How to derive a recurrence relation from explicit form

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Discussion Overview

The discussion revolves around deriving a recurrence relation from a given explicit formula. Participants explore the relationship between the closed form and the recurrence relation, discussing methods and potential challenges in the derivation process.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant requests clarification on what constitutes a closed or explicit form.
  • Another participant provides the explicit form as \( P_N=\frac{\frac{A^N}{N!}}{\sum\limits_{x=0}^N \frac{A^x}{x!}} \) and the corresponding recurrence relation \( P_i = \frac{A p_{i-1}}{i+A p_{i-1}} \) for \( i=1 \) to \( N \).
  • A different participant expresses a desire for a closed form solution in the format \( A_n=\sum_{k=0}^m\left(c_kr_k^n\right) \) that would lead to a linear recurrence.
  • Another participant suggests that the recurrence relation may need to be adjusted to reflect the notation correctly, indicating \( P_{i-1} \) should be capitalized.
  • One participant proposes that deriving the recurrence formula may involve substituting the closed formula into the recurrence relation, suggesting an alternative expression \( \frac{1}{P_i}=\frac{i}{AP_{i-1}}+1 \).
  • A participant questions whether the derivation can be achieved solely through algebraic manipulation, hinting at a potential method they believe they have discovered.

Areas of Agreement / Disagreement

The discussion does not reach a consensus on the best method for deriving the recurrence relation, with multiple approaches and ideas being proposed without resolution.

Contextual Notes

Participants express uncertainty about the derivation process and the notation used, indicating that assumptions about the forms and relationships may not be fully clarified.

find_the_fun
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I am given a formula in explicit form and as a recurrence relation. It is asked to derive the recurrence relation from the explicit form. How is this done?
 
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What is the closed, or explicit, form?
 
closed: [math]P_N=\frac{\frac{A^N}{N!}}{\sum\limits_{x=0}^N \frac{A^x}{x!}}[/math]

recurrence: [math]P_i = \frac{A p_{i-1}}{i+A p_{i-1}}[/math] for i=1 to N
 
Last edited:
I was hoping you would post a closed form solution in the form:

$$A_n=\sum_{k=0}^m\left(c_kr_k^n\right)$$

Leading to a linear recurrence.

I will have to defer here to someone more knowledgeable in this field. :D
 
find_the_fun said:
recurrence: [math]P_i = \frac{A p_{i-1}}{i+A p_{i-1}}[/math] for i=1 to N
I assume $P_{i-1}$ should be written with a capital $P$ in the right-hand side.

I'll have to think more how to derive the recurrence formula, but it is easy to prove it once it is known: just substitute the closed formula into it. It is probably easier to work with
\[
\frac{1}{P_i}=\frac{i}{AP_{i-1}}+1.
\]
 
Can this be done using nothing but algebraic manipulation? If so, I think I found a way.
 

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