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# Stability of Positivity Preserving Patankar-Type Schemes

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[![License: MIT](https://img.shields.io/badge/License-MIT-success.svg)](https://opensource.org/licenses/MIT)

This repository contains the code for the reproducibility of "A new Stability Approach for Positivity-Preserving Patankar-type Schemes" by D. Torlo, P. Öffner and H. Ranocha [arXiv preprint](https://arxiv.org).

We provide some *Julia* codes and Jupyter notebooks to reproduce the simulations of the paper, some *Mathematica* notebook to reproduce the symbolic computations and the results of the paper, and the supplementary material file with all the results that could not fit in the article.

### Julia codes
In folder [simulations](simulations/) there are:
* the implementations of all the Patankar and modified Patankar schemes ([Patankar Runge--Kutta](simulations/mPRK.jl) and [modified Patankar Deferred Correction](simulations/mPDeC.jl)) and some auxiliary functions ([Lagrange polynomials file](simulations/Lagrange_polynomials.jl))
* the study of the general **linear $2\times 2$ system** searching for the maximum $\Delta t$ bound for which we do not observe oscillations for all the schemes is in the [CFL notebook](simulations/SearchingCFLconditions.ipynb)
* the study of the spurious steady state for all the methods is in the [Inconsistency notebook](simulations/Inconsistency.ipynb)
* the test of the stability bound previously found on **scalar nonlinear problem** is in [Nonlinear Problem notebook](simulations/Nonlinear_Problems.ipynb)
* the test of the inconsistency on **Robertson problem** is in [Robertson notebook](simulations/Robertson_Problem.ipynb)
* the test of the inconsistency on **HIRES problem** is in [HIRES notebook](simulations/Problem_HIRES.ipynb)


### Mathematica notebooks
In folder [symbolic_computations](simbolic_computations/) contains:
* the computations of the $\Delta t$ bound on the general **linear $2\times 2$ system** for the **MPRK(2,2,1)** scheme is in the [MPRK(2,2,1) notebook](simbolic_computations/MPRK_2_2_1_generalSystem.nb)
* the computations of the $\Delta t$ bound and the Taylor expansion to understand which scheme has inconsistency on the **symmetric linear $2\times 2$ system** for some of the **MPRK(2,2,$\alpha$)** schemes is in the [MPRK(2,2,$\alpha$) notebook](simbolic_computations/MPRK_2_2_alpha.nb)
* the Taylor expansion to understand if **MPRK(3,2)** has inconsistency on the **symmetric linear $2\times 2$ system** is in the [MPRK(3,2) notebook](simbolic_computations/MPRK_3_2.nb)
* the Taylor expansion to understand if **mPDeC2** and **mPDeC3** have inconsistencies on the **symmetric linear $2\times 2$ system** is in the [MPDeC notebook](simbolic_computations/MPDeC.nb)

### Supplementary material
In folder [supplementary_material](supplementary_material/) there is the [file](supplementary_material/supplementary_material.pdf) with the supplementary results which are only described in the report 


## Disclaimer

Everything is provided as-is and without warranty. Use at your own risk!