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{| class="wikitable" style="margin:0 0 0 1em; text-align:right; float:right;"
|+ Selected members of the factorial [[sequence]] {{OEIS|id=A000142}}; values specified in scientific notation are rounded to the displayed precision
|-
! ''n''
! ''n''!
|-
| 0 || 1
|-
| 1 || 1
|-
| 2 || 2
|-
| 3 || 6
|-
| 4 || 24
|-
| 5 || 120
|-
| 6 || 720
|-
| 7 || {{gaps|5|040}}
|-
| 8 || {{gaps|40|320}}
|-
| 9 || {{gaps|362|880}}
|-
| 10 || {{gaps|3|628|800}}
|-
| 11 || {{gaps|39|916|800}}
|-
| 12 || {{gaps|479|001|600}}
|-
| 13 || {{gaps|6|227|020|800}}
|-
| 14 || {{gaps|87|178|291|200}}
|-
| 15 || {{gaps|1|307|674|368|000}}
|-
| 16 || {{gaps|20|922|789|888|000}}
|-
| 17 || {{gaps|355|687|428|096|000}}
|-
| 18 || {{gaps|6|402|373|705|728|000}}
|-
| 19 || {{gaps|121|645|100|408|832|000}}
|-
| 20 || {{gaps|2|432|902|008|176|640|000}}
|-
| 25 || {{val|1.5511210043|e=25}}
|-
| 42 || {{val|1.4050061178|e=51}}
|-
| 50 || {{val|3.0414093202|e=64}}
|-
| 70 || {{val|1.1978571670|e=100}}
|-
| 100 || {{val|9.3326215444|e=157}}
|-
| 450 || {{val|1.7333687331|e=1000}}
|-
| {{gaps|1|000}} || {{val|4.0238726008|e=2567}}
|-
| {{gaps|3|249}} || {{val|6.4123376883|e=10000}}
|-
| {{gaps|10|000}} || {{val|2.8462596809|e=35659}}
|-
| {{gaps|25|206}} || {{val|1.2057034382|e=100000}}
|-
| {{gaps|100|000}} || {{val|2.8242294080|e=456573}}
|-
| {{gaps|205|023}} || {{val|2.5038989317|e=1000004}}
|-
| {{gaps|1|000|000}} || {{val|8.2639316883|e=5565708}}
|-
| {{val|1.0248383838|e=98}} || [[googolplex|10<sup>{{val|e=100}}</sup>]]
|-
| [[googol|{{val|e=100}}]] || 10<sup>{{val|9.9565705518|e=101}}</sup>
|}

In [[mathematics]], the '''factorial''' of a [[non-negative integer]] ''n'', denoted by ''n''!, is the [[Product (mathematics)|product]] of all positive integers less than or equal to ''n''. For example,
:<math>5 ! = 5  \times  4  \times  3  \times  2  \times  1 = 120.  \ </math>

The value of 0! is 1, according to the convention for an [[empty product]].<ref>Ronald L. Graham, Donald E. Knuth, Oren Patashnik (1988) ''[[Concrete Mathematics]]'', Addison-Wesley, Reading MA. {{ISBN|0-201-14236-8}}, p.&nbsp;111</ref>

The factorial operation is encountered in many different areas of mathematics, notably in [[combinatorics]], [[algebra]] and [[mathematical analysis]]. Its most basic occurrence is the fact that there are ''n''! ways to arrange ''n'' distinct objects into a sequence (i.e., [[permutation]]s of the set of objects). This fact was known at least as early as the 12th century, to Indian scholars.<ref>N. L. Biggs, ''The roots of combinatorics'', Historia Math. 6 (1979) 109−136</ref>
The notation ''n''<nowiki>!</nowiki> was introduced by [[Christian Kramp]] in 1808.<ref>{{Citation |title=Number Story: From Counting to Cryptography |last=Higgins |first=Peter |year=2008 |publisher=Copernicus |location=New York |isbn=978-1-84800-000-1 |page=12 |pages= }} says Krempe though.</ref>

The definition of the factorial function can also be [[#Extension of factorial to non-integer values of argument|extended to non-integer arguments]], while retaining its most important properties; this involves more advanced mathematics, notably techniques from mathematical analysis.

==Definition==
The factorial function is formally defined by the [[Multiplication#Capital_Pi_notation|product]]

:<math> n!=\prod_{k=1}^n k \!</math>

or [[Recursion|recursively]] defined by

:<math> n! = \begin{cases}
1 & \text{if } n = 0, \\
(n-1)!\times n & \text{if } n > 0.
\end{cases}
</math>

Both of the above definitions incorporate the instance

:<math>0! = 1, \ </math>

in the first case by the convention that the [[empty product|product of no numbers at all]] is 1.  This is convenient because:

* There is exactly one permutation of zero objects (with nothing to permute, "everything" is left in place).
* The [[recurrence relation]] {{nowrap|(''n'' + 1)! {{=}} ''n''! × (''n'' + 1)}}, valid for ''n'' > 0, extends to ''n'' = 0.
* It allows for the expression of many formulae, such as the [[exponential function]], as a power series:
:: <math> e^x = \sum_{n = 0}^{\infty}\frac{x^n}{n!}.</math>
* It makes many identities in [[combinatorics]] valid for all applicable sizes. The number of ways to choose 0 elements from the [[empty set]] is <math>\tbinom{0}{0} = \tfrac{0!}{0!0!} = 1</math>. More generally, the number of ways to choose (all) ''n'' elements among a set of ''n'' is <math>\tbinom nn = \tfrac{n!}{n!0!} = 1</math>.

The factorial function can also be defined for non-integer values using more advanced mathematics, detailed in the [[#Extension of factorial to non-integer values of argument|section below]]. This more generalized definition is used by advanced [[calculator]]s and [[mathematical software]] such as [[Maple (software)|Maple]] or [[Mathematica]].

