In mathematics, the Stieltjes constants are the numbers that occur in the Laurent series expansion of the Riemann zeta function:
The zero'th constant is known as the Euler–Mascheroni constant.
The Stieltjes constants are given by the limit
(In the case n = 0, the first summand requires evaluation of 00, which is taken to be 1.)
Cauchy's differentiation formula leads to the integral representation
Various representations in terms of integrals and infinite series are given in works of Jensen, Franel, Hermite, Hardy, Ramanujan, Ainsworth, Howell, Coppo, Connon, Coffey, Choi, Blagouchine and some other authors. In particular, Jensen-Franel's integral formula, often erroneously attributed to Ainsworth and Howell, states that
where δn,k is the Kronecker symbol (Kronecker delta). Among other formulae, we find
As concerns series representations, a famous series implying an integer part of a logarithm was given by Hardy in 1912
Israilov gave semi-convergent series in terms of Bernoulli numbers
Connon, Blagouchine and Coppo gave several series with the binomial coefficients
where Gn are Gregory's coefficients, also known as reciprocal logarithmic numbers (G1=+1/2, G2=−1/12, G3=+1/24, G4=−19/720,... ). Oloa and Tauraso showed that series with harmonic numbers may lead to Stieltjes constants
Blagouchine obtained slowly-convergent series involving unsigned Stirling numbers of the first kind
as well as semi-convergent series with rational terms only
where m=0,1,2,... In particular, series for the first Stieltjes constant has a surprisingly simple form
where Hn is the nth harmonic number. More complicated series for Stieltjes constants are given in works of Lehmer, Liang, Todd, Lavrik, Israilov, Stankus, Keiper, Nan-You, Williams, Coffey.
Bounds and asymptotic growth
The Stieltjes constants satisfy the bound
given by Berndt in 1972. Better bounds in terms of elementary functions were obtained by Lavrik
with k=1,2,... and C(1)=1/2, C(2)=7/12,... , by Nan-You and Williams
where Bn are Bernoulli numbers, and by Matsuoka
As concerns estimations resorting to non-elementary functions and solutions, Knessl, Coffey and Fekih-Ahmed obtained quite accurate results. For example, Knessl and Coffey give the following formula that approximates the Stieltjes constants relatively well for large n. If v is the unique solution of
with , and if , then
Up to n = 100000, the Knessl-Coffey approximation correctly predicts the sign of γn with the single exception of n = 137.
The first few values are:
||approximate value of γn
||−4.2534015717080269623144385197278358247028931053 × 1017
||−1.5709538442047449345494023425120825242380299554 × 10486
||−2.2104970567221060862971082857536501900234397174 × 106883
||+1.9919273063125410956582272431568589205211659777 × 1083432
For large n, the Stieltjes constants grow rapidly in absolute value, and change signs in a complex pattern.
Further information related to the numerical evaluation of Stieltjes constants may be found in works of Keiper, Kreminski, Plouffe and Johansson. The latter author provided values of the Stieltjes constants up to n = 100000, accurate to over 10000 digits each. The numerical values can be retrieved from the LMFDB .
Generalized Stieltjes constants
More generally, one can define Stieltjes constants γn(a) that occur in the Laurent series expansion of the Hurwitz zeta function:
Here a is a complex number with Re(a)>0. Since the Hurwitz zeta function is a generalization of the Riemann zeta function, we have γn(1)=γn The zero'th constant is simply the digamma-function γ0(a)=-Ψ(a), while other constants are not known to be reducible to any elementary or classical function of analysis. Nevertheless, there are numeorous representations for them. For example, there exists the following asymptotic representation
due to Berndt and Wilton. The analog of Jensen-Franel's formula for the generalized Stieltjes constant is the Hermite formula
Generalized Stieltjes constants satisfy the following recurrence relation
as well as the multiplication theorem
where denotes the binomial coefficient (see and, pp. 101–102).
First generalized Stieltjes constant
The first generalized Stieltjes constant has a number of remarkable properties.
- Malmsten's identity (reflection formula for the first generalized Stieltjes constants): the reflection formula for the first generalized Stieltjes constant has the following form
where m and n are positive integers such that m<n. This formula has been long-time attributed to Almkvist and Meurman who derived it in 1990s. However, it was recently reported that this identity, albeit in a slightly different form, was first obtained by Carl Malmsten in 1846.
- Rational arguments theorem: the first generalized Stieltjes constant at rational argument may be evaluated in a quasi closed-form via the following formula
see Blagouchine. An alternative proof was later proposed by Coffey and several other authors.
- Finite summations: there are numerous summation formulae for the first generalized Stieltjes constants. For example,
For more details and further summation formulae, see.
- Some particular values: some particular values of the first generalized Stieltjes constant at rational arguments may be reduced to the gamma-function, the first Stieltjes constant and elementary functions. For instance,
At points 1/4, 3/4 and 1/3, values of first generalized Stieltjes constants were independently obtained by Connon and Blagouchine
At points 2/3, 1/6 and 5/6
These values were calculated by Blagouchine. To the same author are also due
Second generalized Stieltjes constant
The second generalized Stieltjes constant is much less studied than the first constant. Similarly to the first generalized Stieltjes constant, the second generalized Stieltjes constant at rational argument may be evaluated via the following formula
see Blagouchine. An equivalent result was later obtained by Coffey by another method.
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