Questions tagged [differential-equations]
For questions about solving, analyzing, or creating differential equations to model some system. If possible, include specific tags about the type of differential equation (e.g. [tag:pde], [tag:ode], [tag:stochastic-ode]).
161
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-2
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38
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Setting up boundary conditions to solve PDEs using method of lines
Objective: To add boundary/initial conditions (BCs/ICs) to a system of ODEs
I have used the method of lines to convert a system of PDEs into a system of ODEs. The ODEs themselves involve a lot of ...
1
vote
0
answers
57
views
Implementation of integration schemes for ordinary differential equations in Python and peformance comparison
I look for a book/manual where I can find implementations of different integration schemes for ordinary differential equations (like 4-th order Runge-Kutta) in Python with Numba.
To be more specific, ...
- 161
0
votes
0
answers
69
views
How to estimate stability and stiffness of a system of coupled ODEs?
I'm running into issues with Python/Julia ODE solvers requiring prohibitively small timesteps to evolve a system of 4 coupled ODEs (the order of magnitude of the state variables and time unit span ~40-...
- 111
2
votes
1
answer
107
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2nd order differential equation coupled to integro-differential equation in python
I'm trying to solve the following equations numerically in python
$$\begin{align}
12\pi\int_0^\infty drf(r)\phi(r)r^4&=E\\
f(r)-\frac{1}{2\mu}\bigg(\frac{d^2\phi(r)}{dr^2}+\frac{2}{r}\frac{d\phi(r)...
- 23
2
votes
1
answer
93
views
Boundary value problem with singularity and boundary condition at infinity
I'm trying to solve the following boundary value problem on $[0,\infty]$:
$$f^{\prime \prime}=-\frac{1}{r} f^{\prime}+\frac{1}{r^{2}} f+m^{2} f+2 \lambda f^{3}$$
$$f(0)=0 \ ; f(\infty)=\sqrt{-m^2/(2\...
- 195
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votes
1
answer
78
views
How to deal with solving coupled ODE systems where variables are updated multiple times within each timestep?
I'm solving a system of coupled ODEs using Euler integration for simplicity. To make this concrete, please see the (extremely simplified) minimal working example below in Python. Imagine we have a box ...
- 111
2
votes
0
answers
49
views
Implementation of Jeffery equation in FVM solver
I am trying to implement the Jeffery equation in a Finite Volume Method solver:
The equation reads:
$$
\frac{\mathrm{d} \mathbf{A}}{\mathrm{d} t} = \mathbf{W} \cdot \mathbf{A} - \mathbf{A} \cdot \...
- 21
1
vote
0
answers
71
views
Well-conditioned pseudospectral for computing eigenvalues to (partial) differential equations
I am working on writing a Chebyshev pseudospectral method (see for example "Chebsyhev and Fourier spectral methods" by John Boyd) to solve for the eigenvalues of differential equations of ...
0
votes
0
answers
18
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Transparent Boundary Conditions relationship with intermediate BCs in ADI-PR method
I have read some materials about ADI - PR method with the aim to understand how to put boundary conditions in my 2D scheme which solves the Time-Dependent Schrodinger Equation.
All the theory I read ...
- 131
0
votes
0
answers
69
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Finite difference solver for the 2D Poisson's equation with an integral boundary condition
I wanted to attempt an implementation of a finite-difference-based solver for the 2D elctrostatic Poisson equation when metallic objects are present. Also, I hope to take as input, the location of ...
1
vote
1
answer
67
views
solving a Algebraic Differential Equation in Julia using modelingToolKit.JL
I'm trying to solve a differential algebraic equation in Julia's modelingTookKit.JL, where the vector field has the form f(X) = 0.
I found an example of a DAE in the below link
modelingToolkit.JL DAE](...
- 43
2
votes
1
answer
111
views
DG method for solving Hyperbolic Partial Differential Equation with Dirichlet Boundary Conditions
Consider the following partial differential equation
\begin{align}
\frac{\partial u}{\partial t}+\frac{\partial f}{\partial x} &= g(x,t), \ \ x\in \Omega = [x_{L},x_{R}] \\
u(x,0) &= u_{0}(x) ...