The n-factorial can also be defined using the power rule, in which
:<math> \frac({d}{dx})^n x^n = n! </math>  


==Applications==

Although the factorial function has its roots in [[combinatorics]], formulas involving factorials occur in many areas of mathematics.

* There are ''n''! different ways of arranging ''n'' distinct objects into a sequence, the [[permutation]]s of those objects.

* Often factorials appear in the [[denominator]] of a formula to account for the fact that ordering is to be ignored. A classical example is counting ''k''-[[combination]]s (subsets of ''k'' elements) from a set with ''n'' elements. One can obtain such a combination by choosing a ''k''-permutation: successively selecting and removing an element of the set, ''k'' times, for a total of
::<math>n^{\underline k}=n(n-1)(n-2)\cdots(n-k+1)</math>
:possibilities. This however produces the ''k''-combinations in a particular order that one wishes to ignore; since each ''k''-combination is obtained in ''k''! different ways, the correct number of ''k''-combinations is
::<math>\frac{n^{\underline k}}{k!}=\frac{n(n-1)(n-2)\cdots(n-k+1)}{k(k-1)(k-2)\cdots1}.</math>
:This number is known as the [[binomial coefficient]] <math>\tbinom nk</math>, because it is also the coefficient of ''X''<sup>''k''</sup> in {{nowrap|(1 + ''X'')<sup>''n''</sup>}}.

* Factorials occur in [[algebra]] for various reasons, such as via the already mentioned coefficients of the [[binomial formula]], or through [[averaging]] over [[permutations]] for [[symmetrization]] of certain operations.

* Factorials also turn up in [[calculus]]; for example they occur in the denominators of the terms of [[Taylor's formula]], where they are used as compensation terms due to the ''n''-th [[derivative]] of ''x''<sup>''n''</sup> being equivalent to ''n''<nowiki>!</nowiki>.

* Factorials are also used extensively in [[probability theory]].

* Factorials can be useful to facilitate expression manipulation. For instance the number of ''k''-permutations of ''n'' can be written as
::<math>n^{\underline k}=\frac{n!}{(n-k)!};</math>
:while this is inefficient as a means to compute that number, it may serve to prove a symmetry property of binomial coefficients:
::<math>\binom nk=\frac{n^{\underline k}}{k!}=\frac{n!}{(n-k)!k!}=\frac{n^{\underline{n-k}}}{(n-k)!}=\binom n{n-k}.</math>

==Number theory==
Factorials have many applications in [[number theory]]. In particular, ''n''<nowiki>!</nowiki> is necessarily divisible by all [[prime number]]s up to and including&nbsp;''n''. As a consequence, ''n'' > 5 is a [[composite number]] [[if and only if]]
:<math>(n-1)!\ \equiv\ 0 \pmod n.</math>

A stronger result is [[Wilson's theorem]], which states that
:<math>(p-1)!\ \equiv\ -1 \pmod p</math>
if and only if ''p'' is prime.

[[Adrien-Marie Legendre]] found that the multiplicity of the prime ''p'' occurring in the prime factorization of ''n''<nowiki>!</nowiki> can be expressed exactly as
:<math>\sum_{i=1}^{\infty} \left \lfloor \frac{n}{p^i} \right \rfloor .</math>

This fact is based on counting the number of factors ''p'' of the integers from 1 to&nbsp;''n''. The number of multiples of ''p'' in the numbers 1 to ''n'' are given by <math>\textstyle \left \lfloor \frac{n}{p} \right \rfloor</math>; however, this formula counts those numbers with two factors of ''p'' only once. Hence another <math>\textstyle \left \lfloor \frac{n}{p^2} \right \rfloor</math> factors of ''p'' must be counted too. Similarly for three, four, five factors, to infinity. The sum is finite since ''p''<sup>&nbsp;''i''</sup> can only be less than or equal to ''n'' for finitely many values of&nbsp;''i'', and the [[Floor and ceiling functions|floor function]] results in 0 when applied for&nbsp;''p''<sup>&nbsp;''i''</sup>&nbsp;>&nbsp;''n''.

The only factorial that is also a prime number is 2, but there are many primes of the form&nbsp;''n''!&nbsp;±&nbsp;1, called [[factorial prime]]s.

All factorials greater than 1! are [[Parity (mathematics)|even]], as they are all multiples of&nbsp;2. Also, all factorials from 5! upwards are multiples of 10 (and hence have a [[trailing zero]] as their final digit), because they are multiples of 5 and&nbsp;2.

Also note that the [[multiplicative inverse|reciprocal]]s of factorials produce a [[convergent series]]: (see [[e (mathematical constant)|''e'']])
:<math>\sum_{n=0}^{\infty} \frac{1}{n!} = \frac{1}{1} + \frac{1}{1} + \frac{1}{2} + \frac{1}{6} + \frac{1}{24}  + \frac{1}{120} + \ldots = e\,.</math>

==Rate of growth and approximations for large n==
[[File:Log-factorial.svg|300px|thumb|right|Plot of the natural logarithm of the factorial]]
As ''n'' grows, the factorial ''n''<nowiki>!</nowiki> increases faster than all [[polynomial]]s and [[exponential growth|exponential function]]s (but slower than [[double exponential function]]s) in ''n''.

