---
tag: TER6MWXBS8
kind: concept
name: Double factorial
aliases: [double-factorials, "n", odd-factorial, semifactorial]
sources:
  - wikipedia: en:507208@r1350553238
    locator: Double factorial
    note: adopted
    mention: wikipedia://en/507208
    sections: [Summary]
same_as:
  wikidata: Q13520655
  wikipedia: Double factorial
inventory: [M0F0CAGRRD]
---

## Summary

In mathematics, the **double factorial**, or **semifactorial**, $n\text{‼}$ of a positive integer $n$ is the product of all the positive integers up to $n$ that have the same parity (odd or even) as $n$. That is,