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3
votes
1
answer
121
views
How to solve a BVP with known parameters?
I need to solve a boundary value problem (BVP) of second order, where the equation depends on several know parameters, which are geometric parameters and material constants.
I would like to solve this ...
- 510
3
votes
1
answer
117
views
Solving DAE in Julia using GPUs
I'm trying to solve a Differential Algebraic Equation (DAE) in Julia which is very computationally expensive using GPUs. I'm brand new to Julia and don't have much experience coding with GPUs. The ...
- 43
0
votes
0
answers
71
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Solving an apparently tricky geodesic BVP in Matlab
I want to be able to solve the BVP
$$\ddot \mu_k = -\frac{\mu_k}{2} \left ( \sum_{i=1}^n \frac{\dot \mu_i^2}{\mu_i} - \frac{\dot \mu_k^2}{\mu_k^2} + \frac{ \left [ \sum_{i=1}^n \dot \mu_i \right ]^2}{\...
- 123
0
votes
0
answers
56
views
Solve simultaneous differential equations with embedded functions and a parameter estimation
The aim is to solve the below equations and plot $m$ with time, i.e. $\frac{dm}{dt}$
$k$ is unknown and needs to be estimated. For the parameter estimation, the below values in the table for m versus ...
3
votes
2
answers
185
views
Is there a Python version of the ODE tool pplane?
This is the same question as this one, except for Python instead of Mathematica. Basically, the MATLAB software PPLANE is a staple in ODE courses. Is there a Python equivalent?
I don't know much about ...
2
votes
1
answer
85
views
Implicit integrator for ODE with quadratic right-hand side
I have an ODE for an unknown $x(t):[0,\infty)\to\mathbb R^n$ of the following form:
$$
x_i'(t)=a_i^\top x(t) + x(t)^\top Q_i x(t),
$$
for $i\in\{1,\ldots,n\}$. Here, the vectors $a_i\in\mathbb R^n$ ...
- 1,331
1
vote
2
answers
373
views
Recommendations for ODE solvers for stiff equations
I'm continuing the research of a former Ph.D. student in my group requiring the solution of a system of ODEs. On a technical note, they wrote:
The system of Boltzmann equations behaves numerically ...
- 105
2
votes
0
answers
112
views
Floquet theory for periodic delay differential equations: current numerical routines
I would like to determine the stability of a system of periodic delay differential equations (a seasonal host-parasite model). I've tried to implement the method described in Lemma 2.5 in this paper:
...
- 21
1
vote
1
answer
586
views
solve_ivp from scipy does not integrate the whole range of tspan
I'm trying to use solve_ivp from scipy in Python to solve an IVP. I specified the tspan ...
0
votes
1
answer
59
views
Logistic growth curve using scipy is not quite right
I'm trying to fit a simple logistic growth model to dummy data using Python's Scipy package. The code is shown below, along with the output that I get. The correct ...
9
votes
1
answer
631
views
How to solve a second order differential equation (diffusion) with boundary conditions using Python
I am having trouble implementing a model from a publication.
Huang, K-L.; Holsen, T.M.; Selman, J.R. Ind. Eng. Chem. Res. 2003, 42, 15, 3620–3625
scihub link: https://sci-hub.se/10.1021/ie030109q
I ...
- 91
3
votes
4
answers
172
views
Solving the eigenvalue from a set of coupled second order differential equation numerically
I met a problem in solving a set of coupled differential equation, as shown below:
$$A_1\psi_1(z)+A_2\frac{d^2\psi_1(z)}{dz^2}+A_3\frac{d\psi_2(z)}{dz}=\lambda\psi_1(z)$$
$$A_4\psi_2(z)+A_5\frac{d^2\...