Most approximations for ''n''! are based on approximating its [[natural logarithm]]
:<math>\log n! = \sum_{x=1}^n \log x.</math>

The graph of the function ''f''(''n'') = log ''n''! is shown in the figure on the right. It looks approximately [[linear function|linear]] for all reasonable values of ''n'', but this intuition is false.
We get one of the simplest approximations for log ''n''! by bounding the sum with an [[integral]] from above and below as follows:

:<math> \int_1^n \log x \, dx \leq \sum_{x=1}^n \log x \leq \int_0^n \log (x+1) \, dx</math>

which gives us the estimate

:<math> n\log\left(\frac{n}{e}\right)+1 \leq \log n! \leq (n+1)\log\left( \frac{n+1}{e} \right) + 1.</math>

Hence log ''n''! is Θ(''n'' log ''n'') (see [[Big_O_notation#Family_of_Bachmann.E2.80.93Landau_notations|Big ''O'' notation]]). This result plays a key role in the analysis of the [[computational complexity theory|computational complexity]] of [[sorting algorithm]]s (see [[comparison sort]]). From the bounds on log&nbsp;''n''! deduced above we get that

:<math>e\left(\frac ne\right)^n \leq n! \leq e\left(\frac{n+1}e\right)^{n+1}.</math>

It is sometimes practical to use weaker but simpler estimates. Using the above formula it is easily shown that for all ''n'' we have <math>(n/3)^n < n!</math>, and for all ''n'' ≥ 6 we have <math>n! < (n/2)^n</math>.

For large ''n'' we get a better estimate for the number ''n''<nowiki>!</nowiki> using [[Stirling's approximation]]:

:<math>n!\approx \sqrt{2\pi n}\left(\frac{n}{e}\right)^n.</math>

In fact, it can be proved that for all ''n'' we have

:<math>n! > \sqrt{2\pi n}\left(\frac{n}{e}\right)^n.</math>

Another approximation for {{nowrap|log ''n''!}} is given by [[Srinivasa Ramanujan]] {{harv|Ramanujan|1988}}

:<math>\log n! \approx n\log n - n + \frac {\log(n(1+4n(1+2n)))} {6} + \frac {\log(\pi)} {2}</math>
:<math>= n\log n - n + \frac {\log(1 +1/(2n) +1/(8n^2))} {6} + \frac {3\log (2n)} 6 + \frac {\log(\pi)} {2}.</math>
Thus it is even smaller than the next correction term <math>\tfrac 1 {12n}</math> of Stirling's formula.

==Computation==

If efficiency is not a concern, computing factorials is trivial from an algorithmic point of view: successively multiplying a variable initialized to 1 by the integers 2 up to ''n'' (if any) will compute ''n''<nowiki>!</nowiki>, provided the result fits in the variable. In functional languages, the recursive definition is often implemented directly to illustrate recursive functions.

The main practical difficulty in computing factorials is the size of the result. To assure that the exact result will fit for all legal values of even the smallest commonly used integral type (8-bit signed integers) would require more than 700 bits, so no reasonable specification of a factorial function using fixed-size types can avoid questions of overflow. The values 12! and 20! are the largest factorials that can be stored in, respectively, the 32-bit and 64-bit integers commonly used in [[personal computer]]s. [[Floating-point]] representation of an approximated result allows going a bit further, but this also remains quite limited by possible overflow. Most [[calculator]]s use [[scientific notation]] with 2-digit decimal exponents, and the largest factorial that fits is then 69!, because 69!&nbsp;<&nbsp;10<sup>100</sup>&nbsp;<&nbsp;70!. Calculators that use 3-digit exponents can compute larger factorials, up to, for example, 253! ≈ 5.2{{e|499}} on [[Hewlett-Packard|HP]] calculators and 449! ≈ 3.9{{e|997}} on the [[TI-86]]. The calculator seen in [[Mac OS X]], [[Microsoft Excel]] and [[Google Calculator]], as well as the freeware Fox Calculator, can handle factorials up to 170!, which is the largest factorial whose floating-point approximation can be represented as a [[IEEE 754-1985#Double-precision 64 bit|64-bit IEEE 754 floating-point]] value. The scientific calculator in Windows 7 and Windows 8 is able to calculate factorials up to 3248!.

Most software applications will compute small factorials by direct multiplication or table lookup. Larger factorial values can be approximated using [[Stirling's formula]]. [[Wolfram Alpha]] can calculate exact results for the [[ceiling function]] and [[floor function]] applied to the [[binary logarithm|binary]], [[natural logarithm|natural]] and [[common logarithm]] of ''n''<nowiki>!</nowiki> for values of ''n'' up to 249999, and up to 20,000,000! for the integers.

If the exact values of large factorials are needed, they can be computed using [[arbitrary-precision arithmetic]]. Instead of doing the sequential multiplications <math>((1 \times 2) \times 3) \times 4\dots</math>, a program can partition the sequence into two parts, whose products are roughly the same size, and multiply them using a [[divide-and-conquer algorithm|divide-and-conquer]] method. This is often more efficient.<ref>[[GNU MP]] software manual, [http://gmplib.org/manual/Factorial-Algorithm.html "Factorial Algorithm"] (retrieved 22 January 2013).</ref>

The asymptotically best efficiency is obtained by computing ''n''<nowiki>!</nowiki> from its prime factorization. As documented by [[Peter Borwein]], prime factorization allows ''n''<nowiki>!</nowiki> to be computed in time [[Big O notation|O]](''n''(log&nbsp;''n''&nbsp;log&nbsp;log&nbsp;''n'')<sup>2</sup>), provided that a fast [[multiplication algorithm]] is used (for example, the [[Schönhage&ndash;Strassen algorithm]]).<ref>Peter Borwein. "On the Complexity of Calculating Factorials". ''Journal of Algorithms'' 6, 376&ndash;380 (1985)</ref> Peter Luschny presents source code and benchmarks for several efficient factorial algorithms, with or without the use of a [[prime sieve]].<ref>Peter Luschny, [http://www.luschny.de/math/factorial/FastFactorialFunctions.htm ''Fast-Factorial-Functions: The Homepage of Factorial Algorithms''].</ref>

==Extension of factorial to non-integer values of argument==

===The Gamma and Pi functions===
{{Main|Gamma function}}

[[File:Generalized factorial function.svg|thumb|right|325px|The factorial function, generalized to all real numbers except negative integers. For example, 0!&nbsp;=&nbsp;1!&nbsp;=&nbsp;1, (&minus;0.5)!&nbsp;=&nbsp;&radic;''&pi;'', (0.5)!&nbsp;=&nbsp;&radic;''&pi;''/2.]]

