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In mathematics, **Stirling's approximation** (or **Stirling's formula**) is an approximation for factorials. It is a good approximation, leading to accurate results even for small values of . It is named after James Stirling, though a related but less precise result was first stated by Abraham de Moivre.^{[1]}^{[2]}^{[3]}

One way of stating the approximation involves the logarithm of the factorial:

where the big O notation means that, for all sufficiently large values of , the difference between and will be at most proportional to the logarithm. In computer science applications such as the worst-case lower bound for comparison sorting, it is convenient to use instead the binary logarithm, giving the equivalent form

The error term in either base can be expressed more precisely as , corresponding to an approximate formula for the factorial itself,

Here the sign means that the two quantities are asymptotic, that is, that their ratio tends to 1 as tends to infinity. The following version of the bound holds for all , rather than only asymptotically:

Roughly speaking, the simplest version of Stirling's formula can be quickly obtained by approximating the sum

The full formula, together with precise estimates of its error, can be derived as follows. Instead of approximating , one considers its natural logarithm, as this is a slowly varying function:

The right-hand side of this equation minus

and the error in this approximation is given by the Euler–Maclaurin formula:

where is a Bernoulli number, and *R*_{m,n} is the remainder term in the Euler–Maclaurin formula. Take limits to find that

Denote this limit as . Because the remainder *R*_{m,n} in the Euler–Maclaurin formula satisfies

where big-O notation is used, combining the equations above yields the approximation formula in its logarithmic form:

Taking the exponential of both sides and choosing any positive integer , one obtains a formula involving an unknown quantity . For *m* = 1, the formula is

The quantity can be found by taking the limit on both sides as tends to infinity and using Wallis' product, which shows that . Therefore, one obtains Stirling's formula:

An alternative formula for using the gamma function is

A complex-analysis version of this method^{[4]} is to consider as a Taylor coefficient of the exponential function , computed by Cauchy's integral formula as

This line integral can then be approximated using the saddle-point method with an appropriate choice of contour radius . The dominant portion of the integral near the saddle point is then approximated by a real integral and Laplace's method, while the remaining portion of the integral can be bounded above to give an error term.

Stirling's formula is in fact the first approximation to the following series (now called the **Stirling series**):^{[5]}

An explicit formula for the coefficients in this series was given by G. Nemes.^{[6]} Further terms are listed in the On-Line Encyclopedia of Integer Sequences as A001163 and A001164. The first graph in this section shows the relative error vs. , for 1 through all 5 terms listed above.

As *n* → ∞, the error in the truncated series is asymptotically equal to the first omitted term. This is an example of an asymptotic expansion. It is not a convergent series; for any *particular* value of there are only so many terms of the series that improve accuracy, after which accuracy worsens. This is shown in the next graph, which shows the relative error versus the number of terms in the series, for larger numbers of terms. More precisely, let *S*(*n*, *t*) be the Stirling series to terms evaluated at . The graphs show

Writing Stirling's series in the form

More precise bounds, due to Robbins,^{[7]} valid for all positive integers are

For all positive integers,

However, the gamma function, unlike the factorial, is more broadly defined for all complex numbers other than non-positive integers; nevertheless, Stirling's formula may still be applied. If Re(*z*) > 0, then

Repeated integration by parts gives

where is the th Bernoulli number (note that the limit of the sum as is not convergent, so this formula is just an asymptotic expansion). The formula is valid for large enough in absolute value, when |arg(*z*)| < π − *ε*, where ε is positive, with an error term of *O*(*z*^{−2N+ 1}). The corresponding approximation may now be written:

where the expansion is identical to that of Stirling's series above for , except that is replaced with *z* − 1.^{[8]}

A further application of this asymptotic expansion is for complex argument z with constant Re(*z*). See for example the Stirling formula applied in Im(*z*) = *t* of the Riemann–Siegel theta function on the straight line 1/4 + *it*.

For any positive integer , the following notation is introduced:

Then^{[9]}^{[10]}

For further information and other error bounds, see the cited papers.

Thomas Bayes showed, in a letter to John Canton published by the Royal Society in 1763, that Stirling's formula did not give a convergent series.^{[11]} Obtaining a convergent version of Stirling's formula entails evaluating Binet's formula:

One way to do this is by means of a convergent series of inverted rising exponentials. If

The approximation

Gergő Nemes proposed in 2007 an approximation which gives the same number of exact digits as the Windschitl approximation but is much simpler:^{[13]}

An alternative approximation for the gamma function stated by Srinivasa Ramanujan (Ramanujan 1988^{[clarification needed]}) is

The approximation may be made precise by giving paired upper and lower bounds; one such inequality is^{[14]}^{[15]}^{[16]}^{[17]}

The formula was first discovered by Abraham de Moivre^{[2]} in the form

De Moivre gave an approximate rational-number expression for the natural logarithm of the constant. Stirling's contribution consisted of showing that the constant is precisely .^{[3]}

