What is double factorial? - Definition from WhatIs.com
Part of the Computing fundamentals glossary:

The double factorial, symbolized by two exclamation marks (!!), is a quantity defined for all integer s greater than or equal to -1. For an even integer n , the double factorial is the product of all even integers less than or equal to n but greater than or equal to 2. For an odd integer p , the double factorial is the product of all odd integers less than or equal to p and greater than or equal to 1. The double factorial values of 0 and -1 are defined as equal to 1. Double factorial values for integers less than -1 are not defined.

Mathematically, the formulas for double factorial are as follows. If n is even, then

n !! = n ( n - 2)( n - 4)( n - 6) ... (4)(2)

If p is odd, then

p !! = p ( p - 2)( p - 4)( p - 6) ... (3)(1)

If q = 0 or q = -1, then q !! = 1 by convention.

The double factorial is mainly of interest to number theorists. It occasionally arises in statistics, combinatorics, calculus, and physics. Compare factorial .

This was last updated in September 2005
Posted by: Margaret Rouse

Related Terms

Definitions

  • IS (information system or information services)

    - An information system (IS) is the collection of technical and human resources that provide the storage, computing, distribution, and communication for the information required by all or some part o... (WhatIs.com)

  • distributed

    - Computing is said to be "distributed" when the computer programming and data that computers work on are spread out over more than one computer, usually over a network.Cloud computing is a particula... (WhatIs.com)

  • greedy algorithm

    - A greedy algorithm is a mathematical process that looks for simple, easy-to-implement solutions to complex, multi-step problems by deciding which next step will provide the most obvious benefit. (WhatIs.com)

Glossaries

  • Computing fundamentals

    - Terms related to computer fundamentals, including computer hardware definitions and words and phrases about software, operating systems, peripherals and troubleshooting.

  • Internet applications

    - This WhatIs.com glossary contains terms related to Internet applications, including definitions about Software as a Service (SaaS) delivery models and words and phrases about web sites, e-commerce ...

Ask a Question About double factorialPowered by ITKnowledgeExchange.com

Get answers from your peers on your most technical challenges

Tech TalkComment

Share
Comments

    Results

    Contribute to the conversation

    All fields are required. Comments will appear at the bottom of the article.