Differential Equations : Integrating factors

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Discussion Overview

The discussion revolves around the concept of integrating factors in differential equations, particularly focusing on their definitions, conditions for existence, and their application in different types of equations, including exact differential equations.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant states that if u(x,y) and v(x,y) are two integrating factors of a differential equation, then u/v is not a constant, leading to the conclusion that u(x,y) = cv(x,y) is a solution for any constant c.
  • Another participant seeks clarification on the meaning of an integrating factor.
  • A different participant defines an integrating factor as a function that, when multiplied by the differential equation, simplifies the integration process.
  • There is a question raised about the strict conditions that must be satisfied for integrating factors.
  • One participant suggests that integrating factors are more of a technique than a strictly defined concept, noting that different types of equations may have varying conditions for their integrating factors.
  • Another participant emphasizes the context of exact differential equations and questions the implications of integrating factors in this scenario.
  • A later reply asserts that every first-order differential equation theoretically has an integrating factor, but only linear equations have a general formula for it, explaining the relationship between integrating factors and exactness of the differential equation.
  • Finally, a participant inquires about the second mixed derivatives related to the first partial derivatives discussed.

Areas of Agreement / Disagreement

Participants express differing views on the definition and application of integrating factors, with some focusing on specific types of equations and others discussing general principles. The discussion remains unresolved regarding the strict conditions and implications of integrating factors in various contexts.

Contextual Notes

Participants highlight that integrating factors may depend on the type of differential equation being considered, and the discussion includes various assumptions about the nature of these factors and their applications.

pooja mehta
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If u(x,y) and v(x,y) are two integrating factors of a diff eqn M(x,y)dx + N(x,y)dy,
u/v is not a constant, then
u(x,y) = cv(x,y) is a solution to the differential eqn, for every constant c.
 
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What does an integrating factor mean?
 
an "integrating factor" is a function F ( for example ) so that when you multiply the differential equation out by F, the result is easier to integrate ( i.e. with a product rule or something )
 
What is the strict condition that has to be satisfied?
 
the integrating factors are more of a technique than something that is explicitly defined; certain types of equations will have different conditions for the integrating factors ( as the thing the OP wants to show is a condition about the integrating factors ).. or for an equation of the form y ' + p( t )y = g( t ), an integrating factor can be of the form e^integral ( p( t ) ) [ when you multiply the equation out by this, you can easily integrate because the expression will be in the "product rule form" ]
 
But, we are considering a different kind of equation here, namely the so called exact differential equation:

[tex] M(x, y) \, dx + N(x, y) \, dy = 0[/tex]

What does it mean in this context?
 
Theoretically, every first order differential equation has an "integrating factor". It is only for linear equations that it is possible to give a general formula for that integrating factor.

Saying that u(x,y) is an integrating factor for the diffrerential equation M(x,y)dx+ N(x,y)dy= 0 means that u(x,y)M(x,y)dx+ u(x,y)N(x,y)dy= d(f(x,y)) for some differentiable function f(x,y). That, of course, means that u(x,y)M(x,y)dx+ u(x,y)N(x,y)dy is "exact". In particular that
[tex]\frac{\partial f}{\partial x}= u(x,y)M(x,y)[/tex]
and that
[tex]\frac{\partial f}{\partial y}= u(x,y)N(x,y)[/tex]
 
So, if those are the first partial derivatives, what are the second mixed derivatives?
 

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