**^**Dutka, Jacques (1991), "The early history of the factorial function",*Archive for History of Exact Sciences*,**43**(3): 225–249, doi:10.1007/BF00389433- ^
^{a}^{b}Le Cam, L. (1986), "The central limit theorem around 1935",*Statistical Science*,**1**(1): 78–96, doi:10.1214/ss/1177013818, JSTOR 2245503, MR 0833276; see p. 81, "The result, obtained using a formula originally proved by de Moivre but now called Stirling's formula, occurs in his 'Doctrine of Chances' of 1733." - ^
^{a}^{b}Pearson, Karl (1924), "Historical note on the origin of the normal curve of errors",*Biometrika*,**16**(3/4): 402–404 [p. 403], doi:10.2307/2331714, JSTOR 2331714,I consider that the fact that Stirling showed that De Moivre's arithmetical constant was does not entitle him to claim the theorem, [...]

**^**Flajolet, Philippe; Sedgewick, Robert (2009),*Analytic Combinatorics*, Cambridge, UK: Cambridge University Press, p. 555, doi:10.1017/CBO9780511801655, ISBN 978-0-521-89806-5, MR 2483235**^**Olver, F. W. J.; Olde Daalhuis, A. B.; Lozier, D. W.; Schneider, B. I.; Boisvert, R. F.; Clark, C. W.; Miller, B. R. & Saunders, B. V., "5.11 Gamma function properties: Asymptotic Expansions",*NIST Digital Library of Mathematical Functions*, Release 1.0.13 of 2016-09-16**^**Nemes, Gergő (2010), "On the coefficients of the asymptotic expansion of ",*Journal of Integer Sequences*,**13**(6): 5**^**Robbins, Herbert (1955), "A Remark on Stirling's Formula",*The American Mathematical Monthly*,**62**(1): 26–29, doi:10.2307/2308012, JSTOR 2308012**^**Spiegel, M. R. (1999),*Mathematical handbook of formulas and tables*, McGraw-Hill, p. 148**^**Schäfke, F. W.; Sattler, A. (1990), "Restgliedabschätzungen für die Stirlingsche Reihe",*Note di Matematica*,**10**(suppl. 2): 453–470, MR 1221957**^**G. Nemes, Error bounds and exponential improvements for the asymptotic expansions of the gamma function and its reciprocal,*Proc. Roy. Soc. Edinburgh Sect. A***145**(2015), 571–596.**^***A letter from the late Reverend Mr. Thomas Bayes, F. R. S. to John Canton, M. A. and F. R. S.*(PDF), 24 November 1763, archived (PDF) from the original on 2012-01-28, retrieved 2012-03-01**^**Toth, V. T.*Programmable Calculators: Calculators and the Gamma Function*(2006) Archived 2005-12-31 at the Wayback Machine.**^**Nemes, Gergő (2010), "New asymptotic expansion for the Gamma function",*Archiv der Mathematik*,**95**(2): 161–169, doi:10.1007/s00013-010-0146-9**^**Karatsuba, Ekatherina A. (2001), "On the asymptotic representation of the Euler gamma function by Ramanujan",*Journal of Computational and Applied Mathematics*,**135**(2): 225–240, doi:10.1016/S0377-0427(00)00586-0, MR 1850542**^**Mortici, Cristinel (2011), "Ramanujan's estimate for the gamma function via monotonicity arguments",*Ramanujan J.*,**25**: 149–154**^**Mortici, Cristinel (2011), "Improved asymptotic formulas for the gamma function",*Comput. Math. Appl.*,**61**: 3364–3369.**^**Mortici, Cristinel (2011), "On Ramanujan's large argument formula for the gamma function",*Ramanujan J.*,**26**: 185–192.

- Abramowitz, M. & Stegun, I. (2002),
*Handbook of Mathematical Functions*[DEAD LINK] - Paris, R. B. & Kaminski, D. (2001),
*Asymptotics and Mellin–Barnes Integrals*, New York: Cambridge University Press, ISBN 978-0-521-79001-7 - Whittaker, E. T. & Watson, G. N. (1996),
*A Course in Modern Analysis*(4th ed.), New York: Cambridge University Press, ISBN 978-0-521-58807-2 - Romik, Dan (2000), "Stirling's approximation for : the ultimate short proof?",
*The American Mathematical Monthly*,**107**(6): 556–557, doi:10.2307/2589351, MR 1767064 - Li, Yuan-Chuan (July 2006), "A note on an identity of the gamma function and Stirling's formula",
*Real Analysis Exchange*,**32**(1): 267–271, MR 2329236

Wikimedia Commons has media related to Stirling's approximation.

- "Stirling_formula",
*Encyclopedia of Mathematics*, EMS Press, 2001 [1994] - Peter Luschny,
*Approximation formulas for the factorial function n!* - Weisstein, Eric W., "Stirling's Approximation",
*MathWorld* - Stirling's approximation at PlanetMath.