Besides nonnegative integers, the factorial function can also be defined for non-integer values, but this requires more advanced tools from [[mathematical analysis]]. One function that "fills in" the values of the factorial (but with a shift of 1 in the argument) is called the [[Gamma function]], denoted Γ(''z''), defined for all complex numbers ''z'' except the non-positive integers, and given when the real part of ''z'' is positive by

:<math>\Gamma(z)=\int_0^\infty t^{z-1} e^{-t}\, \mathrm{d}t. \!</math>

Its relation to the factorials is that for any natural number ''n''
:<math>n!=\Gamma(n+1).\,</math>

[[Leonhard Euler|Euler's]] original formula for the Gamma function was

:<math>\Gamma(z)=\lim_{n\to\infty}\frac{n^zn!}{\displaystyle\prod_{k=0}^n (z+k)}. \!</math>

It is worth mentioning that there is an alternative notation that was originally introduced by [[Carl Friedrich Gauss|Gauss]] which is sometimes used.  The '''Pi function''', denoted Π(''z'') for real numbers ''z'' no less than&nbsp;0, is defined by

:<math>\Pi(z)=\int_0^\infty t^{z} e^{-t}\, \mathrm{d}t\,.</math>

In terms of the Gamma function it is

:<math>\Pi(z) = \Gamma(z+1) \,.</math>

It truly extends the factorial in that

:<math>\Pi(n) = n!\text{ for }n \in \mathbf{N}\, .</math>

In addition to this, the Pi function satisfies the same recurrence as factorials do, but at every complex value ''z'' where it is defined

:<math>\Pi(z) = z\Pi(z-1)\,.</math>

In fact, this is no longer a recurrence relation but a [[functional equation]].
Expressed in terms of the Gamma function this functional equation takes the form

:<math>\Gamma(n+1)=n\Gamma(n)\,.</math>

Since the factorial is extended by the Pi function, for every complex value ''z'' where it is defined, we can write:

:<math>z! = \Pi(z)\,</math>

The values of these functions at [[half-integer]] values is therefore determined by a single one of them; one has

:<math>\Gamma\left (\frac{1}{2}\right )=\left (-\frac{1}{2}\right )!=\Pi\left (-\frac{1}{2}\right ) = \sqrt{\pi},</math>

from which it follows that for&nbsp;''n''&nbsp;∈&nbsp;'''N''',

:<math>\Gamma\left (\frac{1}{2}+n\right ) = \left (-\frac{1}{2}+n\right )! = \Pi\left (-\frac{1}{2}+n\right ) = \sqrt{\pi} \prod_{k=1}^n {2k - 1 \over 2} = {(2n)! \over 4^n n!} \sqrt{\pi} = {(2n-1)! \over 2^{2n-1}(n-1)!} \sqrt{\pi}.</math>

For example,
:<math>\Gamma\left (4.5 \right ) = 3.5! = \Pi\left (3.5\right ) = {1\over 2}\cdot{3\over 2}\cdot{5\over 2}\cdot{7\over 2} \sqrt{\pi} = {8! \over 4^4 4!} \sqrt{\pi} = {7! \over 2^7 3!} \sqrt{\pi} = {105 \over 16} \sqrt{\pi} \approx 11.63.</math>

It also follows that for&nbsp;''n''&nbsp;∈&nbsp;'''N''',

:<math>\Gamma\left (\frac{1}{2}-n\right ) = \left (-\frac{1}{2}-n\right )! = \Pi\left (-\frac{1}{2}-n\right ) = \sqrt{\pi} \prod_{k=1}^n {2 \over 1 - 2k} = {(-4)^n n! \over (2n)!} \sqrt{\pi}.</math>

For example,
:<math>\Gamma\left (-2.5 \right ) = (-3.5)! = \Pi\left (-3.5\right ) = {2\over -1}\cdot{2\over -3}\cdot{2\over -5} \sqrt{\pi} = {(-4)^3 3! \over 6!} \sqrt{\pi} = -{8 \over 15} \sqrt{\pi} \approx -0.9453.</math>

The Pi function is certainly not the only way to extend factorials to a function defined at almost all complex values, and not even the only one that is [[Analytic function|analytic]] wherever it is defined. Nonetheless it is usually considered the most natural way to extend the values of the factorials to a complex function.  For instance, the [[Bohr–Mollerup theorem]] states that the Gamma function is the only function that takes the value 1 at 1, satisfies the functional equation Γ(''n''&nbsp;+&nbsp;1)&nbsp;=&nbsp;''n''Γ(''n''), is [[meromorphic]] on the complex numbers, and is [[log-convex]] on the positive real axis.  A similar statement holds for the Pi function as well, using the Π(''n'')&nbsp;=&nbsp;''n''Π(''n''&nbsp;&minus;&nbsp;1) functional equation.

