Differential equations involving the function composition

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Discussion Overview

The discussion revolves around solving a differential equation involving function composition, specifically the equation (g∘f)f' = g, where g is a known function of x and f is the function to be determined. The participants explore various strategies for addressing this non-linear ordinary differential equation (ODE).

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • Some participants suggest that separation of variables could be applied to solve the differential equation.
  • One participant notes that by inspection, f(x) = x is a solution to the equation.
  • Another participant emphasizes that such ODEs are generally non-linear and lack universal strategies for solving them unless they are separable.
  • A participant proposes multiplying both sides by dx to facilitate integration, leading to the equation G(f(x)) = G(x), where G is a primitive of g.
  • There is a discussion about the equivalence of the integrals ∫ g(f(x)) df = ∫ g(x) dx and the challenge of extracting f(x) or simplifying the composition in the integrand.

Areas of Agreement / Disagreement

Participants express differing views on the methods for solving the differential equation, with some advocating for separation of variables while others highlight the complexities of non-linear ODEs. The discussion remains unresolved regarding the best approach to handle the composition function in the integrand.

Contextual Notes

Participants note the lack of literature on differential equations involving function composition, indicating a potential gap in existing methodologies. The discussion also reflects uncertainty about the generality of solutions and the conditions under which specific strategies may apply.

dftfunctional
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I have not met differential equations involving the composition functions (also not much literature on it).

Assume we know the form of g=g(x), and need to solve the following differential equation, finding f=f(x):

(g∘f)f'=g

Where g∘f=g(f(x)).

Does anybody have a strategy for solving the above-mentioned differential equation, that involves such function composition?
 
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It looks like you can apply separation of variables and integrate
 
dftfunctional said:
I have not met differential equations involving the composition functions (also not much literature on it).

Assume we know the form of g=g(x), and need to solve the following differential equation, finding f=f(x):

(g∘f)f'=g

Where g∘f=g(f(x)).

By inspection, f(x) = x is a solution.

Does anybody have a strategy for solving the above-mentioned differential equation, that involves such function composition?

Such ODEs are in general non-linear, and there are no general strategies for solving non-linear ODEs unless they happen to be separable.
 
mbp said:
It looks like you can apply separation of variables and integrate

pasmith said:
By inspection, f(x) = x is a solution.


Thank you both,


As a trained material scientist I am not an expert on ODE. Could you please, therefore provide me more details.


mbp said:
It looks like you can apply separation of variables and integrate

pasmith said:
Such ODEs are in general non-linear, and there are no general strategies for solving non-linear ODEs unless they happen to be separable.


If it is separable, how I can proceed to finding a solution considering that there is the composition given in the function?
 
Multiply both sides for dx getting

g(f(x)) f'(x) dx = g(x) dx

Then integrate both sides

G(f(x)) = G(x)

where G is a primitive of g, and you get f(x) = x as pasmith suggested
 
mbp said:
Multiply both sides for dx getting


Then integrate both sides

G(f(x)) = G(x)

where G is a primitive of g




Thank you very much,

as far as I understood G(f(x)) = G(x) would be equivalent to:

∫ g(f(x)df = ∫g(x)dx

And per inspection we could find that one solution is f(x)=x. Empirically I know that there are many solutions to the given equation. Is there a way for "exctracting" f(x) or getting rid of the composition function in the integrand from the (again):

∫ g(f(x)df = ∫g(x)dx
 

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