# Closed Formula

A closed formula is one that doesn't contain a $\sum$ or $\prod$.

Example:

$a_n = \frac{3^n - 1}{n}$

A closed formula can have a the sum or product of a *fixed* number of parts, but not a *changing* number of parts.

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A closed formula is one that doesn't contain a $\sum$ or $\prod$.

Example:

$a_n = \frac{3^n - 1}{n}$

A closed formula can have a the sum or product of a *fixed* number of parts, but not a *changing* number of parts.