product of all the integers from 1 up to the integral input of the function that have the same parity as this input
DoubleFactorialReIm
{\displaystyle (2n-1)!!=\sum _{k=0}^{n-1}{\binom {n}{k+1}}(2k-1)!!(2n-2k-3)!!.}
In mathematics, the double factorial n‼ 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. … Wikipedia
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Wikipedia
en.wikipedia.org › wiki › Double_factorial
Double factorial - Wikipedia
1 week ago - The final expression is defined for all complex numbers except the negative even integers, and its reciprocal is well defined for all complex numbers. This double factorial satisfies (z + 2)!! = (z + 2) · z!! everywhere it is defined. As with the gamma function that extends the ordinary factorial function, this double factorial function is logarithmically convex in the sense of the Bohr–Mollerup theorem.
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Quora
quora.com › What-is-the-value-of-2-factorials
What is the value of 2 factorials? - Quora
Answer (1 of 2): Factorial function is denoted by the Symbol “!” Factorial of any number means finding the value of the product of all the numbers “starting from 1 …till the given number..” Lets say 5!(Five factorial) 5!=1×2×3×4×5=120 ...
Discussions

ELI5: What is a factorial and how does it work
To calculate the factorial of a number, multiply it by all whole numbers below it.  For example, 5! = 5x4x3x2x1 = 120. One use of factorials is to find the number of permutations (orders).  For example, if 5 people run in a race, any of the 5 people can finish first, any of the 4 remaining people can finish second, any of the remaining 3 people can finish third, any of the 2 remaining people can finish fourth, and the last remaining person finishes last.  So there are 5! = 5x4x3x2x1 = 120 different possible race results. More on reddit.com
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December 31, 2024
Factorials - Explain like im a two year old
The notation "1*2*...*(n-1)*n" simply means to multiply all the integers from 1 to n. If n = 1, then n-1 is not included since it is not between 1 and n. You may be more satisfied with a recursive definition: Define 0! = 1 Define n! = n*(n-1)! for any positive integer n. More on reddit.com
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May 12, 2019
What is the factorial of 1/2? The surprising answer: (1/2)! = (√π)/2
I had a really protracted argument on here before about this with someone. They didn't really change my mind. (1/2)! factorial is not (√π)/2 - it's undefined. If you extend factorial to non-counting numbers using gamma (yes i get it it's a unique "natural" extension) then fine. But for the factorial n!, the thing that counts the number permutations of a set of n elements, (1/2)! simply does not make sense. It's exactly like saying the sum of 1,2,3,4,... = -1/12 and then going "omg wow". No it doesn't. For a particular interpration of sum it does but for the "take the limit of adding these things together using regular binary addition" it does not. More on reddit.com
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December 10, 2014
What significance do double factorials have?
They appear in some power series, such as the power series for √(1-x²) More on reddit.com
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June 4, 2024
People also ask

What is the symbol of factorial?
The factorial function is a mathematical formula represented by an exclamation mark “!”. For example, the factorial of 8 can be represented as 8! and it is read as eight factorial.
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byjus.com
byjus.com › maths › factorial
Factorial
What is the meaning of 5 factorial?
The meaning of 5 factorial is that we need to multiply the numbers from 1 to 5. That means, 5! = 5 × 4 × 3 × 2 × 1 = 120.
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byjus.com
byjus.com › maths › factorial
Factorial
What is the value of 7!?
The value of 7! is 5040, i.e. 7! = 7 × 6 × 5 × 4 × 3 × 2 × 1 = 5040.
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byjus.com
byjus.com › maths › factorial
Factorial
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CalculatorSoup
calculatorsoup.com › calculators › discretemathematics › factorials.php
Factorial Calculator n!
... In mathematics, there are n! ways to arrange n objects in sequence. "The factorial n! gives the number of ways in which n objects can be permuted."[1] For example: 2 factorial is 2!
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Math is Fun
mathsisfun.com › numbers › factorial.html
Factorial Function !
The factorial function (symbol: !) says to multiply all whole numbers from our chosen number down to 1. Examples:
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Brilliant
brilliant.org › wiki › double-factorials-and-multifactorials
Double Factorials and Multifactorials | Brilliant Math & Science Wiki
The double factorial of a positive integer \(n\) is the generalization of the factorial \(n!;\) this type of factorial is denoted by \(n!!\). It is a type of multifactorial which will be discussed later. As far as double factorial is concerned, it ends with \(2\) for an even number, and \(1\) ...
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Reddit
reddit.com › r/explainlikeimfive › eli5: what is a factorial and how does it work
r/explainlikeimfive on Reddit: ELI5: What is a factorial and how does it work
December 31, 2024 - A way to understand what a factorial represents is that it's the number of ways you can arrange X items. for example if you had 6 teddy bears, and you lined them up, you'd have 6 choices for the first bear, 5 choices for the second bear, and so on. So the end result is that you had 6 * 5 * 4 * 3 * 2 * 1 = 6!
