First Order Non-Linear ODE (what method to use?)

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Discussion Overview

The discussion centers around solving the first-order non-linear ordinary differential equation (ODE) given by dy/dx = −[2x + ln(y)]*(y/x). Participants explore various methods for solving the equation, including checking for exactness, homogeneity, and potential substitutions.

Discussion Character

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

Main Points Raised

  • One participant notes that the equation is not exact and struggles to find a suitable function to make it exact, while also stating it is not separable or homogeneous.
  • Another participant suggests guessing a solution, proposing forms like ##a^x## that might relate to the logarithmic function present in the equation.
  • A different participant agrees with the initial approach of checking for exactness and homogeneity, recommending a substitution of the form y=et to facilitate finding an integrating factor.
  • Another participant shares their approach using y = exp(qx), leading to a separation of variables and deriving a relationship for q as (A/x2) - 1, where A is a constant of integration.

Areas of Agreement / Disagreement

Participants express various methods and approaches to tackle the problem, but there is no consensus on a single method or solution. Multiple competing views remain regarding the best approach to take.

Contextual Notes

Some participants indicate limitations in their attempts, such as the challenges in finding an exact form or suitable substitutions, but these remain unresolved.

Who May Find This Useful

Readers interested in differential equations, particularly those dealing with non-linear ODEs and various solution methods, may find this discussion relevant.

Apothem
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Hi,

The problem is to solve:
dy/dx = −[2x + ln(y)]*(y/x)

Attempt:
I have tried to see if it is exact, I found it not to be, I can't easily find a function to multiply by to make it exact either (unless I am missing something obvious). It clearly isn't seperable, nor is it homogenous (I know although some non-homogenous ODEs can be solved using the homogenous method I don't think this one can be), it also isn't a linear equation so we can't use integrating factors.

I'm looking for pointers in the right direction/which method to use.

Thanks for your time
 
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Have you tried guessing a solution?

Something like ##a^x## or something that works well with the natural log function.
 
I think you are going the right way; checking the exact form, homogeneity etc.
Here since there is a logarithmic function, I recommend substituting y=et where t is parameter and then be solved by finding integrating factor.
 
Using y = exp(qx), where q is a function of x , I got it to separate, giving
q = (A/x2) - 1, where A is a constant of integration.
It seems to check.
 

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