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
and so on. For non-positive integers, the gamma function is not defined.
For positive half-integers, the function values are given exactly by
or equivalently, for non-negative integer values of n:
where n!! denotes the double factorial. In particular,
and by means of the reflection formula,
General rational argument
In analogy with the half-integer formula,
where n!(q) denotes the qth multifactorial of n. Numerically,
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 Γ(1/3) and Γ(1/4) were shown to be transcendental by G. V. Chudnovsky. Γ(1/4) / 4√π has also long been known to be transcendental, and Yuri Nesterenko proved in 1996 that Γ(1/4), π, and eπ are algebraically independent.
The number Γ(1/4) is related to the lemniscate constant ϖ by
and it has been conjectured by Gramain that
where δ is the Masser–Gramain constant OEIS: A086058, although numerical work by Melquiond et al. indicates that this conjecture is false.[1]
Borwein and Zucker have found that Γ(n/24) can be expressed algebraically in terms of π, K(k(1)), K(k(2)), K(k(3)), and K(k(6)) where 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:
No similar relations are known for Γ(1/5) or other denominators.
In particular, where AGM() is the arithmetic–geometric mean, we have[2]
Other formulas include the infinite products
and
where A is the Glaisher–Kinkelin constant and G is Catalan's constant.
The following two representations for Γ(3/4) were given by I. Mező[3]
and
where θ1 and θ4 are two of the Jacobi theta functions.
Certain values of the gamma function can also be written in terms of the hypergeometric function. For instance,
and
however it is an open question whether this is possible for all rational inputs to the gamma function. [4]
Products
Some product identities include:
In general:
From those products can be deduced other values, for example, from the former equations for , and , can be deduced:[5]
Other rational relations include
and many more relations for Γ(n/d) where the denominator d divides 24 or 60.[7]
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:
Imaginary and complex arguments
The gamma function at the imaginary unit i = √−1 gives OEIS: A212877, OEIS: A212878:
It may also be given in terms of the Barnes G-function:
Curiously enough, appears in the below integral evaluation:[9]
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
with the value
For a series for the minimum ( https://math.stackexchange.com/questions/4832923/is-there-a-clear-pattern-for-this-coefficients-in-gamma-function-expansion )
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:
x | Γ(x) | OEIS |
---|---|---|
−0.5040830082644554092582693045 | −3.5446436111550050891219639933 | OEIS: A175472 |
−1.5734984731623904587782860437 | 2.3024072583396801358235820396 | OEIS: A175473 |
−2.6107208684441446500015377157 | −0.8881363584012419200955280294 | OEIS: A175474 |
−3.6352933664369010978391815669 | 0.2451275398343662504382300889 | OEIS: A256681 |
−4.6532377617431424417145981511 | −0.0527796395873194007604835708 | OEIS: A256682 |
−5.6671624415568855358494741745 | 0.0093245944826148505217119238 | OEIS: A256683 |
−6.6784182130734267428298558886 | −0.0013973966089497673013074887 | OEIS: A256684 |
−7.6877883250316260374400988918 | 0.0001818784449094041881014174 | OEIS: A256685 |
−8.6957641638164012664887761608 | −0.0000209252904465266687536973 | OEIS: A256686 |
−9.7026725400018637360844267649 | 0.0000021574161045228505405031 | OEIS: A256687 |
See also
References
- ↑ Melquiond, Guillaume; Nowak, W. Georg; Zimmermann, Paul (2013). "Numerical approximation of the Masser–Gramain constant to four decimal places". Math. Comp. 82 (282): 1235–1246. doi:10.1090/S0025-5718-2012-02635-4.
- ↑ "Archived copy". Retrieved 2015-03-09.
- ↑ Mező, István (2013), "Duplication formulae involving Jacobi theta functions and Gosper's q-trigonometric functions", Proceedings of the American Mathematical Society, 141 (7): 2401–2410, doi:10.1090/s0002-9939-2013-11576-5
- ↑ Johansson, F. (2023). Arbitrary-precision computation of the gamma function. Maple Transactions, 3(1). https://doi.org/10.5206/mt.v3i1.14591
- ↑ Pascal Sebah, Xavier Gourdon. "Introduction to the Gamma Function" (PDF).
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(help) - ↑ Weisstein, Eric W. "Gamma Function". MathWorld.
- ↑ Raimundas Vidūnas, Expressions for Values of the Gamma Function
- ↑ math.stackexchange.com
- ↑ The webpage of István Mező
- Gramain, F. (1981). "Sur le théorème de Fukagawa-Gel'fond". Invent. Math. 63 (3): 495–506. Bibcode:1981InMat..63..495G. doi:10.1007/BF01389066. S2CID 123079859.
- Borwein, J. M.; Zucker, I. J. (1992). "Fast Evaluation of the Gamma Function for Small Rational Fractions Using Complete Elliptic Integrals of the First Kind". IMA Journal of Numerical Analysis. 12 (4): 519–526. doi:10.1093/imanum/12.4.519. MR 1186733.
- X. Gourdon & P. Sebah. Introduction to the Gamma Function
- S. Finch. Euler Gamma Function Constants
- Weisstein, Eric W. "Gamma Function". MathWorld.
- Vidunas, Raimundas (2005). "Expressions for values of the gamma function". Kyushu Journal of Mathematics. 59 (2): 267–283. arXiv:math.CA/0403510. doi:10.2206/kyushujm.59.267. S2CID 119623635.
- Vidunas, Raimundas (2005). "Expressions for values of the gamma function". Kyushu J. Math. 59 (2): 267–283. arXiv:math/0403510. doi:10.2206/kyushujm.59.267. MR 2188592. S2CID 119623635.
- Adamchik, V. S. (2005). "Multiple Gamma Function and Its Application to Computation of Series" (PDF). The Ramanujan Journal. 9 (3): 271–288. arXiv:math/0308074. doi:10.1007/s11139-005-1868-3. MR 2173489. S2CID 15670340.
- Duke, W.; Imamoglu, Ö. (2006). "Special values of multiple gamma functions" (PDF). Journal de Théorie des Nombres de Bordeaux. 18 (1): 113–123. doi:10.5802/jtnb.536. MR 2245878.