Particular values of the gamma function#Imaginary and complex arguments
{{Short description|Mathematical constants}}
The gamma function is an important special function in mathematics. Its particular values can be expressed in closed form for integer and half-integer arguments, but no simple expressions are known for the values at rational points in general. Other fractional arguments can be approximated through efficient infinite products, infinite series, and recurrence relations.
Integers and half-integers
For positive integer arguments, the gamma function coincides with the factorial. That is,
:
and hence
:
\Gamma(1) &= 1, \\
\Gamma(2) &= 1, \\
\Gamma(3) &= 2, \\
\Gamma(4) &= 6, \\
\Gamma(5) &= 24,
\end{align}
and so on. For non-positive integers, the gamma function is not defined.
For positive half-integers where is an odd integer greater or equal , the function values are given exactly by
:
or equivalently, for non-negative integer values of {{mvar|n}}:
:
\Gamma\left(\tfrac12+n\right) &= \frac{(2n-1)!!}{2^n}\, \sqrt{\pi} = \frac{(2n)!}{4^n n!} \sqrt{\pi} \\
\Gamma\left(\tfrac12-n\right) &= \frac{(-2)^n}{(2n-1)!!}\, \sqrt{\pi} = \frac{(-4)^n n!}{(2n)!} \sqrt{\pi}
\end{align}
where {{math|n!!}} denotes the double factorial. In particular,
:
| | |{{OEIS2C|A002161}} |
| | |{{OEIS2C|A019704}} |
| | |{{OEIS2C|A245884}} |
| | |{{OEIS2C|A245885}} |
and by means of the reflection formula,
:
| | |{{OEIS2C|A019707}} |
| | |{{OEIS2C|A245886}} |
| | |{{OEIS2C|A245887}} |
General rational argument
In analogy with the half-integer formula,
:
\Gamma \left(n+\tfrac13 \right) &= \Gamma \left(\tfrac13 \right) \frac{(3n-2)!!!}{3^n} \\
\Gamma \left(n+\tfrac14 \right) &= \Gamma \left(\tfrac14 \right ) \frac{(4n-3)!!!!}{4^n} \\
\Gamma \left(n+\tfrac{1}{q} \right ) &= \Gamma \left(\tfrac{1}{q} \right ) \frac{\big(qn-(q-1)\big)!^{(q)}}{q^n} \\
\Gamma \left(n+\tfrac{p}{q} \right) &= \Gamma \left(\tfrac{p}{q}\right) \frac{1}{q^n} \prod _{k=1}^n (k q+p-q)
\end{align}
where {{math|n!(q)}} denotes the {{mvar|q}}th multifactorial of {{mvar|n}}. Numerically,
: {{OEIS2C|A073005}}
: {{OEIS2C|A068466}}
: {{OEIS2C|A175380}}
: {{OEIS2C|A175379}}
: {{OEIS2C|A220086}}
: {{OEIS2C|A203142}}.
As tends to infinity,
:
where is the Euler–Mascheroni constant and denotes asymptotic equivalence.
It is unknown whether these constants are transcendental in general, but {{math|Γ({{sfrac|1|3}})}} and {{math|Γ({{sfrac|1|4}})}} were shown to be transcendental by G. V. Chudnovsky. {{math|Γ({{sfrac|1|4}}) / {{radic|π|4}}}} has also long been known to be transcendental, and Yuri Nesterenko proved in 1996 that {{math|Γ({{sfrac|1|4}})}}, {{math|π}}, and {{math|eπ}} are algebraically independent.
