Solving nonlinear singular differential equations

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Discussion Overview

The discussion revolves around methods for solving nonlinear singular differential equations, specifically referencing the Lane-Emden equation. Participants explore strategies for addressing singularities in both theoretical and computational contexts.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant inquires about methods for tackling singularities while coding solutions to nonlinear singular differential equations.
  • Another participant suggests that for specific cases, such as the equation \(\frac{1}{r^{n-1}} \frac{d}{dr}\left(r^{n-1}\frac{d\theta}{dr}\right) = -\theta^m\), singular points can be managed by appropriately choosing boundary conditions, thus avoiding difficulties at the singular point.
  • A different viewpoint emphasizes the conceptualization of solutions as a vector field, proposing that avoiding singularities can be achieved by selecting initial values that navigate through the field without encountering them. This participant also discusses the classification of singularities and the implications for numerical approaches.
  • Another participant expresses appreciation for the vector field perspective, noting its impact on their understanding of ordinary differential equations (ODEs).

Areas of Agreement / Disagreement

Participants present varying approaches to handling singularities, with no consensus on a single method. The discussion includes both theoretical and numerical perspectives, indicating multiple competing views on the topic.

Contextual Notes

Participants do not fully resolve the implications of different methods for handling singularities, nor do they clarify the robustness of numerical solutions in varying initial conditions.

wasi-uz-zaman
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TL;DR
how to tackle singularity in the differential equations
hi, i am going through differential equations which are nonlinear and singular - like Lane-Emden equation etc.
my question is how to tackle singularity - while coding.
regards
 
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What method are you using to solve these equations?

If you solve <br /> \frac{1}{r^{n-1}} \frac{d}{dr}\left(r^{n-1}\frac{d\theta}{dr}\right) = -\theta^m for n = 2 or 3 subject to \theta(0) =1 and \theta&#039;(0) = 0 then there is no difficulty: The singular point is at the origin, but you aren't enforcing the ODE there because \theta and \theta&#039; are fixed by the boundary conditions.
 
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I like to think of the solutions of an ODE as a vector field. @pasmith's answer means to avoid singularities by choosing initial values, i.e. trajectories which lead you through the field without encountering singularities.

If you still consider the entire field, then there are certain types of singularities: attractors, repellers, or intersecting trajectories, maybe even isolated points. Such a classification allows handling different types of singularities differently. It's always the same: what is the situation and what is the goal?

If you approach it numerically, then you have to consider how robust a solution is. This means if you vary the initial values, how much do the trajectories vary? Again a matter of the given situation and goal.
 
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Yes, I remember the idea of solutions as a vector field being a revelation for me. I was fortunate that this notion was introduced in my initial ODE course.
 
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