Can this equation be solved analytically?

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Discussion Overview

The discussion revolves around the analytical solvability of a specific differential equation involving a cubic polynomial. Participants explore methods for solving the equation, the implications of the constants involved, and the nature of potential solutions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a differential equation and seeks assistance in solving it analytically.
  • Another participant suggests separating the variables and integrating both sides as a method to approach the solution.
  • Some participants argue that for arbitrary values of A, B, and C, an analytic solution may not exist, noting that even if the polynomial can be factored, it leads to complex logarithmic terms.
  • There is a mention of "reducing to quadratures" as a technique that could be useful when other methods fail.
  • One participant expresses doubt about finding an explicit solution using separation of variables, suggesting that an implicit solution may be possible instead.
  • A participant introduces the concept of the cusp catastrophe, relating it to the cubic differential equation and inquires about the understanding of "shocks" in dynamics.
  • Another participant challenges the assertion that a nice analytic solution cannot be obtained, arguing that while factoring leads to logarithmic terms, exponentiating could yield a polynomial in y, with the caveat regarding double roots.

Areas of Agreement / Disagreement

Participants express differing views on the existence of an analytic solution, with some asserting that it is unlikely while others propose that it may be possible under certain conditions. The discussion remains unresolved regarding the feasibility of finding an explicit solution.

Contextual Notes

Participants note the complexity introduced by the arbitrary constants A, B, and C, and the implications of polynomial roots on the solvability of the equation. The discussion does not resolve these complexities.

sccv
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Dear all,
I have this differential equation:

dy
---- = K*(A * y^3 + B * y^2 + C * y + 1)
dx

where A, B, C, K are non-zero constants

I tried to solve it analytically but achieve no result so far. Could anyone help me to solve this equation ?
 
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you can separate your equation

\frac{dy}{Ay^3+By^2+Cy+1}=dx

all you have to do is integrate both sides

\int_{y(x_0)}^{y(x)} \frac{d\xi}{A\xi^3+B\xi^2+C\xi+1}=K(x-x_0)
 
Last edited:
For arbitrary values of A, B and C you will not get a nice analytic solution. Even if you could factor the polynomial, you would get three different logarithmic terms (if the factors are distinct) and you would not be able to solve for y = y(x).
 
P.S. What Chiquito has shown you is sometimes referred to as "reducing to quadratures" and is useful when all else fails!
 
You are right, Tide. That is why I have to find another way to solve for y = y(x)
 
Yup, in principle, you can find the primitive of the integral above, and that will give you an implicit solution for your ode, but to find a explicit solution? i doubt youll be able to, at least with separation of variables.
 
The cusp catastrophe

Nice! It's the cubic differential equation, the canonical form of the cusp catastrophe. Are you aware of this? Know what "shocks" are in regards to dynamics?

SD
 
For arbitrary values of A, B and C you will not get a nice analytic solution. Even if you could factor the polynomial, you would get three different logarithmic terms (if the factors are distinct) and you would not be able to solve for y = y(x).

You sure? You can always factor the polynomial (over C, of course), and while you get three logarithmic terms, once you exponentiate both sides, you're left with a polynomial in y.

The only problem is when the polynomial has a double root. (A triple root is fine)
 

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