- 131
3
votes
0
answers
88
views
Numerical Soultion to Background equations of cosmology
I am trying to solve the background equations of cosmology numerically using Runge-Kutta Dormand Prince method with simplified assumption $8\pi G=1$ and $c=1$. The equations are
$$\ddot a = - \frac{1}{...
- 39
0
votes
0
answers
29
views
How to solve nonlinear second order ODE in Matlab? [duplicate]
I am working on simulating a car suspension system using Matlab.
Specifically, I have to derive equation of motion using the Lagrange method and then use ode 45 to solve it. However, while using ...
Luận Trần Công
7
votes
0
answers
155
views
Is there a graphical interpretation or explanation of automatic differentiation compared to numerical differentiation
I have been looking at automatic differentiation for solving differential equations lately. I understand the basic ideas of using Dual numbers and such for finding derivatives, etc. However, I feel ...
- 245
0
votes
1
answer
39
views
How to decrease error in (FTCS) forward time centered space method?
I am using the FTCS method for solving differential equations. I know that the condition for stable output is
$$
\frac{\alpha \Delta t}{\Delta x ^2} < \frac{1}{2}
$$
But when I use the distance ...
- 13
4
votes
1
answer
90
views
Finite Volume on Cubed Sphere
The US weather model uses an uncommon (?) discretization called 'Finite Volume on Cubed Sphere'.
To avoid the singularities that occur at the poles when using lat/lon discretization, they instead ...
- 43
0
votes
0
answers
23
views
Why does this Non-Standard FDTD implementation lead to infinite increase in the magnitude of an EM pulse?
I have been working on a Particle-In-Cell Framework in Python and have noticed an issue where the magnitude of a EM pulse increasing infinitely as the simulation updates.
Currently, I am using the Non-...
5
votes
2
answers
589
views
Specifying ode solver options to speed up compute time
I'm specifying the 'JPattern', sparsity_pattern in the ode options to speed up the compute time of my actual system. I am sharing a sample code below to show how I ...
- 581
3
votes
1
answer
514
views
Solving numerically the 1D Kuramoto-Sivashinsky Equation using spectral methods
Lately, I've been trying to solve numerically the 1D Kuramoto-Sivashinsky Equation using spectral methods.
Let $\nu$ be the viscosity and $[0,L]$ the domain. The 1D equation is,
$$
u_t + uu_x + u_{xx} ...
- 153
4
votes
1
answer
88
views
Geometric integrators besides midpoint/Crank-Nicolson?
I have a first-order ODE
$$
\dot{x} = a(t) \times x, \quad x(0) \in\mathbb{R}^3.
$$
with $\|a(t)\| = 1 \;\forall t$.
Consequently, $\|x(t)\|=\|x(0)\|$ for all $t>0$. I would like this to be ...
- 3,058
3
votes
2
answers
340
views
How to solve second order coupled non linear differential equations
For a project I am doing, I have to solve the following system of differential equations numerically using my own code:
$$
x^2K'' = KH^2 + K(K^2-1)
$$
and,
$$
x^2H'' = 2K^2H + \alpha H(H^2-x^2)
$$
...
- 41
1
vote
1
answer
98
views
Numerical integrator for $a'(t)=e^{-a(t)}f(t)$
Suppose I know a function $f(t)$ and all its derivatives in $t$ in closed form. Given $a(0)$ and some $t_0>0$, I'm looking for an explicit integrator that can estimate $a(t_0)$, where $a(\cdot)$ ...
- 1,331
1
vote
1
answer
239
views
Finite Element Method for 1D Poisson Equation with Inhomogeneous Boundary Conditions
Im trying to solve the Poisson equation in 1D: $$-u_{xx} = f(x), \hspace{6mm} u(a) = d1, \hspace{2mm} u(b) = d2$$Assuming a uniform partition such that $x_n = a + nh$, where $h = (b-a)/N$ and $n \in [...
- 123
2
votes
0
answers
137
views
Find time step for Euler method in numerical solving of a system of non linear differential equations
I have a system of non linear differential equations in the form :
$$\frac{dy_i}{dt}=\sum_j a_{ij} y_i y_j $$.
