resources-for-differentiable-programming/85369 "2022-08-08T13:19:06Z")

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Hello everyone, I’m looking for workshops or another type of resource for learning differentiable programming with Julia (other than julia’s documentation on differentiable programming packages like Zygote). Any resour…

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## [SciMLBenchmarks.jl: Comparison with Fixed timestep methods](https://discourse.julialang.org/t/scimlbenchmarks-jl-comparison-with-fixed-timestep-methods/85246)

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**Author:** [@Mikel\_Antonana](https://discourse.julialang.org/u/Mikel_Antonana)\
**Replies:** 12\
**Last updated:** [August 5, 2022, 5:40pm UTC](https://discourse.julialang.org/t/scimlbenchmarks-jl-comparison-with-fixed-timestep-methods/85246 "2022-08-05T17:40:54Z")

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Hi, I have run a benchmark to compare adaptive and fixed step size method DPRKN12(): tols=abstols=reltols=\[1e-8, 1e-9, 1e-10, 1e-11, 1e-12, 1e-13, 1e-14, 1e-15\] dts= \[15., 25.,35., 50., 65., 75., 100., 125.\] setups …

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## [Can stiff ODE methods have an interpolant of the same order?](https://discourse.julialang.org/t/can-stiff-ode-methods-have-an-interpolant-of-the-same-order/85278)

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**Author:** [@xzackli](https://discourse.julialang.org/u/xzackli)\
**Replies:** 4\
**Last updated:** [August 4, 2022, 8:25pm UTC](https://discourse.julialang.org/t/can-stiff-ode-methods-have-an-interpolant-of-the-same-order/85278 "2022-08-04T20:25:00Z")

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Are there implicit methods that generate a continuous extension (i.e. dense output) at the same order as the integration? Many non-stiff solvers have this property, but I don’t know of any 4th or 5th order implicit solve…

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## [Symbolics.jl | Interfacing - building an expression tree to use custom rules](https://discourse.julialang.org/t/symbolics-jl-interfacing-building-an-expression-tree-to-use-custom-rules/85264)

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**Author:** [@Audrius-St](https://discourse.julialang.org/u/Audrius-St)\
**Replies:** 4\
**Last updated:** [August 4, 2022, 12:44am UTC](https://discourse.julialang.org/t/symbolics-jl-interfacing-building-an-expression-tree-to-use-custom-rules/85264 "2022-08-04T00:44:09Z")

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Hello, I would like to use the SymbolicUtils composing rewriter Fixpoint(rw) which returns a rewriter which applies rw repeatedly until there are no changes to be made to apply a custom rule to a Symbolics.jl expres…

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## [Can not choose alg for lyapunovspectrum DynamicalSystems.jl](https://discourse.julialang.org/t/can-not-choose-alg-for-lyapunovspectrum-dynamicalsystems-jl/85173)

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**Author:** [@Sergey\_Novak](https://discourse.julialang.org/u/Sergey_Novak)\
**Replies:** 9\
**Last updated:** [August 3, 2022, 7:46am UTC](https://discourse.julialang.org/t/can-not-choose-alg-for-lyapunovspectrum-dynamicalsystems-jl/85173 "2022-08-03T07:46:32Z")

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I have system and parametres: function HR!(du, u, p, t) function sigma(x) return 1.0 / ( 1.0 + exp( -10.0 \* ( x - ( - 0.25 ) ) ) ) end a, b, c, d, s, xr, r, I, vs, k1, k2, el\_link = …

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## [Dirac Delta with DifferentialEquations.jl](https://discourse.julialang.org/t/dirac-delta-with-differentialequations-jl/84835)

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**Author:** [@cmdenis](https://discourse.julialang.org/u/cmdenis)\
**Replies:** 4\
**Last updated:** [August 2, 2022, 8:18pm UTC](https://discourse.julialang.org/t/dirac-delta-with-differentialequations-jl/84835 "2022-08-02T20:18:55Z")

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Hi, I want to integrate a differential equations system numerically, but some equations involve Dirac delta functions. Is there a built-in delta function to do this? Or should I be mimicking one with a continuous funct…

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## [Linearizing ModelingToolkit ODESystem](https://discourse.julialang.org/t/linearizing-modelingtoolkit-odesystem/61387)

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**Author:** [@baggepinnen](https://discourse.julialang.org/u/baggepinnen)\
**Replies:** 3\
**Last updated:** [August 2, 2022, 11:58am UTC](https://discourse.julialang.org/t/linearizing-modelingtoolkit-odesystem/61387 "2022-08-02T11:58:48Z")

