Discrete Math: Linear Inhomogeneous Recurrence

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Discussion Overview

The discussion revolves around solving a linear inhomogeneous recurrence relation of the form an = 6an−2 + 8an−3 + 3an−4 + 64·3^n−4, with specific initial conditions. Participants explore methods for finding the characteristic roots, the homogeneous solution, and the particular solution, as well as the closed form of the generating function for the sequence.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • Some participants seek clarification on the notation n>=4 and the identification of characteristic roots.
  • Several participants propose the characteristic equation and discuss its roots, with some suggesting the general form of the homogeneous solution.
  • There is a discussion about the need for linear independence of terms in the homogeneous solution due to repeated roots.
  • Participants suggest forms for the particular solution, with some disagreement on the correct approach.
  • One participant mentions using the method of undetermined coefficients and seeks further explanation on its application to the problem.
  • There is a proposal for the general solution using the principle of superposition, incorporating both the homogeneous and particular solutions.
  • Participants work through setting up a system of equations to solve for the constants in the general solution based on initial conditions.
  • Final expressions for the closed form of the solution are discussed, with some participants questioning the meaning of "solving" the recurrence relation.

Areas of Agreement / Disagreement

Participants generally agree on the methods to approach the problem, but there are multiple competing views on the forms of the particular solution and the specifics of the homogeneous solution. The discussion remains unresolved regarding the interpretation of "solving" the recurrence relation.

Contextual Notes

Some participants express uncertainty about the method of undetermined coefficients in the context of recurrence relations, and there are unresolved details regarding the derivation of the particular solution.

  • #31
I have again merged threads...please post questions pertaining to this problem in this thread. :D
 
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  • #32
Unfortunately, I am also not very good with generating functions and have little time. I can only recommend the following formulas.
\begin{align}
(x^{n+1})'&=nx^n+x^n&&\text{to find }\sum(-1)^nnx^n\\
(x^{n+2})''&=n^2x^n+3nx^n+2x^n&&\text{to find }\sum(-1)^nn^2x^n\\
\sum n(3x)^n&=\sum n(3^nx^n)
\end{align}
 
  • #33
The calculation of a closed form for the generating function G(z) is straight forward. This is a rational function in z. If you're really ambitious (I'm not), you can apply partial fraction decomposition to G(z), use the geometric series and it's derivatives and find the closed form solution for $a_n$.

jph07t.png
 

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