Double Factorial Calculator
\[ \small{ n!!= \begin{cases} n\left(n-2\right)\cdots3\cdot1&\text{if n is odd,}\\ n\left(n-2\right)\cdots4\cdot2&\text{if n is even.} \end{cases} } \]
- You can input an integer from 1 or more to less than 1000 for \(n\).
- Factorial Calculator is available at this link.
What is double factorial?
Double factorial is a calculation defined as follows
\[ n!!= \begin{cases} n\left(n-2\right)\cdots3\cdot1&\text{if n is odd,}\\ n\left(n-2\right)\cdots4\cdot2&\text{if n is even.} \end{cases} \]
The double factorial is the product of \( n,\left(n-2\right),\left(n-4\right)\cdots \).
Therefore \( n!! =\left(n!\right)! \) does not hold.
Example
Example 1
Calculate \(7!!\).
Solution
\[ 7!!=7\cdot5\cdot3\cdot1=105 \]
Example 2
Calculate \(8!!\).
Solution
\[ 8!!=8\cdot6\cdot4\cdot2=384 \]