28
votes
Nonlinear differential equation: numerical solution
Introduction
I think there are several questions on this site about ODEs of the form
$$(x-a)^2 u''(x) = F(x,u,u')$$
with an initial condition at $x=a$.
There is no general guarantee that solutions ...
- 233k
25
votes
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How do I use the new nonlinear finite element in Mathematica 12 for this equation?
OK, there are a few things going on here. Let me explain them in turn. First, as the message suggests, this should be written in Inactive form (we'll get to the why later). If you click on the three ...
- 39k
23
votes
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17
votes
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Linearization of a nonlinear system
a) To find equilibria, use Solve:
eq = Solve[{A - B*x - x*y^2, A*(x*y^2 - y)} == {0, 0}, {x, y}]
b) Linearizing around the ...
- 19.5k
16
votes
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Is there any predictor-corrector method in Mathematica for solving nonlinear system of algebraic equations?
Since @hesam asked about a command, and to get a better understanding of @DanielLichtblau's approach, I tried to generalize it and package it in a function. Feedback would be appreciated!
...
- 19.5k
16
votes
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Curve tracing for a given data set
You have to fit implicitly, for example fit a conic section (or ellipse )
...
- 49.1k
15
votes
Plotting a Bifurcation diagram
Please note that though cosmetically appealing this is not rigorous or the best insight into basin of attraction of system as pointed out by bbgodfrey in comment below. I leave it as, perhaps, a road ...
- 58.7k
15
votes
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Singularity error. What is actually causing the problem here?
Description of the issues
It is a bit unusual to discuss an ODE system as a function of a parameter ν but with a fixed initial condition ...
- 233k
14
votes
NonlinearModelFit's fit is atrocious
We need to select another fit function( shift the function Sin[a*x] to Sin[a (x + p)] + q)
...
- 63.5k
13
votes
Is there any predictor-corrector method in Mathematica for solving nonlinear system of algebraic equations?
As noted by @ChrisK, this works better starting at the top. Reason being there are no real solutions below the parameter value of 48.
Using FoldList one can ...
- 58.4k
13
votes
What can one do with extremely stiff problem in NDSolve?
EDIT #2
My error was useful. It brought me to the conclusion that the difficulties in solving the PDE of the OP are due to the drift term
$$\frac{\partial (x u(x,t))}{\partial x}$$
If the drift ...
13
votes
Conservation of area solving a PDE via finite difference scheme
Partly NDSolve-based solution
Use a higher even order spatial grid to discretize the PDE to an ODE set seems to be a good approach. The definition of ...
- 63.5k
13
votes
How to apply different equations to different parts of a geometry in PDE?
Denote the disk by $\varOmega$ and its boundary by $\varGamma = \partial \varOmega$. I'd prefer to denote the function residing on the boundary by $u \colon \varGamma \to \mathbb{R}$; the function on ...
- 104k
13
votes
Accepted
Finding the Period of a Limit Cycle
Although it's primarily designed for ecological models, my EcoEvo package can help. First, you need to install it with
...
- 19.5k
13
votes
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NonlinearModelFit's fit is atrocious
Nonlinear optimization problems almost never just magically work without a push in the right direction. This is especially true for problems with multiple local minima like this one. You need to give ...
- 21.9k
12
votes
Numerical solution of coupled ODEs with boundary conditions
This is the most difficult of the nearly two dozen nonlinear ODE separatrix computations that I have encountered on Mathematica.SE. Nonetheless, it can be can be solved by a systematically refined ...
- 60.7k
12
votes
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Heat convection differential equations from 1952 - Mathematica "fails to converge"
The problem is with the default starting initial conditions used by the shooting method in NDSolve. The shooting method is where ...
- 233k
12
votes
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DSolve doesn't work
Edit: Derivation of symbolic solution
As commented by Mariusz Iwaniuk), the initial conditions in the question are inconsistent. Setting ...
- 60.7k
12
votes
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Problem with optimal control and Pontryagin's maximum principle
Pontryagin's minimum principle means that we have to use Euler-Lagrange equations. Therefore code looks like this
...
- 41.9k
12
votes
Routh-Hurwitz criterion not giving correct answer when done manually?
I can't say why your approach didn't work, but my RouthHurwitzCriteria function uses a simplified test for 3x3 matrices due to Fuller (1968), which I first learned about from Gandolfo (1997):
There ...
- 19.5k
12
votes
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Are there practical Mathematica tools for detection of limit cycles in two dimensional dynamical systems?
In general, I'm not sure there's a good algorithm to find all limit cycles for a given set of equations. But if there's one in particular you want to find, then it's not too hard with a decent ...
- 19.5k
12
votes
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11
votes
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Nonlinear model fit not fitting with good parameters. I can find the coefficients manually and origin manages to fit anyway
You need to use a constraint for c to avoid complex numbers.
...
- 3,861
11
votes
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Need to fit curve to 5 parameters: what's a problem with NonlinearModelFit?
Because there is a term x^d, I dropped the point {0, 0} from the data (see here).
Next, the terms ...
- 23.7k
11
votes
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How to stop DSolve from solving equations
It seems that Solve will be called during the DSolve calculation, so if you stop Solve, the <...
- 1,933
11
votes
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How to apply different equations to different parts of a geometry in PDE?
Since I have the code to solve the original problem described in the article GDI-Mediated Cell Polarization in Yeast Provides Precise Spatial and Temporal Control of Cdc42 Signaling, I will give here ...
- 41.9k
11
votes
Finding the Period of a Limit Cycle
Here is a simple approach to get the period of the unknown limit cycle.
The idea is to approximate the limitcycle by a circle (1st harmonic) around the mean of the limitcycle:
solution ...
- 49.1k
11
votes
Accepted
Steffensen's Method Implementation in Mathematica
I think you did everything right. The problem is that {Vx[1,1],Vy[1,1]} equals {253, 292} which is very far from ...
- 104k
11
votes
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Fitting a power law on linear and log scale
The difference is in the way you treat the residuals. The real model is usually:
Y == a X^b + error
where error follows some ...
- 21.9k
10
votes
Accepted
Solve stiff system by shooting method
The goal of this question, I believe, is selecting Φ[r0] so that the solution connects smoothly to the asymptotic solution at large ...
- 60.7k
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