However, there exist complex functions that are probably simpler in the sense of analytic function theory and which interpolate the factorial values.  For example, [[Jacques Hadamard|Hadamard's]] 'Gamma'-function {{harv|Hadamard|1894}} which, unlike the Gamma function, is an [[entire function]].<ref>Peter Luschny, [http://www.luschny.de/math/factorial/hadamard/HadamardsGammaFunction.html ''Hadamard versus Euler - Who found the better Gamma function?''].</ref>

Euler also developed a convergent product approximation for the non-integer factorials, which can be seen to be equivalent to the formula for the Gamma function above:

:<math>\begin{align}n! = \Pi(n) &= \prod_{k = 1}^\infty \left(\frac{k+1}{k}\right)^n\!\!\frac{k}{n+k} \\ &= \left[ \left(\frac{2}{1}\right)^n\frac{1}{n+1}\right]\left[ \left(\frac{3}{2}\right)^n\frac{2}{n+2}\right]\left[ \left(\frac{4}{3}\right)^n\frac{3}{n+3}\right]\cdots. \end{align}</math>

However, this formula does not provide a practical means of computing the Pi or Gamma function, as its rate of convergence is slow.

===Applications of the Gamma function===

The [[volume]] of an [[Dimension|''n''-dimensional]] [[N-sphere|hypersphere]] of radius ''R'' is

:<math>V_n=\frac{\pi^{n/2}}{\Gamma((n/2)+1)}R^n.</math>

===Factorial at the complex plane===
[[File:Factorial05.jpg|400px|thumb|Amplitude and phase of factorial of complex argument.]]
Representation through the Gamma-function allows evaluation of factorial of complex argument. Equilines of amplitude and phase of factorial are shown in figure. Let <math>\ f=\rho \exp({\rm i}\varphi)=(x+{\rm i}y)!=\Gamma(x+{\rm i}y+1) </math>. Several levels of constant modulus (amplitude) <math>\rho =\rm const</math> and constant phase <math>\varphi=\rm const</math> are shown. The grid covers range
<math>~-3 \le x \le 3~</math>,
<math>~-2 \le y \le 2~</math>
with unit step. The scratched line shows the level <math>\varphi=\pm \pi</math>.

Thin lines show intermediate levels of constant modulus and constant phase. At poles <math> x+ {\rm i}y \in \rm (negative ~ integers)</math>, phase and amplitude are not defined. Equilines are dense in vicinity of singularities along negative integer values of the argument.

For <math>|z|<1</math>, the Taylor expansions can be used:
:<math>z!=\sum_{n=0}^{\infty} g_n z^n.</math>
The first coefficients of this expansion are

<table border="2" cellpadding="4" cellspacing="2" style="margin:1em 1em 1em 0; background:#fff; border:1px #aaa solid; border-collapse:collapse;">
<tr>
<th><math>n</math></th>
<th><math>g_n</math></th>
<th>approximation</th>
</tr>

<tr>
<td>0</td>
<td><math>1</math></td>
<td><math>  1</math></td>
</tr>

<tr>
<td>1</td>
<td><math>-\gamma</math></td>
<td><math>- 0.5772156649</math></td>
</tr>

<tr>
<td>2</td>
<td><math>\frac{\pi^2}{12}+\frac{\gamma^2}{2}</math></td>
<td><math>  0.9890559955</math></td>
</tr>

<tr>
<td>3</td>
<td><math>-\frac{\zeta(3)}{3}-\frac{\pi^2\gamma}{12}-\frac{\gamma^3}{6}</math></td>
<td><math>-0.9074790760</math></td>
</tr>
</table>

where <math>\gamma</math> is the [[Euler–Mascheroni constant|Euler constant]] and <math>\zeta</math> is the [[Riemann zeta function]]. [[Computer algebra system]]s such as [[Sage (mathematics software)]] can generate many terms of this expansion.

===Approximations of factorial===
For the large values of the argument,
factorial can be approximated through the integral of the
[[digamma function]], using the [[continued fraction]] representation.
This approach is due to T. J. [[Stieltjes]] (1894). Writing ''z''! = exp(P(''z'')) where P(''z'') is
: <math> P(z) = p(z) + \log(2\pi)/2 - z + \left(z+\frac{1}{2}\right)\log(z),</math>
Stieltjes gave a continued fraction for p(''z'')
: <math>
p(z)=\cfrac{a_0}{z+
     \cfrac{a_1}{z+
     \cfrac{a_2}{z+
     \cfrac{a_3}{z+\ddots}}}}
</math>
The first few coefficients a<sub>n</sub> are <ref name="dlmf5.10" >Digital Library of Mathematical Functions, http://dlmf.nist.gov/5.10</ref>
<table border="2" cellpadding="4" cellspacing="2"
style="margin-left:2em; background:#fff; border:1px #aaa solid; border-collapse:collapse;">
<tr><th>n</th><th>a<sub>n</sub></th></tr>
<tr><td>0</td><td>1 / 12</td></tr>
<tr><td>1</td><td>1 / 30</td></tr>
<tr><td>2</td><td>53 / 210</td></tr>
<tr><td>3</td><td>195 / 371</td></tr>
<tr><td>4</td><td>22999 / 22737</td></tr>
<tr><td>5</td><td>29944523 / 19733142</td></tr>
<tr><td>6</td><td>109535241009 / 48264275462</td></tr>
</table>

There is common [[List of common misconceptions|misconception]], that <math>\displaystyle\log(z!)=P(z)</math> or <math>\log(\Gamma(z\!+\!1))=P(z)</math>
for any complex ''z''&nbsp;≠&nbsp;0. Indeed, the relation through the logarithm is valid only for specific range of values of ''z'' in vicinity of the real axis, while <math>|\Im(\Gamma(z\!+\!1))| < \pi </math>. The larger is the real part of the argument, the smaller should be the imaginary part.  However, the inverse relation, ''z''!&nbsp;=&nbsp;exp(''P''(''z'')), is valid for the whole complex plane apart from zero. The convergence is poor in vicinity of the negative part of the real axis. (It is difficult to have good convergence of any approximation in vicinity of the singularities).  While <math>|\Im(z)| >2 </math> or <math>\Re(z)>2</math>, the 6 coefficients above are sufficient for the evaluation of the factorial with the complex<double> precision. For higher precision more coefficients can be computed by a rational QD-scheme ([[H. Rutishauser]]'s [[QD algorithm]]).<ref>Peter Luschny, [http://www.luschny.de/math/factorial/approx/continuedfraction.html ''On Stieltjes' Continued Fraction for the Gamma Function.''].</ref>