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I am having my bi-monthly go at ModelingToolkit and have reached a point where I can create an ODESystem from ControlSystems.jl StateSpace and connect a few of those togeher and simulate, nice :slight\_smile: As a next s…

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## [Agent Based Modeling resources other than Agents.jl documentation](https://discourse.julialang.org/t/agent-based-modeling-resources-other-than-agents-jl-documentation/85035)

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**Author:** [@franfram](https://discourse.julialang.org/u/franfram)\
**Replies:** 2\
**Last updated:** [August 2, 2022, 8:39am UTC](https://discourse.julialang.org/t/agent-based-modeling-resources-other-than-agents-jl-documentation/85035 "2022-08-02T08:39:32Z")

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Hello everyone. I’m starting to use Agents.jl for a PhD project on animal movement and epidemiology. I was wondering if anyone knew of additional resources to learn other than those of Agents.jl. George Datseris youtu…

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## [Symbolics.jl | Lie series expansion for PDEs](https://discourse.julialang.org/t/symbolics-jl-lie-series-expansion-for-pdes/84885)

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**Author:** [@Audrius-St](https://discourse.julialang.org/u/Audrius-St)\
**Replies:** 2\
**Last updated:** [August 1, 2022, 7:43pm UTC](https://discourse.julialang.org/t/symbolics-jl-lie-series-expansion-for-pdes/84885 "2022-08-01T19:43:31Z")

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Hello, I’ve used Sympy for several years now and am interested in giving Symbolics.jl a go with an eye to switching over completely. I’ve previously used Sympy to solve systems of ODES using the symbolic Lie series met…

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## [Modelling uncertainty propagation in forecasts 🔮](https://discourse.julialang.org/t/modelling-uncertainty-propagation-in-forecasts/85089)

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**Author:** [@agucova](https://discourse.julialang.org/u/agucova)\
**Replies:** 1\
**Last updated:** [August 1, 2022, 2:58pm UTC](https://discourse.julialang.org/t/modelling-uncertainty-propagation-in-forecasts/85089 "2022-08-01T14:58:21Z")

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Hi everyone, I’ve been playing with Causal, a business modeling platform which does automatic uncertainty calculations in forecasts. This works by allowing one to set model parameters in terms of ranges, which are real…

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## [Ensemble simulations with random initial conditions that sum to a fixed value](https://discourse.julialang.org/t/ensemble-simulations-with-random-initial-conditions-that-sum-to-a-fixed-value/84951)

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**Author:** [@Dragolonth](https://discourse.julialang.org/u/Dragolonth)\
**Replies:** 6\
**Last updated:** [August 1, 2022, 6:51am UTC](https://discourse.julialang.org/t/ensemble-simulations-with-random-initial-conditions-that-sum-to-a-fixed-value/84951 "2022-08-01T06:51:59Z")

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Hi all. For my ensemble problem, I would like to randomize the initial conditions for each trajectory. However, I would like the sum of certain species to remain a fixed number. For example, if u0 is my set of initial co…

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## [BifurcationKit MethodError](https://discourse.julialang.org/t/bifurcationkit-methoderror/84904)

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**Author:** [@Dragolonth](https://discourse.julialang.org/u/Dragolonth)\
**Replies:** 3\
**Last updated:** [August 1, 2022, 6:14am UTC](https://discourse.julialang.org/t/bifurcationkit-methoderror/84904 "2022-08-01T06:14:37Z")

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Hi all. Currently, I have a reaction network defined in Catalyst, and I’ve been following the tutorial at Bifurcation Diagrams · Catalyst.jl. However, I’ve been running into an error and am not entirely sure how to resol…

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## [Solve many first-order ODEs](https://discourse.julialang.org/t/solve-many-first-order-odes/85060)

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**Author:** [@qwerty](https://discourse.julialang.org/u/qwerty)\
**Replies:** 6\
**Last updated:** [July 31, 2022, 6:25pm UTC](https://discourse.julialang.org/t/solve-many-first-order-odes/85060 "2022-07-31T18:25:21Z")

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What is the best way to solve so many decoupled ODEs? Possibly in parallel. Should I use Modeling Toolkit or DifferentialEquations? The equations are of the type: u’ = -a^2k^2u where a is the same for all, then I have…

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## [What is the proper way to handle a discontinuity in Differential Equations?](https://discourse.julialang.org/t/what-is-the-proper-way-to-handle-a-discontinuity-in-differential-equations/84963)

