Express $a_n$ in $n$: Recursive Formula

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In summary, a recursive formula is a mathematical expression that defines a sequence or series by relating each term to one or more previous terms. It differs from an explicit formula in that it does not directly calculate a specific term, but rather defines each term in terms of previous ones. To express a term in terms of n using a recursive formula, the first term must be defined and a rule must be established to relate subsequent terms to the previous one. To use a recursive formula to find a specific term, the first term, recursive rule, and the value of n must be known. However, not all sequences or series can be expressed using a recursive formula, as some may have patterns that cannot be expressed recursively.
  • #1
Albert1
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$a_0=1, a_n=\dfrac {a_{n-1}}{1+(n-1)\times a_{n-1}}$
for all $n\geq 1$
express $a_n$ in $n$
 
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  • #2
My solution:

Computation of the first few terms suggests the closed form is:

\(\displaystyle a_n=\frac{2}{n^2-n+2}\)

As a check we may write:

\(\displaystyle \frac{a_{n-1}}{1+(n-1)a_{n-1}}=\frac{\dfrac{2}{(n-1)^2-(n-1)+2}}{1+(n-1)\dfrac{2}{(n-1)^2-(n-1)+2}}=\frac{2}{(n-1)^2-(n-1)+2+2(n-1)}=\frac{2}{n^2-2n+1-n+1+2+2n-2}=\frac{2}{n^2-n+2}=a_n\)

This closed form satisfies the given initial value, and the given non-linear recurrence, and so we may state:

\(\displaystyle a_n=\frac{2}{n^2-n+2}\)
 
  • #3
nice try !
 

FAQ: Express $a_n$ in $n$: Recursive Formula

What is a recursive formula?

A recursive formula is a mathematical expression or rule that defines a sequence or series by relating each term to one or more of the previous terms. It is a way of expressing a sequence or series without explicitly listing out each term.

How is a recursive formula different from an explicit formula?

An explicit formula directly calculates a specific term in a sequence or series, while a recursive formula defines each term in terms of previous terms. Recursive formulas are often used when an explicit formula is difficult to find or when the sequence or series has a recursive structure.

How do you express $a_n$ in terms of $n$ using a recursive formula?

To express $a_n$ in terms of $n$ using a recursive formula, you need to define the first term, $a_1$, and then create a rule that relates each subsequent term, $a_n$, to the previous term, $a_{n-1}$. This can be written as $a_n = \text{recursive rule involving } a_{n-1}$.

What information is needed to use a recursive formula to find a specific term?

To use a recursive formula to find a specific term, you need to know the first term, $a_1$, and the recursive rule that relates each term to the previous term. Additionally, you need to know the value of $n$ for the term you want to find.

Can all sequences or series be expressed using a recursive formula?

No, not all sequences or series can be expressed using a recursive formula. Some sequences or series have patterns or relationships that cannot be expressed recursively. In these cases, an explicit formula or another method must be used to define the sequence or series.

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