===Non-extendability to negative integers===
The relation ''n''&nbsp;!&nbsp;=&nbsp;(''n''&nbsp;&minus;&nbsp;1)!&nbsp;&times;&nbsp;''n'' allows one to compute the factorial for an integer given the factorial for a ''smaller'' integer.  The relation can be inverted so that one can compute the factorial for an integer given the factorial for a ''larger'' integer:

:<math>n! = \frac{(n+1)!}{n+1}.</math>

Note, however, that this recursion does not permit us to compute the factorial of a negative integer; use of the formula to compute (&minus;1)<nowiki>!</nowiki> would require a division by zero, and thus blocks us from computing a factorial value for every negative integer.  (Similarly, the Gamma function is not defined for non-positive integers, though it is defined for all other complex numbers.)

==Factorial-like products and functions==
There are several other integer sequences similar to the factorial that are used in mathematics:

===Primorial===
The [[primorial]] {{OEIS|id=A002110}} is similar to the factorial, but with the product taken only over the [[prime number]]s.

=== Double factorial ===
The product of all odd integers up to some odd positive integer ''n'' is often called the '''double factorial''' of ''n'' (even though it only involves about half the factors of the ordinary factorial, and its value is therefore closer to the square root of the factorial). It is denoted by
''n''<nowiki>!!</nowiki>.

For an odd positive integer ''n'' = 2''k'' - 1, ''k'' ≥ 1, it is

:<math>(2k-1)!! = \prod_{i=1}^k (2i-1).</math>

For example, 9!!&nbsp;=&nbsp;1&nbsp;&times;&nbsp;3&nbsp;&times;&nbsp;5&nbsp;&times;&nbsp;7&nbsp;&times;&nbsp;9 =&nbsp;945. This notation creates a notational ambiguity with the composition of the factorial function with itself (which for ''n''&nbsp;>&nbsp;2 gives much larger numbers than the double factorial); this may be justified by the fact that composition arises very seldom in practice, and could be denoted by (''n''<nowiki>!)!</nowiki> to circumvent the ambiguity. The double factorial notation is not essential; it can be expressed in terms of the ordinary factorial by

:<math>(2k-1)!!=\frac{(2k)!}{2^k k!},</math>

since the denominator equals <math>\displaystyle\prod_{i=1}^k 2i</math> and cancels the unwanted even factors from the numerator. The introduction of the double factorial is motivated by the fact that it occurs rather frequently in combinatorial and other settings, for instance
* (2''n''&nbsp;&minus;&nbsp;1)<nowiki>!!</nowiki> is the number of [[permutation]]s of 2''n'' whose [[cycle type]] consists of ''n'' parts equal to&nbsp;2; these are the [[Involution (mathematics)|involutions]] without fixed points.
* (2''n''&nbsp;&minus;&nbsp;1)<nowiki>!!</nowiki> is the number of [[perfect matching]]s in a complete graph ''K''(2''n'').
* (2''n''&nbsp;&minus;&nbsp;5)<nowiki>!!</nowiki> is the number of [[unrooted binary tree]]s with ''n'' labeled leaves.
* The value <math>\textstyle\Gamma(n+\frac12)</math> is equal to <math>\textstyle\frac{(2n-1)!!}{2^n}\sqrt\pi</math> (see above)

Sometimes ''n''<nowiki>!!</nowiki> is defined for non-negative even numbers as well. One choice is a definition similar to the one for odd values

:<math>(2k)!!= \prod_{i=1}^k (2i) = 2^k k!.</math>

For example, with this definition, 8<nowiki>!!</nowiki> =&nbsp;2&nbsp;&times;&nbsp;4&nbsp;&times;&nbsp;6&nbsp;&times;&nbsp;8 =&nbsp;[[384 (number)|384]].
However, note that this definition does not match the expression above, of the double factorial in terms of the ordinary factorial, and is also inconsistent with the extension of the definition of <math>n!!</math> to complex numbers <math>n</math> that is achieved via the [[Gamma function]] as indicated [[#Alternative extension of the double factorial|below]]. Also, for even numbers, the double factorial notation is hardly shorter than expressing the same value using ordinary factorials.  For combinatorial interpretations (the value gives, for instance, the size of the [[hyperoctahedral group]]), the latter expression can be more informative (because the factor 2<sup>''n''</sup> is the order of the kernel of a projection to the [[symmetric group]]). Even though the formulas for the odd and even double factorials can be easily combined into

:<math>n!!=\prod_{i;~0\leq 2i<n}(n-2i),</math>

the only known interpretation for the sequence of all these numbers {{OEIS|id=A006882}} is somewhat artificial: the number of down-up permutations of a set of {{nowrap|''n'' + 1}} elements for which the entries in the even positions are increasing.

The sequence of double factorials for ''n''&nbsp;=&nbsp;1,&nbsp;3,&nbsp;5,&nbsp;7,&nbsp;... {{OEIS|id=A001147}} starts as
: 1, 3, 15, 105, 945, 10395, 135135, ....