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**Author:** [@Ge0rges](https://discourse.julialang.org/u/Ge0rges)\
**Replies:** 2\
**Last updated:** [July 29, 2022, 12:24pm UTC](https://discourse.julialang.org/t/what-is-the-proper-way-to-handle-a-discontinuity-in-differential-equations/84963 "2022-07-29T12:24:38Z")

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Hello all, I have a system of two differential equations that I am able to solve using Rosenbrock23(). I now would like to model this system with one modification, where at time t I manually change the value of one of …

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## [ODE solver slowed down by interpolation](https://discourse.julialang.org/t/ode-solver-slowed-down-by-interpolation/84913)

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**Author:** [@ap2013](https://discourse.julialang.org/u/ap2013)\
**Replies:** 1\
**Last updated:** [July 28, 2022, 12:04pm UTC](https://discourse.julialang.org/t/ode-solver-slowed-down-by-interpolation/84913 "2022-07-28T12:04:18Z")

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I have an ODE system defined through modelingtoolkit. The system is formed of multiple ODESystems and simplified with system = structural\_simplify(all). One of the ODESystem is defined by the following function functio…

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## [Save and load the model generated by surrogates.jl](https://discourse.julialang.org/t/save-and-load-the-model-generated-by-surrogates-jl/83302)

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**Author:** [@Jack\_Tang](https://discourse.julialang.org/u/Jack_Tang)\
**Replies:** 4\
**Last updated:** [July 28, 2022, 10:31am UTC](https://discourse.julialang.org/t/save-and-load-the-model-generated-by-surrogates-jl/83302 "2022-07-28T10:31:58Z")

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Hello Community, How can I save and load the model generated by surrogates.jl?

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## [Overfitting when building kriging surrogate model](https://discourse.julialang.org/t/overfitting-when-building-kriging-surrogate-model/84857)

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**Author:** [@chenr86](https://discourse.julialang.org/u/chenr86)\
**Replies:** 6\
**Last updated:** [July 28, 2022, 7:37am UTC](https://discourse.julialang.org/t/overfitting-when-building-kriging-surrogate-model/84857 "2022-07-28T07:37:26Z")

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Hello, I’m using Surrogates.jl to build a surrogate model based on data in a CSV file. There are 24 input parameters and 1 output variables in my data. I have 500 samples. 80% samples are used for training set and other…

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## [Iterate fitting linear model and extract coefficients over grouped DataFrame](https://discourse.julialang.org/t/iterate-fitting-linear-model-and-extract-coefficients-over-grouped-dataframe/84814)

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**Author:** [@guysutton](https://discourse.julialang.org/u/guysutton)\
**Replies:** 4\
**Last updated:** [July 27, 2022, 10:45am UTC](https://discourse.julialang.org/t/iterate-fitting-linear-model-and-extract-coefficients-over-grouped-dataframe/84814 "2022-07-27T10:45:44Z")

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I am trying to iteratively fit many models (e.g. a linear model \[GLM.jl\] or mixed model \[MixedModels.jl\]) to a DataFrame, whereby a new model is fit to each level of a grouping column. For example: if I wanted to fit one…

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## [How to get the variable names of a PDESystem if using a symbolic array](https://discourse.julialang.org/t/how-to-get-the-variable-names-of-a-pdesystem-if-using-a-symbolic-array/84776)

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**Author:** [@hpieper14](https://discourse.julialang.org/u/hpieper14)\
**Replies:** 7\
**Last updated:** [July 26, 2022, 8:40pm UTC](https://discourse.julialang.org/t/how-to-get-the-variable-names-of-a-pdesystem-if-using-a-symbolic-array/84776 "2022-07-26T20:40:36Z")

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I’m having a hard time understanding exactly how a PDESystem with independent array variables differs from a PDESystem initialized with other independent variables. For example, suppose I have the following two PDESystem…

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## [How can I save surrogate model in Julia and use it in Python](https://discourse.julialang.org/t/how-can-i-save-surrogate-model-in-julia-and-use-it-in-python/84746)

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**Author:** [@Jack\_Tang](https://discourse.julialang.org/u/Jack_Tang)\
**Replies:** 4\
**Last updated:** [July 26, 2022, 2:29am UTC](https://discourse.julialang.org/t/how-can-i-save-surrogate-model-in-julia-and-use-it-in-python/84746 "2022-07-26T02:29:09Z")

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Hello, In my project, I would like to save surrogate model in Julia and use it in Python. HDF5 is the best file format? If yes, How can I read the surrogate model and use it in Python? Thanks!