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Wolfram MathWorld
mathworld.wolfram.com › DoubleFactorial.html
Double Factorial -- from Wolfram MathWorld
February 1, 2009 - The double factorial of a positive integer n is a generalization of the usual factorial n! defined by n!!={n·(n-2)...5·3·1 n>0 odd; n·(n-2)...6·4·2 n>0 even; 1 n=-1,0. (1) Note that -1!!=0!!=1, by definition (Arfken 1985, p. 547).
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Quora
quora.com › What-is-the-factorial-of-2
What is the factorial of -2? - Quora
Answer (1 of 4): * In mathematics, the factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n. For example, The value of 0! is 1, according to the convention for an empty product.
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Answers
math.answers.com › math-and-arithmetic › What_is_2_factorial
What is 2 factorial? - Answers
3! = 3 factorial 3 factorial means 1x2x3 = 6 2! or 2 factorial means 1x2 = 2 4! or 4 factorial means 1x2x3x4 = 24 ... To calculate the number of zeros in a factorial number, we need to determine the number of factors of 5 in the factorial.
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BYJUS
byjus.com › maths › factorial
Factorial
In short, a factorial is a function that multiplies a number by every number below it till 1. For example, the factorial of 3 represents the multiplication of numbers 3, 2, 1, i.e.
Published   October 6, 2021
Views   1K
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GeeksforGeeks
geeksforgeeks.org › mathematics › factorial
Factorial in Maths: Definition, Formulas & Applications - GeeksforGeeks
The factorial of a number is the product of all positive integers from that number down to 1. It plays a key role in many mathematical concepts, such as permutations, combinations, probability, and more.
Published   December 29, 2025
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Athometuition
athometuition.com › factorials.aspx
Factorials
This means that the factorial of any natural number (also called the counting numbers or positive integers; meaning all integers from 1 to infinity) is equal to the product of all of the integers from 1 up to that number.
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Wikipedia
en.wikipedia.org › wiki › Factorial
Factorial - Wikipedia
1 week ago - {\displaystyle \vert \!{\underline {\,n}}} , in which the argument of the factorial was half-enclosed by the left and bottom sides of a box, was popular for some time in Britain and America but fell out of use, perhaps because it is difficult to typeset. The word "factorial" (originally French: factorielle) was first used in 1800 by Louis François Antoine Arbogast, in the first work on Faà di Bruno's formula, but referring to a more general concept of products of arithmetic progressions. The "factors" that this name refers to are the terms of the product formula for the factorial. ... {\displaystyle n!=1\cdot 2\cdot 3\cdots (n-2)\cdot (n-1)\cdot n.} This may be written more concisely in product notation as
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Edumentors
edumentors.co.uk › answer › maths › gcse › what-is-a-factorial-and-why-is-it-useful-in-mathematics
What is a factorial and why is it useful in mathematics?
A factorial is the product of an integer and all the integers below it. It's denoted with the symbol !. For example, 5! = 54321 = 120. Factorials are
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STEM
stem.org.uk › resources › library › collection › 20252 › factorials
Factorials
Yannay Khaikin explains how factorials allow us to pinpoint the exact (very large) number of permutations in a standard deck of cards. ... SupportAccessibilityCookiesPrivacy policySafeguardingTerms and conditionsStatement on Modern Slavery · © STEM Learning Ltd. is a not-for-profit company (05081097)National STEM Learning Centre, University of York, York YO10 5DD01904 948711
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ZeptoMath
zeptomath.com › calculators › factorial.php
2! - Factorial of 2
To determine the number of zeros at the end of a factorial, recursively divide the number by 5 until the quotient is less than 5, and sum the results after applying the greatest integer function. The greatest integer function (usually denoted by brackets) is the rounded down integer of a value. For example, [5] = 5, [4.5] = 4, [-4.5] = -5. For example, the number of trailing zeros in 2!
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Stat Trek
stattrek.com › online-calculator › factorial
Factorial Calculator
The Factorial Calculator can compute a factorial for any number up to 10,000. Beyond 10,000, the calculator reports a lower-bound estimate. Here's the logic. The value of 10,000! is approximately 2.846 x 1035659. Therefore, we know the factorial of any integer greater than 10,000 will be at ...
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CK-12 Foundation
ck12.org › all subjects › cbse math › multiplying and dividing integers - steps, rules, interactives and examples › what is the factorial of 0?
Flexi answers - What is the factorial of 0? | CK-12 Foundation
September 11, 2025 - The factorial of a number is the product of all positive integers less than or equal to that number. By convention, the factorial of 0, denoted as 0!, is defined as 1. This makes sense in terms of the definition of the factorial function if we extend it from positive integers to non-positive ...