Some identities involving double factorials are:

:<math>(2n+1)!! = {(2n+1)! \over 2^n n!} = 2^n n! {n+\frac 1 2 \choose n} = (-2)^{n+1} (n+1)! {-\frac 1 2 \choose n+1}.</math>

:<math>(2n-1)!! = {(2n)! \over 2^n n!} = 2^n n! {n-\frac 1 2 \choose n} = (-2)^n n! {-\frac 1 2 \choose n}.</math>

====Alternative extension of the double factorial====
<!-- This section name is an anchor for a "see below" link above. -->

Disregarding the above definition of ''n''<nowiki>!!</nowiki> for even values of&nbsp;''n'', the double factorial for odd integers can be extended to most real and complex numbers ''z'' by noting that when ''z'' is a positive odd integer then
:<math>z!! = z(z-2)\cdots (3)
= 2^{(z-1)/2}\left(\frac{z}{2}\right)\left(\frac{z-2}{2}\right)\cdots \left(\frac{3}{2}\right)
= 2^{(z-1)/2} \frac{\Gamma\left(\frac{z}{2}+1\right)}{\Gamma\left(\frac{1}{2}+1\right)}
= \sqrt{\frac{2^{z+1}}{\pi}} \Gamma\left(\frac{z}{2}+1\right)\,.</math>
The expressions obtained by taking one of the above formulas for <math>(2n+1)!!</math> and <math>(2n-1)!!</math> and expressing the occurring factorials in terms of the gamma function can both be seen (using the [[multiplication theorem#Gamma function|multiplication theorem]]) to be equivalent to the one given here.

The expression found for ''z''<nowiki>!!</nowiki> is defined for all complex numbers except the negative even numbers. Using it as the definition, the [[volume]] of an ''n''-[[dimension]]al [[hypersphere]] of radius ''R'' can be expressed as

:<math>V_n=\frac{2 (2\pi)^{(n-1)/2}}{n!!} R^n.</math>

===Multifactorials===<!-- This section is linked from [[Catalan number]] -->
A common related notation is to use multiple exclamation points to denote a '''multifactorial''', the product of integers in steps of two (<math>n!!</math>), three (<math>n!!!</math>), or more. The double factorial is the most commonly used variant, but one can similarly define the triple factorial (<math>n!!!</math>) and so on.   One can define the ''k''-th factorial, denoted by <math>n!^{(k)}</math>, recursively for non-negative integers as

:<math>
  n!^{(k)}=
  \left\{
   \begin{matrix}
    1,\qquad\qquad\ &&\mbox{if }0\le n<k,
   \\
    n((n-k)!^{(k)}),&&\mbox{if }n\ge k\,,\quad\ \ \,
   \end{matrix}
  \right.
 </math>
though see [[#Alternative extension of the multifactorial|the alternative definition below]].

Some mathematicians have suggested an alternative notation of <math>n!_2</math> for the double factorial and similarly <math>n!_k</math> for other multifactorials, but this has not come into general use.

The factorial operation is encountered in many different areas of mathematics, notably in [[combinatorics]], [[algebra]] and [[mathematical analysis]]. Its most basic occurrence is the fact that there are ''n''! ways to arrange ''n'' distinct objects into a sequence (i.e., [[permutation]]s of the set of objects). This fact was known to Indian scholars at least as early as the 12th century.

In the same way that <math>n!</math> is not defined for negative integers, and <math>n!!</math> is not defined for negative even integers, <math>n!^{(k)}</math> is not defined for negative integers divisible by <math>k</math>.

====Alternative extension of the multifactorial====

Alternatively, the multifactorial ''z''!<sup>(''k'')</sup> can be extended to most real and complex numbers ''z'' by noting that when ''z'' is one more than a positive multiple of ''k'' then
:<math>z!^{(k)} = z(z-k)\cdots (k+1)
= k^{(z-1)/k}\left(\frac{z}{k}\right)\left(\frac{z-k}{k}\right)\cdots \left(\frac{k+1}{k}\right)
= k^{(z-1)/k} \frac{\Gamma\left(\frac{z}{k}+1\right)}{\Gamma\left(\frac{1}{k}+1\right)}\,.</math>
This last expression is defined much more broadly than the original; with this definition, ''z''!<sup>(''k'')</sup> is defined for all complex numbers except the negative real numbers evenly divisible by ''k''.  This definition is consistent with the earlier definition only for those integers ''z'' satisfying&nbsp;''z''&nbsp;≡&nbsp;1&nbsp;mod&nbsp;''k''.

In addition to extending ''z''!<sup>(''k'')</sup> to most complex numbers&nbsp;''z'', this definition has the feature of working for all positive real values of&nbsp;''k''.  Furthermore, when ''k''&nbsp;=&nbsp;1, this definition is mathematically equivalent to the Π(''z'') function, described above.  Also, when ''k''&nbsp;=&nbsp;2, this definition is mathematically equivalent to the [[#Alternative extension of the double factorial|alternative extension of the double factorial]], described above.

===Quadruple factorial===
The so-called [[Catalan number#Quadruple factorial|quadruple factorial]], however, is not the multifactorial&nbsp;''n''!<sup>(4)</sup>; it is a much larger number given by&nbsp;(2''n'')!/''n''!, starting as

:1, 2, 12, 120, 1680, 30240, 665280, ... {{OEIS|id=A001813}}.

It is also equal to

: <math>
\begin{align}
2^n\frac{(2n)!}{n!2^n} & = 2^n \frac{(2\cdot 4\cdots 2n) (1\cdot 3\cdots (2n-1))}{2\cdot 4\cdots 2n} \\[8pt]
& = (1\cdot 2)\cdot (3 \cdot 2) \cdots((2n-1)\cdot 2)=(4n-2)!^{(4)}.
\end{align}
</math>

===Superfactorial===
{{Main|Large numbers}}
{{redirect|N$|the currency|Namibian dollar}}
[[Neil Sloane]] and [[Simon Plouffe]] defined a '''superfactorial''' in The Encyclopedia of Integer Sequences (Academic Press, 1995) to be the product of the first <math>n</math> factorials.  So the superfactorial of 4 is

:<math> \mathrm{sf}(4)=1! \times 2! \times 3! \times 4!=288. \,</math>

In general

:<math>
  \mathrm{sf}(n)
  =\prod_{k=1}^n k! =\prod_{k=1}^n k^{n-k+1}
  =1^n\cdot2^{n-1}\cdot3^{n-2}\cdots(n-1)^2\cdot n^1.
 </math>

Equivalently, the superfactorial is given by the formula
:<math>
  \mathrm{sf}(n)
  =\prod_{0 \le i < j \le n} (j-i)
 </math>
which is the [[determinant]] of a [[Vandermonde matrix]].

