Questions tagged [nonlinear]

Used to mark questions about nonlinear differential equations, `NonlinearModelFit`, and related to nonlinear dynamics.

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1answer
39 views

Optimization of nonlinear equation

I m try to solve non linear equations, and I m blocked ...
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0answers
82 views

How do I create a piecewise function using Do[] or While[]?

The following non-linear circuit is an uncontrolled three-phase full-wave 6-pulse rectifier. The inputs are $v_{an}(t) = V_m \sin (\omega t)$, $v_{bn}(t) = V_m \sin (\omega t-120°)$ and $v_{cn}(t) = ...
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1answer
103 views

Solving symbolic system of non-linear equations takes too long

I am trying to solve a set of system of symbolic non-linear equations: ...
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37 views

Is it possible to find a solution to this equation, reducing it to the Fredholm equation? [migrated]

To continue the questions: https://math.stackexchange.com/questions/3743374/approximation-of-the-convolution-operator?noredirect=1#comment7697800_3743374 https://math.stackexchange.com/questions/...
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3answers
157 views

Problem plotting a bifurcation diagram of logistic map

I would like to plot a bifurcation diagram from the logistic map (difference equation) of the type $$ x_{t+1} = r\,(1-x_{t})\,x_{t}. $$ The code I used for that was ...
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2answers
136 views

FindFit problem, some models work — and some do not

I asked this question. Answers and comments were appreciated. But (some) of the comments did not make satisfied. [Maybe due to my bad English I did not describe my problem very well]. Let me try ...
2
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1answer
97 views

Calculate the Lyaponov exponent for a driven dampend spherical pendulum?

I’m relative new to Mathematica and also new to this forum. In fact this is my first question so please apologise if I do some formatting errors. I want to calculate the Lyapunov exponent of a driven ...
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3answers
116 views

FindFit, FindFormula, Fit, NonlinearModelFit - Which one to use? (I got no good curves & plots)

See this code, what mistake I did? (I got no plot) ...
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1answer
123 views

Approximate solution of a nonlinear ODE in the form of a Fourier series containing the coefficients of the initial ODE

In this topic we considering nonlinear ODE: $\frac{dx}{dt}= (x^4) \cdot a_1 \cdot sin(\omega_1 \cdot t)-a_1 \cdot sin(\omega_1 \cdot t + \frac{\pi}{2})$ - Chini ODE https://www.maplesoft.com/support/...
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1answer
72 views

Modern methods for ODE with complex harmonic oscillations and phasors

We have a differential equation of the following form: $\frac{dx}{dt}=f(x) \cdot a \cdot \sin(\omega \cdot t)-a \cdot \sin(\omega \cdot t + \frac{\pi}{2})$ where $f(x)$ - arbitrary function from state ...
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1answer
86 views

How to run NDSolve over a large range without freezing?

I am trying to solve a set of coupled differential eqns. I need the solution over a large range of the variable t. But the notebook freezes and stops working. ...
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1answer
85 views

NonLinearModelFit is overlooping throught constraints

I have a NonLinearModelFit in which a constraint is this formula (I have added the Print function to test) ...
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2answers
65 views

Find best nonlinear equation for data [closed]

I have the following data: https://pastebin.com/APjy2G3Q which plotted with ListLinePlot[data] looks like: Does anybody know what will be the best non-linear ...
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1answer
99 views

NonlinearModelFit with some shared parameters

I have a big set of data (more than 800 spectra) which need to be fit with a Maxwell-Boltzmann function. Let's start with only two spectra ...
2
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1answer
52 views

Calculating the bounce solution numerically

I would like to obtain a numerical solution to the following example bounce equations, $$\begin{align*} \frac{\partial^2 a}{\partial t^2} &= \frac{1}{t^2} a(1-a)(1-3a)-\frac{b^2}{2}(1-a)\\ \frac{\...
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1answer
126 views

1-D PDE with nonlinear ODE as boundary condition

Recently, I am trying to solve a 1-D PDE with a nonlinear boundary condition using the function NDSolveValue. However, it seems that MMA (12) cannot solve it directly with some computational issues. ...
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0answers
70 views

How to plot Poincaré map for system consists of 3 degree of freedom

here the equations of motion and data I need to plot Poincaré map for this system: ...
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0answers
62 views

How to plot x[t] vs x'[t]?

Hello please help me to plot my code like the right side plot where I cannot plot the phase plane plot Displacement VS velocity, see plz the attached picture.. Here is my drafted code , Where I got ...
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0answers
37 views

NDSolve for closed-loop system with Unit Delay (attempt to compare with Simulink) [closed]

I'm trying to reproduce a scheme from Simulink with a unit delay. To do this, I use the NDSolve command with an additional equation that implements a unit delay loop. The results do not match, and I ...
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0answers
22 views

Using NonlinearModelFit with NIntegration

Before actually elaborate my question, just a little bit of contextualization: i'm trying to reproduce a result published in a paper. The experimental data presented by the paper's author are as ...
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2answers
111 views

Multi-Peak fitting with specified fit function of overlapped peaks

I have been trying to adapt some of the previous suggestions on other questions similar to my problem for over two weeks now but I have not been able to solve my problem so far. So, I would like to ...
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0answers
25 views

Using Mathematica to solve a nonlinear system of parameters

I'm looking for some sets of solutions for this nonlinear system. I only have to respect the conditions, getting numerical values to use for another thing: $\delta_1>10 |\lambda_1| (1)$ $\delta_2&...
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1answer
40 views

Find unknown parameters from NonLinear fitted data

I obtained the fitted model from the image using the following code: ...
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0answers
37 views

NonlinearModelFit does not appear to be updating

I have the following issue when trying to use NonlinearModelFit. Consider the following snippet of the data set: ...
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0answers
98 views

