# Multifactorial number

## Definition

A Factorial number is defined by the product

- [math]n! = 1 \cdot 2 \cdot 3 \cdots (n{-}2) \cdot (n{-}1) \cdot n[/math]

for [math]n ≥ 1[/math].

A **Multifactorial number** is denoted by

- [math]n!! = (n) \cdot (n-2) \cdot (n-4) \cdots[/math]
- [math]n!!! = (n) \cdot (n-3) \cdot (n-6) \cdots[/math]

and so on.

To clarify and make reading easier [math]n!!!!![/math] is displayed as [math]n!_5[/math].