I first tried to solve it with Python suing ...
- 171
0
votes
1
answer
73
views
2 point BVP solver: how to compute errors
Background
I am working with chapter 2 in LeVeque's book: https://faculty.washington.edu/rjl/fdmbook/
I have build my own solver in Python to solve the 2 point BVP:
$$
\epsilon u''+u(u'-1) =0 , \\
u(0)...
- 255
1
vote
0
answers
102
views
Stochastic differential equation system (SDE) : overflow encountered in double-scalars
I'm trying to integrate the following SDE system from Dekker et al. [1]
$$\begin{cases}
\frac{dx}{dt}=a_1x^3+a_2x+\phi+\zeta_x\\
\frac{dy}{dt}=b_1z+b_2(\kappa(x)-(y^2+z^2))y+\zeta_y\\
\frac{dz}{dt}=...
- 153
4
votes
2
answers
1k
views
CUDA & Python for numerical integration and solving differential equations
Can anyone please suggest some libraries which allow use CUDA in Python for numerical integration and/or solving of differential equations?
My goal is to solve large (~1000 equations) of coupled non-...
- 161
3
votes
0
answers
70
views
Difference between wave vector and density matrix in numerical calculation of Schrödinger equation
I solved Schrödinger equation for a following tow-level time-dependent Hamiltonian numerically in two ways:
...
- 31
0
votes
0
answers
365
views
Numerov method for solving Schrödinger equation
I have just begun learning computer science to apply it to Physics and I am trying to write a code for solving Schrödinger's equation of the harmonic oscillator (setting $V=\frac{x^2}{2}$) in one ...
- 27
-2
votes
1
answer
64
views
Solving differential equation by setting vectorization `on` in MATLAB
This is a follow up to my previous question posted here.
I've set up an ode system in MATLAB and I'm trying to vectorize the code to increase the speed of computation.
The follow is the code for my ...
- 581
1
vote
1
answer
60
views
solving differential equations with jacobian pattern
I'm trying to compare the simulation time for solving a system of differential equations with and without jacobian pattern for a toy model using ode15s in MATLAB.
...
- 581
2
votes
0
answers
74
views
Linearising Nonlinear Coupled Partial Differential Equations - Alfvénic Diffusion
I am trying to solve the following coupled partial differential equations with a finite difference scheme:
$$\partial_tf+v\partial_zf+\partial_z\frac{1}{W}\partial_zf=0$$
$$\partial_tW+v\partial_zW-\...
- 21
0
votes
2
answers
875
views
Solve a system of coupled differential equations in Python
I have a system of two coupled differential equations, one is a third-order and the second is second-order. I am looking for a way to solve it in Python.
I would be extremely grateful for any advice ...
- 33
7
votes
1
answer
160
views
Which finite difference better approximates $uu'$?
I want to approximate $uu'$ with a finite difference. On the one hand, it seems to be
$$(uu')_i=u_i\frac{u_{i+1}-u_{i-1}}{2\Delta t}=\frac{u_iu_{i+1}-u_iu_{i-1}}{2\Delta t}$$
On the other hand,
$$(uu')...
-1
votes
1
answer
2k
views
ODEintWarning: Excess work done on this call (perhaps wrong Dfun type)
I was messing around with some numerical integration functions. I wrote an arbitrary differential equation to test my understanding, the code is as follows:
...
1
vote
0
answers
69
views
Book recommendation on numerical methods for solving Integro-Differential equations
I was wondering if anyone could recommend a good book or resource on numerical methods for solving integro-differential equations? Of course I am familiar with the methods for solving ODEs and PDEs ...
- 245
1
vote
1
answer
538
views
Trouble with backwards time integration in Python
I am struggling with a rather basic numerical integration task: Using Python's scipy.integrate.solve_ivp module to integrate an ODE sytem backwards in time. As a test, I am using the following ODE ...
- 143