Using a differential equation to fit 2D data in time

I'm trying to fit some data. The data is a series of snapshots along a radius which is visualising a decay. It seems to work, but the fits are pretty terrible and take very a long time. The data is a ...
2
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1answer
71 views

Having error with solving BVP problem

I am trying to solve this differential equation but using my approach: ...
5
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1answer
114 views

Getting errors in NonlinearModelFit

I need to fit a system of ODEs to data. The solutions which could work for me are given in the following posts How to fit 3 data sets to a model of 4 differential equations? ODE fitting to dataset ...
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1answer
34 views

NSolve and MemoryAllocationFailure

I have a code like, ...
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0answers
53 views

NDSolve error for system of two non-linear equations

I'm trying to solve this system of non-linear ODE's $$\left\{y'(s) \left(x''(s) y'(s)-x'(s) y''(s)\right)=0,\mu x'(s) \left(x'(s) y''(s)-x''(s) y'(s)\right)=g \lambda \left(x'(s)^2+y'(s)^2\right)^{3/...
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0answers
50 views

Coupled nonlinear differential system with discontinuous functions which values are given by list elements, boundary conditions, NDSolve

I am very new to mathematica, I have googled my problem in vain, hope you can help me. I am currently trying to solve a non linear system of two coupled differential equations given by (41) and (42). ...
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1answer
74 views

Not happy with my fit results. How to improve it?

I am currently working on a topic involving nuclear theory (the study of deformed nuclei). So for the past two days, I've been trying to fit some data; I developed a theoretical model that aims at ...
2
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1answer
114 views

Solving delay differential equation. DSolve returns `Indeterminate`

I want to solve the following systems of delay differential equations $$ \text{SYSTEM 1}\\ x'(t) = -y'(t)-z'(t)\\ y'(t) = -z'(t) + x(t)y(t) \tanh(t)\\ z'(t) = -\Theta(t-2)x'(t-2) $$ and $$ \text{...
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1answer
73 views

Non linear fit with shared parameters and a known variable

I am new to Mathematica and I am able to do only basic stuff. Here is the situation: I have some datasets (2 in the following for simplicity) that I would like to fit. The function that I am trying ...
2
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2answers
220 views

Got Solutions to a set of non liner equations in Matlab but not in Mathematica. Why?

I was trying to solve a set of non linear equations using NSolve in mathematica. I got a result saying that no solution( {} ). My friend tried it in Matlab he was able to get a solution to the same ...
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0answers
125 views

What does “Solved to an acceptable level” mean in the context of NonlinearModelFit in M12

I made a simple cosine fit to some data: ...
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2answers
69 views

Shooting with updating BC's

So I have a 'fairly simple' problem that needs to be 'solved'. I have been able to solve this running 2 loops in Matlab but I am sure Mathematica should be able to handle this. I have the ODE $$F'(x)+\...
6
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1answer
123 views

Nonlinear differential equation with unknown parameter and integral form boundary condition

I'm trying to solve numerically a non-linear problem in order to determine the velocity field ($U$) and the film thickness ($h$) of a non-Newtonian fluid over an inclined plane. The equations are, $$\...
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0answers
68 views

Fit a three-pulse sequence

I am wondering how to properly fit the three-pulse sequence shown below. I have tried a method which is fitting the three pulses with three polynominal functions seperately and then finding the x-...
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0answers
68 views

Big Confidence Interval for seemingly rather accurate variable, NonLinearModelFit [closed]

I used NonLinearModelFit to fit a function of the form a*Abs(Cos(x))^b to a set of 5 points, that each have a uncertainty. My ...
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2answers
113 views

Alternative way for getting the roots of a transcendental equation and verifying them

I have the following transcendental equation: $$2 \cot(x)=\frac{kx}{h(L/N)}-\frac{h(L/N)}{kx}$$ I use the following code to solve it ...
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1answer
85 views

I can't get DSolve to work on coupled equations

In trying to solve a geodesic equation, I have these coupled equations: ...
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1answer
93 views

Output response for state-space with piecewise nonlinear elements

In addition to the previous topic: System model merge with multiple input and one output For example, I am trying to reproduce a system from simulink, a diagram of which is shown in the figure below:...
2
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2answers
162 views

Error test failure when solving two coupled ODEs

by NDSolve; this is work until y=6.22 ...
2
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1answer
181 views

Problem with numerical solution of a system of ODEs

Background Good evening, I have a big problem with num. solution NDSolve of differential equation. To start with, the model: There is a fast-moving rope in closed path. Where $T$ is tension, $a$ ...
9
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1answer
234 views

Solving nonlinear PDE with Method Of Lines

I'm trying to master the method of lines for solving nonlinear PDEs. Currently, I've faced the PDE, that I'm not able to solve. Firstly, I have started with this equation: $$\partial_t U(t,x) =\...
2
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1answer
141 views

Issues with NDSolve when solving a system of two coupled PDEs

I'm trying to solve a system of non-linear reaction-diffusion-advection PDEs numerically using NDSolve. The two equations are (taken from here): where $H(x)$ is ...
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0answers
42 views

Solving a nonlinear equation system

I have to solve a non-linear equation system which describes the kinematics of a five pieces mechanism. ...
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0answers
27 views

Access to quantities other than parameter values in in StepMonitor for NonlinearModelFit?

NonlinearModelFit obviously does a lot of internal calculations at each step, including determining the Response (the value of ...
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3answers
120 views

Solving 5 nonlinear equations, is there a suitable way?

I'm trying to solve a set of 5 nonlinear equations using NSolve: ...
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1answer
136 views

Works with NDSolve, not DSolve

EDIT1: ...

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