Here, represents the floor function. The numerical value of this constant, to 50 decimal places, is
- 0.57721566490153286060651209008240243104215933593992… A001620.
|Continued fraction||[0; 1, 1, 2, 1, 2, 1, 4, 3, 13, 5, 1, 1, 8, 1, 2, 4, 1, 1, … ](This continued fraction is not known to be finite or periodic. Shown in linear notation)|
The constant first appeared in a 1734 paper by the Swiss mathematician Leonhard Euler, titled De Progressionibus harmonicis observationes (Eneström Index 43). Euler used the notations C and O for the constant. In 1790, Italian mathematician Lorenzo Mascheroni used the notations A and a for the constant. The notation appears nowhere in the writings of either Euler or Mascheroni, and was chosen at a later time because of the constant's connection to the gamma function. For example, the German mathematician Carl Anton Bretschneider used the notation in 1835 and Augustus De Morgan used it in a textbook published in parts from 1836 to 1842.
The Euler–Mascheroni constant appears, among other places, in the following ('*' means that this entry[clarification needed] contains an explicit equation):
- Expressions involving the exponential integral*
- The Laplace transform* of the natural logarithm
- The first term of the Taylor series expansion for the Riemann zeta function*, where it is the first of the Stieltjes constants*
- Calculations of the digamma function
- A product formula for the gamma function
- An inequality for Euler's totient function
- The growth rate of the divisor function
- The calculation of the Meissel–Mertens constant
- The third of Mertens' theorems*
- Solution of the second kind to Bessel's equation
- In Dimensional regularization of Feynman diagrams in Quantum Field Theory
- The mean of the Gumbel distribution
- The information entropy of the Weibull and Lévy distributions, and, implicitly, of the chi-squared distribution for one or two degrees of freedom.
- The answer to the coupon collector's problem*
- A definition of the cosine integral*
For more information of this nature, see Gourdon and Sebah (2004).
The number has not been proved algebraic or transcendental. In fact, it is not even known whether is irrational. Continued fraction analysis reveals that if is rational, its denominator must be greater than 10242080. The ubiquity of revealed by the large number of equations below makes the irrationality of a major open question in mathematics. Also see Sondow (2003a).
For more equations of the sort shown below, see Gourdon and Sebah (2002).
Relation to gamma function 
This is equal to the limits:
Further limit results are (Krämer, 2005):
Relation to the zeta function 
Other series related to the zeta function include:
The error term in the last equation is a rapidly decreasing function of n. As a result, the formula is well-suited for efficient computation of the constant to high precision.
Other interesting limits equaling the Euler–Mascheroni constant are the antisymmetric limit (Sondow, 1998)
Closely related to this is the rational zeta series expression. By peeling off the first few terms of the series above, one obtains an estimate for the classical series limit:
- , where
equals the value of a number of definite integrals:
where is the fractional Harmonic number.
Definite integrals in which appears include:
An interesting comparison by J. Sondow (2005) is the double integral and alternating series
It shows that may be thought of as an "alternating Euler constant".
The two constants are also related by the pair of series (see Sondow 2005 #2)
where N1(n) and N0(n) are the number of 1's and 0's, respectively, in the base 2 expansion of n.
We have also Catalan's 1875 integral (see Sondow and Zudilin)
Series expansions 
Euler showed that the following infinite series approaches :
The series for is equivalent to series Nielsen found in 1897:
In 1910, Vacca found the closely related series:
In 1926 he found a second series:
From the Kummer-expansion of the gamma function we get:
Series of prime numbers:
Asymptotic expansions 
equals the following asymptotic formulas (where is the nth harmonic number.)
The third formula is also called the Ramanujan expansion.
Relations with the reciprocal logarithm 
The reciprocal logarithm function (Krämer, 2005)
has a deep connection with Euler's constant and was studied by James Gregory in connection with numerical integration. The coefficients are called Gregory coefficients; the first six were given in a letter to John Collins in 1670. From the equations
, which can be used recursively to get these coefficients for all , we get the table
|Cn|| A002206 (numerators),
Gregory coefficients are similar to Bernoulli numbers and satisfy the asymptotic relation
and the integral representation
Euler's constant has the integral representations
A very important expansion of Gregorio Fontana (1780) is:
which is convergent for all n.
Weighted sums of the Gregory coefficients give different constants:
This restates the third of Mertens' theorems. The numerical value of eγ is:
- 1.78107241799019798523650410310717954916964521430343 … A073004.
Other infinite products relating to eγ include:
These products result from the Barnes G-function.
We also have
where the nth factor is the (n+1)st root of
This infinite product, first discovered by Ser in 1926, was rediscovered by Sondow (2003) using hypergeometric functions.
Continued fraction 
The continued fraction expansion of is of the form [0; 1, 1, 2, 1, 2, 1, 4, 3, 13, 5, 1, 1, 8, 1, 2, 4, 1, 1, 40, ...] A002852, of which there is no apparent pattern. The continued fraction has at least 470,000 terms, and it has infinitely many terms if and only if is irrational.
