Imagine a shelf. On that shelf, put 3 cards: one that says 1. One that says 2. And one that says 3. Put them on the shelf in some order (e.g. 2,1,3). How many orderings are possible? 3! = 6, right? Exactly. That’s how many different ways my shelf could look different. Now imagine we had 1 card instead of 3. How many ways can the shelf look? It’ll look like this: ____ [ 1 ] ____ with just the one card with a 1 on it. That’s the only possibility for the shelf. 1 factorial is 1. Now imagine we, in fact, have 0 cards. What’s 0 factorial? How many different ways can the shelf look? Only one: ________. There is one way to order zero objects. The way that produces an empty shelf. Answer from R0KK3R on reddit.com
Imagine a shelf. On that shelf, put 3 cards: one that says 1. One that says 2. And one that says 3. Put them on the shelf in some order (e.g. 2,1,3). How many orderings are possible? 3! = 6, right? Exactly. That’s how many different ways my shelf could look different. Now imagine we had 1 card instead of 3. How many ways can the shelf look? It’ll look like this: ____ [ 1 ] ____ with just the one card with a 1 on it. That’s the only possibility for the shelf. 1 factorial is 1. Now imagine we, in fact, have 0 cards. What’s 0 factorial? How many different ways can the shelf look? Only one: ________. There is one way to order zero objects. The way that produces an empty shelf. Answer from R0KK3R on reddit.com
Reddit
reddit.com › r/math › why the factorial of 0 is always 1?
r/math on Reddit: Why the factorial of 0 is always 1?
January 16, 2022 - Now imagine we had 1 card instead of 3. How many ways can the shelf look? It’ll look like this: ____ [ 1 ] ____ with just the one card with a 1 on it. That’s the only possibility for the shelf. 1 factorial is 1.
Wikipedia
en.wikipedia.org › wiki › Factorial
Factorial - Wikipedia
1 week ago - In mathematics, the factorial of a non-negative integer ... {\displaystyle {\begin{aligned}n!&=n\times (n-1)\times (n-2)\times (n-3)\times \cdots \times 3\times 2\times 1\\&={\begin{cases}1,&{\text{if }}n=0\\n\times (n-1)!,&{\text{if }}n\geq 1.\end{cases}}\\\end{aligned}}} For example, ... ...
Videos
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Factorials! What Are They? How to Simplify? | Math with Professor ...
Factorials Explained - YouTube
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a quick explanation for factorials - YouTube
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What is a factorial? - YouTube
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Factorials? 5! 🤔 What is a Factorial? - YouTube
Top answer 1 of 2
4
One definition of the factorial that is more general than the usual
is via the gamma function, where
This definition is not limited to positive integers, and in fact can be taken as the definition of the factorial for non-integers. With this definition, you can quite clearly see that
If you are starting from the "usual" definition of the factorial, in my opinion it is best to take the statement 
as a part of the definition of the factorial function, as anything else would require proofs using the factorial to include special cases for 
and 
. It's a definition that is consistent and makes our lives easier.
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1
There is exactly one permutation of a set of zero elements.
Top answer 1 of 10
85
Yes, precisely there is a unique function 
(with empty graph), which happens to be a bijection (
). Note, that 
is the number of bijections 
.
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72
For positive numbers the factorial function 
is defined as the product of all positive integers less or equal to 
. To define 
we need to "extend" the definition. Another way to define it is to notice that:
Pluging 
we get: 
ThoughtCo
thoughtco.com › why-does-zero-factorial-equal-one-3126598
Why Does Zero Factorial Equal One?
May 18, 2024 - The set with zero elements is called the empty set. To find the value of zero factorial, we ask, “How many ways can we order a set with no elements?” Here we need to stretch our thinking a little bit. Even though there is nothing to put in an order, there is one way to do this. Thus we have 0! = 1.
Quora
quora.com › What-is-the-factorial-of-1
What is the factorial of 1? - Quora
Answer (1 of 7): Here is a process that you can use to figure out the lower factorials if you already know the higher factorials . For example,if you know the value for 5! — then you can figure out the value for 4! * (N-1)! = N! / N * 4! = 5! / 5 = 120 / 5 = 24 * 3! = 4! / 4 = 24 / 4 = 6 * 2...
Math Answers
math.answers.com › other-math › 1_factorial_is_equal_to_1
1 factorial is equal to 1? - Answers
= n x (n-1) x (n-2) x (n-3) x ... ... of a factorial number and by dividing both sidesby n one has: n!/n (n-1)! or (n-1)! = n!/nwhen n=2 one has (2-1)! = 2!/2 or 1! = 2x1/2 or 1! = 1when n=1 one has (1-1)! = 1!/1 or 0! = 1/1 or 0! = 1. =This is ...
ChiliMath
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Zero Factorial | ChiliMath
July 20, 2024 - The general formula of factorial can be written in fully expanded form as ... We know with absolute certainty that 1!=1, where [latex]n = 1[/latex]. If we substitute that value of [latex]n[/latex] into the second formula which is the partially expanded form of [latex]n![/latex], we obtain the following:
The Math Doctors
themathdoctors.org › zero-factorial-why-does-0-1
Zero Factorial: Why Does 0! = 1 ? – The Math Doctors
Since we are multiplying factors from n down to 1, it makes sense (mostly) that \(1!=1\), since we just start and end with 1; but how can you count down from 0 to 1?? Furthermore, the first use of the factorial is to answer permutation problems: \(n!\) is the same as \(_n\text{P}_n\), the number of ways to arrange all of n objects a row.