The sequence of superfactorials starts (from <math>n = 0</math>) as

:1, 1, 2, 12, 288, 34560, 24883200, 125411328000, ... {{OEIS|id=A000178}}

====Alternative definition====
[[Clifford Pickover]] in his 1995 book ''Keys to Infinity'' used a new notation, ''n$'', to define the superfactorial
:<math>n\$\equiv \begin{matrix} \underbrace{ n!^{{n!}^{{\cdot}^{{\cdot}^{{\cdot}^{n!}}}}}} \\ n! \end{matrix}, \,</math>
or as,
:<math>n\$=n!^{(4)}n! \,</math>
where the <sup>(4)</sup> notation denotes the [[Tetration|hyper4]] [[operator (mathematics)|operator]], or using [[Knuth's up-arrow notation]],
:<math>n\$=(n!)\uparrow\uparrow(n!). \,</math>
This sequence of superfactorials starts:
:<math>1\$=1 \,</math>
:<math>2\$=2^2=4 \,</math>
:<math>3\$=6\uparrow\uparrow6={^6}6=6^{6^{6^{6^{6^6}}}}.</math>
Here, as is usual for compound [[exponentiation]], the grouping is understood to be from right to left:
:<math>a^{b^c}=a^{(b^c)}.\,</math>

===Hyperfactorial===
Occasionally the '''hyperfactorial''' of ''n'' is considered. It is written as ''H''(''n'') and defined by

:<math>
  H(n)
  =\prod_{k=1}^n k^k
  =1^1\cdot2^2\cdot3^3\cdots(n-1)^{n-1}\cdot n^n.
 </math>

For ''n'' = 1, 2, 3, 4, ... the values ''H''(''n'') are 1, 4, 108, 27648,... {{OEIS|id=A002109}}.

The asymptotic growth rate is

:  <math>H(n) \sim A n^{(6n^2 + 6n + 1)/12} e^{-n^2/4}</math>

where ''A'' = 1.2824... is the [[Glaisher&ndash;Kinkelin constant]].<ref>{{MathWorld | urlname=Glaisher-KinkelinConstant | title=Glaisher&ndash;Kinkelin Constant}}</ref> ''H''(14)&nbsp;=&nbsp;1.8474...&times;10<sup>99</sup> is already almost equal to a [[googol]], and ''H''(15)&nbsp;=&nbsp;8.0896...&times;10<sup>116</sup> is almost of the same magnitude as the [[Shannon number]], the theoretical number of possible chess games. Compared to the Pickover definition of the superfactorial, the hyperfactorial grows relatively slowly.

The hyperfactorial function can be generalized to [[complex number]]s in a similar way as the factorial function. The resulting function is called the [[K-function]].

==See also==
{{div col|colwidth=30em}}
* [[Alternating factorial]]
* [[Digamma function]]
* [[Exponential factorial]]
* [[Factorial number system]]
* [[Factorial prime]]
* [[Factorion]]
* [[Gamma function]]
* [[List of factorial and binomial topics]]
* [[Pochhammer symbol]], which gives the falling or rising factorial
* [[Stirling's approximation]]
* [[Subfactorial]]
* [[Trailing zeros#Factorial|Trailing zeros]] of factorial
* [[Triangular number]], the additive analogue of factorial
{{div col end}}

==Notes==
{{Reflist|2}}

==References==
*{{citation |first=M. J. |last=Hadamard
|title=Sur L’Expression Du Produit 1·2·3· · · · ·(n&minus;1) Par Une Fonction Entière
|publisher=''OEuvres de Jacques Hadamard'', Centre National de la Recherche Scientifiques, Paris, 1968
|url=http://www.luschny.de/math/factorial/hadamard/HadamardFactorial.pdf|year=1894 |language=French}}
*{{citation|first=Srinivasa|last=Ramanujan|title=The lost notebook and other unpublished papers
|publisher=Springer Berlin |page=339 |year=1988|isbn=3-540-18726-X}}

==External links==
* [http://factorielle.free.fr/index_en.html All about factorial notation n!]
* {{springer|title=Factorial|id=p/f038080}}
* {{MathWorld | urlname=Factorial | title=Factorial}}
* {{PlanetMath | urlname=Factorial | title=Factorial}}
* [http://www.docstoc.com/docs/5606124/Double-Factorials-Selected-Proofs-and-Notes  "Double Factorial Derivations"]
* [http://www.gfredericks.com/main/sandbox/arith/factorial Animated Factorial Calculator]: shows factorials calculated as if by hand using common elementary school algorithms
* [http://www.luschny.de/math/factorial/FastFactorialFunctions.htm Fast Factorial Functions (with source code in Java, C#, C++, Scala and Go)]
* [http://www.gnu.org/software/gsl/manual/html_node/Factorials.html C functions to calculated factorials in the GNU Scientific Library (GSL)]

{{Series (mathematics)}}

[[:Category:Integer sequences]]
[[:Category:Combinatorics]]
[[:Category:Number theory]]
[[:Category:Gamma and related functions]]
[[:Category:Factorial and binomial topics]]