Euler's generalized constants are given by
for 0 < α < 1, with as the special case α = 1. This can be further generalized to
for some arbitrary decreasing function f. For example,
gives rise to the Stieltjes constants, and
where again the limit
A two-dimensional limit generalization is the Masser–Gramain constant.
Published digits 
Euler initially calculated the constant's value to 6 decimal places. In 1781, he calculated it to 16 decimal places. Mascheroni attempted to calculate the constant to 32 decimal places, but made errors in the 20th–22nd decimal places; starting from the 20th digit, he calculated ...1811209008239 when the correct value is ...0651209008240.
|1809||22||Johann G. von Soldner|
|1811||22||Carl Friedrich Gauss|
|1812||40||Friedrich Bernhard Gottfried Nicolai|
|1857||34||Christian Fredrik Lindman|
|1871||99||James W.L. Glaisher|
|1877||262||J. C. Adams|
|1952||328||John William Wrench, Jr.|
|1961||1050||Helmut Fischer and Karl Zeller|
|1962||3,566||Dura W. Sweeney|
|1973||4,879||William A. Beyer and Michael S. Waterman|
|1977||20,700||Richard P. Brent|
|1980||30,100||Richard P. Brent & Edwin M. McMillan|
|2009||29,844,489,545||Alexander J. Yee & Raymond Chan|
- Carl Anton Bretschneider: Theoriae logarithmi integralis lineamenta nova (13 October 1835), Journal für die reine und angewandte Mathematik 17, 1837, pp. 257–285 (in Latin; “γ = c = 0,577215 664901 532860 618112 090082 3..” on p. 260)
- Augustus De Morgan: The differential and integral calculus, Baldwin and Craddock, London 1836–1842 (“γ” on p. 578)
- Havil 2003 p 97.
- http://mathworld.wolfram.com/MertensConstant.html (15)
- Havil, 117-118
- Nagisa – Large Computations
- Borwein, Jonathan M., David M. Bradley, Richard E. Crandall (2000). "Computational Strategies for the Riemann Zeta Function". Journal of Computational and Applied Mathematics 121: 11. Derives γ as sums over Riemann zeta functions.
- Gourdon, Xavier, and Sebah, P. (2002) "Collection of formulas for Euler's constant, γ."
- ----- (2004) "The Euler constant: γ."
- Donald Knuth (1997) The Art of Computer Programming, Vol. 1, 3rd ed. Addison-Wesley. ISBN 0-201-89683-4
- Krämer, Stefan (2005) Die Eulersche Konstante γ und verwandte Zahlen. Diplomarbeit, Universität Göttingen.
- Sondow, Jonathan (1998) "An antisymmetric formula for Euler's constant," Mathematics Magazine 71: 219-220.
- ------ (2002) "A hypergeometric approach, via linear forms involving logarithms, to irrationality criteria for Euler's constant." With an Appendix by Sergey Zlobin, Mathematica Slovaca 59: 307-314.
- ------ (2003) "An infinite product for eγ via hypergeometric formulas for Euler's constant, γ."
- ------ (2003a) "Criteria for irrationality of Euler's constant," Proceedings of the American Mathematical Society 131: 3335-3344.
- ------ (2005) "Double integrals for Euler's constant and ln 4/π and an analog of Hadjicostas's formula," American Mathematical Monthly 112: 61-65.
- ------ (2005) "New Vacca-type rational series for Euler's constant and its 'alternating' analog ln 4/π."
- ------ and Wadim Zudilin (2006), "Euler's constant, q-logarithms, and formulas of Ramanujan and Gosper," Ramanujan Journal 12: 225-244.
- G. Vacca (1926), "Nuova serie per la costante di Eulero, C = 0,577…". Rendiconti, Accademia Nazionale dei Lincei, Roma, Classe di Scienze Fisiche, Matematiche e Naturali (6) 3, 19–20.
- James Whitbread Lee Glaisher (1872), "On the history of Euler's constant". Messenger of Mathematics. New Series, vol.1, p. 25-30, JFM 03.0130.01
- Carl Anton Bretschneider (1837). "Theoriae logarithmi integralis lineamenta nova". Crelle Journal, vol.17, p. 257-285 (submitted 1835)
- Lorenzo Mascheroni (1790). "Adnotationes ad calculum integralem Euleri, in quibus nonnulla problemata ab Eulero proposita resolvuntur". Galeati, Ticini.
- Lorenzo Mascheroni (1792). "Adnotationes ad calculum integralem Euleri. In quibus nonnullae formulae ab Eulero propositae evolvuntur". Galeati, Ticini. Both online at: http://books.google.de/books?id=XkgDAAAAQAAJ
- Havil, Julian (2003). Gamma: Exploring Euler's Constant. Princeton University Press. ISBN 0-691-09983-9.
- E.A. Karatsuba, Fast evaluation of transcendental functions. Probl. Inf. Transm. Vol.27, No.4, pp. 339–360 (1991
- E.A. Karatsuba, On the computation of the Euler constant γ, J. of Numerical Algorithms Vol.24, No.1-2, pp. 83–97 (2000)