For at least one of the two numbers and is transcendental.{{Cite journal |last=Waldschmidt |first=Michel |date=2006 |title=Transcendence of periods: the state of the art |url=https://hal.science/hal-00411301 |journal=Pure and Applied Mathematics Quarterly |volume=2 |issue=2 |pages=435–463|doi=10.4310/PAMQ.2006.v2.n2.a3 }}
The number is related to the lemniscate constant {{mvar|}} by
:
Borwein and Zucker have found that {{math|Γ({{sfrac|n|24}})}} can be expressed algebraically in terms of {{mvar|π}}, {{math|K(k(1))}}, {{math|K(k(2))}}, {{math|K(k(3))}}, and {{math|K(k(6))}} where {{math|K(k(N))}} is a complete elliptic integral of the first kind. This permits efficiently approximating the gamma function of rational arguments to high precision using quadratically convergent arithmetic–geometric mean iterations. For example:
:
\Gamma \left(\tfrac16 \right) &= \frac{\sqrt{\frac{3}{\pi }} \Gamma\left(\frac{1}{3}\right)^2}{\sqrt[3]{2}} \\
\Gamma \left(\tfrac14 \right) &= 2\sqrt{K\left( \tfrac 12 \right)\sqrt{\pi}} \\
\Gamma \left(\tfrac13 \right) &= \frac{2^{7/9} \sqrt[3]{\pi K\left(\frac{1}{4}\left(2-\sqrt{3}\right)\right)}}{\sqrt[12]{3}} \\
\Gamma \left(\tfrac{1}{8}\right) \Gamma \left(\tfrac{3}{8}\right) &= 8 \sqrt[4]{2} \sqrt{\left(\sqrt{2}-1\right) \pi } K\left(3-2 \sqrt{2}\right) \\
\frac{\Gamma \left(\frac{1}{8}\right)}{\Gamma \left(\frac{3}{8}\right)} &= \frac{2 \sqrt{\left(1+\sqrt{2}\right) K\left(\frac{1}{2}\right)}}{\sqrt[4]{\pi }}
\end{align}
No similar relations are known for {{math|Γ({{sfrac|1|5}})}} or other denominators.
In particular, where AGM() is the arithmetic–geometric mean, we have{{cite web|url=https://math.stackexchange.com/q/1631760 |title=Archived copy |accessdate=2015-03-09 }}
:
:
:
Other formulas include the infinite products
:
and
:
where {{mvar|A}} is the Glaisher–Kinkelin constant and {{mvar|G}} is Catalan's constant.
The following two representations for {{math|Γ({{sfrac|3|4}})}} were given by I. Mező{{Citation
| last = Mező
| first = István
| title = Duplication formulae involving Jacobi theta functions and Gosper's q-trigonometric functions
| journal = Proceedings of the American Mathematical Society
| pages = 2401–2410
| volume = 141
| issue = 7
| year = 2013
| doi=10.1090/s0002-9939-2013-11576-5
| doi-access = free
}}
:
and
:
where {{math|θ1}} and {{math|θ4}} are two of the Jacobi theta functions.
There also exist a number of Malmsten integrals for certain values of the gamma function:{{Cite journal |last=Blagouchine |first=Iaroslav V. |date=2014-10-01 |title=Rediscovery of Malmsten's integrals, their evaluation by contour integration methods and some related results |url=https://link.springer.com/article/10.1007/s11139-013-9528-5 |journal=The Ramanujan Journal |language=en |volume=35 |issue=1 |pages=21–110 |doi=10.1007/s11139-013-9528-5 |issn=1572-9303}}
:
:
Products
Some product identities include:
: {{OEIS2C|A186706}}
: {{OEIS2C|A220610}}
:
:
:
:
In general:
:
From those products can be deduced other values, for example, from the former equations for , and , can be deduced:
Other rational relations include
:
:{{MathWorld|GammaFunction|Gamma Function}}
:
and many more relations for {{math|Γ({{sfrac|n|d}})}} where the denominator d divides 24 or 60.[https://arxiv.org/abs/math/0403510 Raimundas Vidūnas, Expressions for Values of the Gamma Function]
Gamma quotients with algebraic values must be "poised" in the sense that the sum of arguments is the same (modulo 1) for the denominator and the numerator.
A more sophisticated example:
:[https://math.stackexchange.com/q/2804457 math.stackexchange.com]
Imaginary and complex arguments
The gamma function at the imaginary unit {{math|i {{=}} {{sqrt|−1}}}} gives {{OEIS2C|A212877}}, {{OEIS2C|A212878}}:
:
It may also be given in terms of the Barnes G-function:
:
Curiously enough, appears in the below integral evaluation:[https://sites.google.com/site/istvanmezo81/monthly-problems The webpage of István Mező]
:
Here denotes the fractional part.
Because of the Euler Reflection Formula, and the fact that , we have an expression for the modulus squared of the Gamma function evaluated on the imaginary axis:
:
The above integral therefore relates to the phase of .
The gamma function with other complex arguments returns
:
:
:
:
:
:
Other constants
The gamma function has a local minimum on the positive real axis
: {{OEIS2C|A030169}}
with the value
: {{OEIS2C|A030171}}.
Integrating the reciprocal gamma function along the positive real axis also gives the Fransén–Robinson constant.