Math is Fun
mathsisfun.com › numbers › factorial.html
Factorial Function !
Let's start with 3! = 3 × 2 × 1 = 6 and go down: And from there on down all integer factorials are undefined.
CK-12 Foundation
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Flexi answers - Why is 0 factorial equal to 1? | CK-12 Foundation
September 11, 2025 - The factorial of any positive integer ... positive integers less than or equal to 0. So why is 0! equals to 1? The answer lies in the principle of mathematical induction and the definition of the empty product....
Reddit
reddit.com › r/explainlikeimfive › eli5: why is the factorial of 0 equal to 1?
r/explainlikeimfive on Reddit: ELI5: Why is the factorial of 0 equal to 1?
October 4, 2023 -
Wouldn't this also mean that 1!=0!, why is this true?
Top answer 1 of 26
640
Others have explained the 0! = "number of ways to rearrange 0 people" part, so I'll try to convince you that 0! shouldn't be 0. You can think of 2 lines of people, with 2 distinct people in first one and 3 in the second. How many distinct way can you rearrange them, assuming that no one leaves their line? Well, as for the first line, there are 2! arrangements. As for the second one, the answer is 3!. Then, since you can pair 1 way of arranging the first line with 1 way of arranging the second line to make 1 distinct arrangement of the 2 lines, the number of distinct arrangements is 2! × 3!. Now, what if the first line is empty? Then the answer should be 0! × 3!. If 0! = 0, this number should be 0. But you should get 3! different arrangements when shuffling the second line, no? Then adding an empty first line shouldn't suddenly make the number of arrangement 0? Look just how convenient it is for the number of permutation of an empty set to be 1!
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There is an excellent video by Eddie Woo on youtube, I don't know if I can link it but I highly recommend you go watch it. However, the proof basically boils down to: 1! = 1 2! = 2 3! = 6 4! = 24 Find the pattern. To get from 4! to 3!, divide by 4. From 3! to 2!, divide by 3. From 2! to 1!, divide by 2. What will be the next answer? Divide it by 1. 0! = 1. It fits the pattern in our made up language of mathematics.
Medium
medium.com › math-simplified › why-is-zero-factorial-1-83cb340fe883
Why is Zero Factorial 1?. Beginning with the definition of… | by Safwan | Math Simplified | Medium
October 16, 2021 - Beginning with the definition of factorials we can work our way to a proof where 0! = 1 is mathematically proven. In the field of combinations and permutations, the explanation given is usually that there is only one way to arrange 0 objects, ...
Homework.Study.com
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Prove that 0! (zero factorial) is 1. | Homework.Study.com
Learn the definition of factorial and how factorials work in mathematics. Explore basic operations, such as multiplying factorials, and practice with examples. ... Prove that x^5 + x + 1 has exactly one zero on [-2, 2].
ScienceABC
scienceabc.com › home › pure sciences › why is the factorial of zero equal to one?
Zero Factorial: Why Is The Value Of Zero Factorial Equal To One?
October 19, 2023 - The factorial of a number n is the product of all numbers starting from one until we reach n. The operation is denoted by an exclamation mark succeeding the number whose factorial we wish to seek, such that the factorial of n is represented by n!.
CalculatorSoup
calculatorsoup.com › calculators › discretemathematics › factorials.php
Factorial Calculator n!
You will get the long integer answer and also the scientific notation for large factorials. You may want to copy the long integer answer result and paste it into another document to view it. n! = n × (n - 1) × (n - 2) × (n - 3) × ...
Top answer 1 of 5
16
Mostly it is based on convention, when one wants to define the quantity $\binom{n}{0} = \frac{n!}{n! 0!}$ for example. An intuitive way to look at it is $n!$ counts the number of ways to arrange $n$ distinct objects in a line, and there is only one way to arrange nothing.
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10
In a combinatorial sense, $n!$ refers to the number of ways of permuting $n$ objects. There is exactly one way to permute 0 objects, that is doing nothing, so $0!=1$.
There are plenty of resources that already answer this question. Also see:
Link
http://wiki.answers.com/Q/Why_is_zero_factorial_equal_to_one
http://en.wikipedia.org/wiki/Factorial#Definition
BYJUS
byjus.com › maths › factorial
Factorial
From the above formulas, the recurrence relation for the factorial of a number is defined as the product of the factorial number and factorial of that number minus 1.
Published October 6, 2021 Views 1K
Math Answers
math.answers.com › other-math › What_is_the_rationale_for_defining_0_factorial_to_be_1
What is the rationale for defining 0 factorial to be 1? - Answers
Only from the definition of a factorial ... n=1 one has (1-1)! = 1!/1 or 0! = 1/1 or 0! = 1. = This is a proof that zero factorial is equal to one to be known....
GeeksforGeeks
geeksforgeeks.org › mathematics › zero-factorial
What is Zero Factorial (0!): Meaning, Calculation, Applications - GeeksforGeeks
December 7, 2023 - So, the way of arranging any data which contains no values is in only one way. So the value of 0! is equal to 1. ... The value of 0! factorial is equal to 1.