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## [Using CellListMap.jl within Agents.jl framework](https://discourse.julialang.org/t/using-celllistmap-jl-within-agents-jl-framework/84709)

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**Author:** [@ehsani63](https://discourse.julialang.org/u/ehsani63)\
**Replies:** 15\
**Last updated:** [July 25, 2022, 5:26pm UTC](https://discourse.julialang.org/t/using-celllistmap-jl-within-agents-jl-framework/84709 "2022-07-25T17:26:02Z")

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Hi, I am using CellListMap in one of my ABM simulations (using Agents.jl). It suits very well what I want to do: There some active agents and some static obstacles. CellListMap finds collisions between agents and obstacl…

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## [Sensitivity of a SDE w.r.t tspan](https://discourse.julialang.org/t/sensitivity-of-a-sde-w-r-t-tspan/83998)

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**Author:** [@stochasticguy](https://discourse.julialang.org/u/stochasticguy)\
**Replies:** 3\
**Last updated:** [July 21, 2022, 3:51pm UTC](https://discourse.julialang.org/t/sensitivity-of-a-sde-w-r-t-tspan/83998 "2022-07-21T15:51:44Z")

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Hi all, I’m using SciMLSensitivity.jl to get the sensitivities of the transformation defined by: f(S\_T) = exp(- r \* T) \* max(S(T) - K, 0.0), where S\_T is the solution at time T of a SDE system of the form: dS\_t = \\mu…

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## [Max Plus Method for Joint Action Selection: Use-Case and Implementation](https://discourse.julialang.org/t/max-plus-method-for-joint-action-selection-use-case-and-implementation/71847)

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**Author:** [@YummyPampers2](https://discourse.julialang.org/u/YummyPampers2)\
**Replies:** 1\
**Last updated:** [July 21, 2022, 12:42am UTC](https://discourse.julialang.org/t/max-plus-method-for-joint-action-selection-use-case-and-implementation/71847 "2022-07-21T00:42:32Z")

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Good Day Folks: Could anyone point me towards a successful implementation (or proxy) of the Max-Plus method for joint selection? If not are there any Markov Decision Process (MDP) or multi-agent Markov decision proce…

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## [ModelingToolkit : Incorparating 2nd Order ODE's into functions](https://discourse.julialang.org/t/modelingtoolkit-incorparating-2nd-order-odes-into-functions/84421)

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**Author:** [@xztt](https://discourse.julialang.org/u/xztt)\
**Replies:** 8\
**Last updated:** [July 20, 2022, 5:30pm UTC](https://discourse.julialang.org/t/modelingtoolkit-incorparating-2nd-order-odes-into-functions/84421 "2022-07-20T17:30:47Z")

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Hi all, I am looking to incorporate the following valve functions in MTK from a paper the images below give the mathematical description of the valves. The paper can be found here The way I am trying to implement…

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## [Passing jacobian generated from ModelingToolkit to ODEFunction for EnsembleGPUArray](https://discourse.julialang.org/t/passing-jacobian-generated-from-modelingtoolkit-to-odefunction-for-ensemblegpuarray/84198)

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**Author:** [@RicardoFR](https://discourse.julialang.org/u/RicardoFR)\
**Replies:** 7\
**Last updated:** [July 19, 2022, 10:52pm UTC](https://discourse.julialang.org/t/passing-jacobian-generated-from-modelingtoolkit-to-odefunction-for-ensemblegpuarray/84198 "2022-07-19T22:52:43Z")

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Hello all! I want to do some tests with GPU Parallel Ensemble Simulation using DifferentialEquations.jl. As a starting point, I was looking into the example in the DiffEqGPU.jl page. Regarding Stiff ODEs, it’s said that…

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## [Repetition of sample blocks the building of Kriging surrogate model](https://discourse.julialang.org/t/repetition-of-sample-blocks-the-building-of-kriging-surrogate-model/84374)

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**Author:** [@chenr86](https://discourse.julialang.org/u/chenr86)\
**Replies:** 1\
**Last updated:** [July 18, 2022, 2:40am UTC](https://discourse.julialang.org/t/repetition-of-sample-blocks-the-building-of-kriging-surrogate-model/84374 "2022-07-18T02:40:14Z")

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Hello, I’m using my own data as input to build a kriging model. There are three columns for input variables, one column for output variable. For input, there are: day\_of\_year: transforming the date into the day of ye…

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