Irrationality and arithmetic nature of the Euler–Mascheroni constant

Euler's constant $\gamma=\lim_{n\to\infty}(H_n-\log n)\approx 0.5772156649$, where $H_n=1+\tfrac12+\cdots+\tfrac1n$ is the $n$-th harmonic number, is one of the most ubiquitous constants in mathematics, yet nothing is known about its arithmetic nature: it is an open problem whether $\gamma$ is irrational, and even less is known about algebraicity versus transcendence. The problem asks for a determination of this arithmetic nature; the cited sources state that irrationality of $\gamma$ is unknown and record conditional evidence such as Papanikolaou's bound that a rational $\gamma$ would need a denominator of at least 242,080 digits.

Background

Euler's (Euler–Mascheroni) constant is defined by $\gamma=\lim_{n\to\infty}(H_n-\log n)$, where $H_n=\sum_{k=1}^n 1/k$ is the $n$-th harmonic number; the limit exists because $H_n-\log n$ decreases monotonically. The constant appears throughout analysis and number theory: in the Laurent expansion $\Gamma(z)=1/z-\gamma+O(z)$ of the gamma function (the gamma function extends the factorial to complex arguments; a Laurent expansion is a power series allowing finitely many negative powers), in the asymptotics $\sum_{k\le n}d(k)=n\log n+(2\gamma-1)n+O(\sqrt{n})$ for the divisor-summatory function, where $d(k)$ denotes the number of positive divisors of $k$, in Mertens' product formula $\prod_{p\le x}(1-1/p)^{-1}\sim e^{\gamma}\log x$ over the primes $p\le x$, in random-matrix and random-permutation problems, and in the theory of the Riemann zeta function, whose Stieltjes constants — the coefficients beyond $\gamma$ in the Laurent expansion of $\zeta(s)$ at $s=1$ — generalize the constant. A real number is called irrational if it is not a quotient of two integers, algebraic if it is a root of a nonzero polynomial with rational coefficients, and transcendental if it is not algebraic (for example $\pi$ and $e$ are transcendental, hence irrational). In striking contrast to neighboring constants such as $\pi$, $e$, $\log 2$, and $\zeta(3)=\sum_{k\ge1}k^{-3}$ (the last proved irrational by Apéry in 1978 by constructing explicit rational approximations $\zeta(3)\approx p_n/q_n$ with error decreasing faster than $q_n^{-1-\varepsilon}$), the arithmetic nature of $\gamma$ is completely undetermined.

What is known is conditional and computational. Brent and McMillan (1980), using rational approximations to $\gamma$ built from modified Bessel functions together with the continued fraction expansion (the sequence of best rational approximations $[0;1,1,2,1,2,1,4,3,13,\dots]$ of $\gamma$), proved that if $\gamma=m/n$ is rational then its denominator satisfies $n>10^{15000}$; Papanikolaou subsequently sharpened the computational evidence to the effect that a rational $\gamma$ would need a denominator of at least 242,080 decimal digits (Connon, arXiv:0710.4032). Aptekarev, Bogolyubskii, Khristoforov, Lysov, and Tulyakov (2007) constructed the best currently known explicit rational approximations to $\gamma$ — ratios of integer solutions of a third-order recurrence with polynomial coefficients — but, as Lagarias's 2013 Bulletin of the AMS survey documents, their quality (error too large relative to denominator growth) is insufficient to certify irrationality. Sondow (2003) gave equivalent integral criteria that would imply irrationality, but no known construction satisfies them. On the positive side, Rivoal (2012), via Siegel's theory of E-functions (entire functions $F(z)=\sum c_n z^n/n!$ satisfying arithmetic growth and differential-equation conditions, whose values at algebraic points are analyzed by the Shidlovskii method, a technique for proving algebraic independence of such values), proved that at least one of $\gamma$ and the Euler–Gompertz constant $\delta=\int_0^\infty e^{-w}/(1+w)\,dw$ is transcendental, improving Aptekarev's observation that at least one of the two is irrational — a coupling result that leaves each constant individually untouched. Lagarias's survey records the expert expectation that $\gamma$ is not even accessible to Apéry-type methods based on G-functions (power-series analogues of E-functions with positive radius of convergence, encoding rational-approximation schemes), so any proof would require genuinely new Diophantine-approximation technology (Diophantine approximation being the study of how well real numbers can be approximated by rationals). The irrationality of $\gamma$ is a standard item on lists of famous open problems; G. H. Hardy is alleged to have offered his Savilian chair at Oxford to anyone who could prove it.

Problem Statement

Determine the arithmetic nature of Euler's constant $\gamma=\lim_{n\to\infty}(H_n-\log n)\approx 0.5772156649\ldots$, where $H_n=\sum_{k=1}^{n}1/k$ is the $n$-th harmonic number. Specifically: (1) Prove that $\gamma$ is irrational (Conjecture 1.0.1 of Lagarias's 2013 survey, the standard formulation), or prove that it is rational. (2) More generally, determine whether $\gamma$ is algebraic or transcendental. (2) is a further determination of the arithmetic nature, which a proof of irrationality alone does not settle. Even the first step — irrationality — is open, and the sources record only conditional evidence (denominator bounds of Brent–McMillan and Papanikolaou under rationality) and coupling results (at least one of $\gamma$ and the Euler–Gompertz constant is transcendental).

References

1. Donal F. Connon, 'Some series and integrals involving the Riemann zeta function, binomial coefficients and the harmonic numbers. Volume VI', arXiv:0710.4032 (2007). https://arxiv.org/abs/0710.4032
2. Jeffrey C. Lagarias, 'Euler's constant: Euler's work and modern developments', Bulletin of the American Mathematical Society 50 (4) (2013), 527–628, DOI 10.1090/S0273-0979-2013-01423-X, arXiv:1303.1856. https://arxiv.org/abs/1303.1856
3. Richard P. Brent and Edwin M. McMillan, 'Some New Algorithms for High-Precision Computation of Euler's Constant', Mathematics of Computation 34 (149) (1980), 305–312, DOI 10.2307/2006237. https://doi.org/10.2307/2006237
4. Jonathan Sondow, 'Criteria for irrationality of Euler's constant', Proceedings of the American Mathematical Society 131 (11) (2003), 3335–3344, DOI 10.1090/S0002-9939-03-07081-3. https://doi.org/10.1090/S0002-9939-03-07081-3
5. A. I. Aptekarev, 'On linear forms containing the Euler constant', arXiv:0902.1768 (2009). https://arxiv.org/abs/0902.1768
6. Tanguy Rivoal, 'On the arithmetic nature of the values of the gamma function, Euler's constant, and Gompertz's constant', Michigan Mathematical Journal 61 (2) (2012), 239–254, DOI 10.1307/mmj/1339011525. https://doi.org/10.1307/mmj/1339011525
7. 'Euler–Mascheroni Constant', MathWorld — A Wolfram Web Resource (accessed 2026-08-24). https://mathworld.wolfram.com/Euler-MascheroniConstant.html