Nonlinear differential equation (Laplace transform?)

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Discussion Overview

The discussion revolves around the challenges of solving a nonlinear ordinary differential equation (ODE) using the Laplace transform. Participants explore the implications of nonlinearity, potential methods for linearization, and numerical approaches for finding solutions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents a nonlinear ODE in the form of sin(τλ) = q^((n+2)/(n+1)) + κq' + q'' and questions the applicability of the Laplace transform for such equations.
  • Another participant suggests that the Laplace transform is typically used for linear differential equations and notes that many sources recommend linearizing the equation before applying the transform.
  • A participant acknowledges that once linearized, the equation could be solved using the Laplace transform, but mentions that analytic solutions for nonlinear ODEs are generally not available unless they take specific forms.
  • It is noted that nonlinear ODEs often lack nice analytic solutions, and numerical methods or approximation schemes may be necessary, particularly for large values of n where the equation behaves more like a linear ODE.
  • One participant proposes a substitution to eliminate the fractional power in the equation, although it introduces complexity with the appearance of a squared derivative.
  • Another participant discusses the potential of using Taylor series approximation for linearization, emphasizing the need to linearize around an equilibrium point.
  • A participant references a paper that claims to solve nonlinear differential equations using the Laplace transform but notes that it requires a specific harmonic input, which does not match their sine input.
  • One participant mentions that the paper only considers nonlinear differential equations involving the first derivative.
  • A later contribution discusses a modification to the exponent that introduces nonlinearity but also allows for specific values of n that yield linear behavior, and inquires about the use of numerical methods in MATLAB for nonlinear systems.

Areas of Agreement / Disagreement

Participants express a range of views on the applicability of the Laplace transform to nonlinear ODEs, with some suggesting linearization as a necessary step while others highlight the limitations of this approach. The discussion remains unresolved regarding the best methods for addressing the original nonlinear equation.

Contextual Notes

Participants note that analytic solutions for nonlinear differential equations are often not available unless specific forms are met, and they discuss the challenges associated with linearization and the need for numerical methods in certain cases.

fnsaceleanu
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Hi,

Part of my research, I nondimensionalized an ODE to eventually arrive at this form:

sin(τλ) = q^((n+2)/(n+1)) + κq' + q''

where q' = dq/dτ

The problem is of course the nonlinear q^n. n is an integer greater than 0.
Is there a Laplace transform for this?
Or what solutions are there for this kind of equation?

Thanks!
 
Last edited:
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I was thinking that the Laplace transform could only be used to solve linear d.e.s but wasn't sure so I "google" "Laplace Transform" and "non-linear differential equation". Somewhat to my surprise, I got a number of hits. Unfortunately, when I opened pages on "solving non-linear differential equations by the Laplace Transform method", I found that the first instruction was to linearize the equation. I didn't read further- I sure they gave further instructions for getting better solutions than just to the linearized version- but it seems that the Laplace Transform cannot be directly used on non-linear differential equations.

If you are still interested, here is the simplest of the pages:
http://wikis.lib.ncsu.edu/index.php/Laplace_Transforms
 
Thanks for the reply.
Once linearized it would be easy to solve with the Laplace transform.
I've also read that there are no analytic solution to nonlinear differential equations unless they have specific forms..
 
Nonlinear ODEs typically don't have nice, analytic solutions. You'll either have to solve it numerically or develop some approximation scheme for the parameter regime of interest. One thing you can note is that for n very large you approximately have a linear ODE, so you might be able to develop some sort of approximation scheme where you solve the linear ODE and then calculate corrections of order 1/n or higher.
 
I doubt that it helps, but if you substitute q = p-n-1 you get rid of the fractional power. It looks a whole lot better but for the appearance of a (p')2.
 
Thanks to those who replied.. looks like linearization via taylor series approximation could work but then i have to linearize about a point, an usually it's the equilibrium.

I have found this as well http://www.new.dli.ernet.in/rawdataupload/upload/insa/INSA_2/20005a65_421.pdf

Looks interesting, as it can solve nonlinear differential equations via the laplace transform, BUT it needs a harmonic input of the form x(t) = exp (s_1 + s_2 + s_3 +...+ s_n)t... whereas my input is of sin form..

Any thoughts there?
 
Last edited by a moderator:
looks like the paper only considers nolinear DE including the first derivative only..
 
I've done a correction to the exponent that makes the DE nonlinear...

It's actually q^(n+2)/((n+1)^2)... this makes it interesting since if n=0.618 or n=-1.618, then the DE is linear... but that n depends on a curve of best fit.

Has anyone used numerical methods in Matlab to get the system response if it's nonlinear?
That seems to work ,and MATLAB has a bunch of functions for stiff/nonstiff systems
 

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