On the negative real axis, the first local maxima and minima (zeros of the digamma function) are:
class="wikitable"
|+ Approximate local extrema of {{math|Γ(x)}} ! {{mvar|x}}!!{{math|Γ(x)}}!!OEIS | ||
{{val|−0.5040830082644554092582693045}} | {{val|−3.5446436111550050891219639933}} | {{OEIS2C|A175472}} |
{{val|−1.5734984731623904587782860437}} | {{0|−}}{{val|2.3024072583396801358235820396}} | {{OEIS2C|A175473}} |
{{val|−2.6107208684441446500015377157}} | {{val|−0.8881363584012419200955280294}} | {{OEIS2C|A175474}} |
{{val|−3.6352933664369010978391815669}} | {{0|−}}{{val|0.2451275398343662504382300889}} | {{OEIS2C|A256681}} |
{{val|−4.6532377617431424417145981511}} | {{val|−0.0527796395873194007604835708}} | {{OEIS2C|A256682}} |
{{val|−5.6671624415568855358494741745}} | {{0|−}}{{val|0.0093245944826148505217119238}} | {{OEIS2C|A256683}} |
{{val|−6.6784182130734267428298558886}} | {{val|−0.0013973966089497673013074887}} | {{OEIS2C|A256684}} |
{{val|−7.6877883250316260374400988918}} | {{0|−}}{{val|0.0001818784449094041881014174}} | {{OEIS2C|A256685}} |
{{val|−8.6957641638164012664887761608}} | {{val|−0.0000209252904465266687536973}} | {{OEIS2C|A256686}} |
{{val|−9.7026725400018637360844267649}} | {{0|−}}{{val|0.0000021574161045228505405031}} | {{OEIS2C|A256687}} |
The only values of {{math|x > 0}} for which {{math|1=Γ(x) = x}} are {{math|1=x = 1}} and {{math|x ≈ {{val|3.5623822853908976914156443427}}}}... {{OEIS2C|A218802}}.
See also
References
Further reading
- {{Cite journal
|first1=F.
|last1=Gramain
|title=Sur le théorème de Fukagawa-Gel'fond
|journal=Invent. Math.
|volume=63
|number=3
|doi=10.1007/BF01389066
|year=1981
|pages=495–506
|bibcode=1981InMat..63..495G
|s2cid=123079859
}}
- {{Cite journal
|first1=J. M.
|last1=Borwein
|first2=I. J.
|last2=Zucker
|title=Fast Evaluation of the Gamma Function for Small Rational Fractions Using Complete Elliptic Integrals of the First Kind
|journal=IMA Journal of Numerical Analysis
|volume=12
|issue=4
|pages=519–526
|year=1992
|mr=1186733
|doi=10.1093/imanum/12.4.519
}}
- X. Gourdon & P. Sebah. [http://numbers.computation.free.fr/Constants/Miscellaneous/gammaFunction.html Introduction to the Gamma Function]
- {{MathWorld|title=Gamma Function|urlname=GammaFunction}}
- {{cite journal
|first1=Raimundas |last1=Vidunas
|title=Expressions for values of the gamma function
|arxiv=math.CA/0403510
|doi=10.2206/kyushujm.59.267
|volume=59
|issue=2
|journal=Kyushu Journal of Mathematics
|pages=267–283|year=2005
|s2cid=119623635
}}
- {{cite journal
|first1=Raimundas | last1=Vidunas
|title=Expressions for values of the gamma function
|journal=Kyushu J. Math. |year=2005
|volume=59 | number=2 | pages=267–283 | mr=2188592 | doi=10.2206/kyushujm.59.267|arxiv=math/0403510| s2cid=119623635
}}
- {{Cite journal
|first1=V. S.
|last1=Adamchik
|url=https://www.cs.cmu.edu/~adamchik/articles/rama.pdf
|title=Multiple Gamma Function and Its Application to Computation of Series
|journal=The Ramanujan Journal
|volume=9
|number=3
|doi=10.1007/s11139-005-1868-3
|year=2005
|pages=271–288
|mr=2173489
|arxiv=math/0308074
|s2cid=15670340
}}
- {{Cite journal
|first1=W.
|last1=Duke
|first2=Ö.
|last2=Imamoglu
|url=https://www.math.ucla.edu/~wdduke/preprints/special-jntb.pdf
|title=Special values of multiple gamma functions
|journal=Journal de Théorie des Nombres de Bordeaux
|volume=18
|issue=1
|year=2006
|mr=2245878
|pages=113–123
|doi=10.5802/jtnb.536
}}