Research Article | Open Access
On Rational Approximations to Euler's Constant and to
The author continues to study series transformations for the Euler-Mascheroni constant . Here, we discuss in detail recently published results of A. I. Aptekarev and T. Rivoal who found rational approximations to and () defined by linear recurrence formulae. The main purpose of this paper is to adapt the concept of linear series transformations with integral coefficients such that rationals are given by explicit formulae which approximate and . It is shown that for every and every integer there are infinitely many rationals for such that and with for tending to infinity.
Let It is well known that the sequence converges to Euler's constant , where Nothing is known on the algebraic background of such mathematical constants like Euler's constant . So we are interested in better diophantine approximations of these numbers, particularly in rational approximations.
In 1995 the author  introduced a linear transformation for the series with integer coefficients which improves the rate of convergence. Let be an additional positive integer parameter.
Proposition 1.1 (see ). For any integers and one has Particularly, by choosing , one gets the following result.
Corollary 1.2. For any integer one has
Proposition (see ). For tending to infinity, one has Kh. Hessami Pilehrood and T. Hessami Pilehrood have found some approximation formulas for the logarithms of some infinite products including Euler's constant . These results are obtained by using Euler-type integrals, hypergeometric series, and the Laplace method .
Proposition (). For tending to infinity the following asymptotic formula holds:
Recently the author has found series transformations involving three parameters , and , . In Propositions 1.5 and 1.6 certain integral representations of the (discrete) series transformations are given, which exhibit important (analytical) tools to estimate the error terms of the transformations.
Proposition 1.5 (see ). Let , and be integers. Additionally one assumes that Then one has
Proposition (see ). Let , and be integers. Additionally one assumes that Then one has with
Setting one gets an explicit upper bound from Proposition 1.6
Corollary 1.7. For integers , , one has where is some constant depending only on . For one gets For an application of Corollary 1.7 let the integers and be defined by denotes the von Mangoldt function. By [5, Theorem 434] one has Then, for , there is some integer such that Multiplying (1.14) by , we deduce the following corollary.
Corollary. There is an integer such that one has for all integers that
2. Results on Rational Approximations to
In 2007, Aptekarev and his collaborators  found rational approximations to , which are based on a linear third-order recurrence. For the sake of brevity, let .
Proposition (see ). Let and be two solutions of the linear recurrence with , , and , , . Then, one has , , and with two positive constants . It seems interesting to replace the fraction by and to estimate the remainder in terms of .
Corollary 2.2. Let . Then there are two positive constants , such that for all sufficiently large integers one has
Recently, Rivoal  presented a related approach to the theory of rational approximations to Euler's constant , and, more generally, to rational approximations for values of derivatives of the Gamma function. He studied simultaneous Padé approximants to Euler's functions, from which he constructed a third-order recurrence formula that can be applied to construct a sequence in that converges subexponentially to for any complex number . Here, is defined by its principal branch. We cite a corollary from .
Proposition 2.3 (see ). (i) The recurrence provides two sequences of rational numbers and with , , and , , such that converges to .(ii) The recurrence provides two sequences of rational numbers and with , , and , , such that converges to .
The goal of this paper is to construct rational approximations to without using recurrences by a new application of series transformations. The transformed sequences of rationals are constructed as simple as possible, only with few concessions to the rate of convergence (see Theorems 2.4 and 6.2 below).
In the following we denote by the Bernoulli numbers, that is, , , , and so on (In Sections 3–6 the Bernoulli numbers cannot be confused with the integers from Corollary 2.2.) In this paper we will prove the following result.
Theorem 2.4. Let , , and be positive integers, and Then, where is some positive constant depending only on .
3. Proof of Theorem 2.4
Lemma 3.1. One has for positive integers and
Proof. Applying the well known inequality , we get This proves the lemma.
takes its maximum value for with which leads to a better bound than in Lemma 3.1. But we are satisfied with Lemma 3.1. A main tool in proving Theorem 2.4 is Euler's summation formula in the form where is a suitable chosen parameter, and the remainder is defined by a periodic Bernoulli polynomial , namely with Applying the summation formula to the function , we get (see [8, equation ( 5)] ) It follows that We prove Theorem 2.4 for . The case is treated similarly. So we have again by the above summation formula that First, we estimate the integral on the right-hand side of (3.8). We have since . Next, we assume that . Hence , and therefore we estimate the integral on the right-hand side in (3.9) by In the sequel we put . Moreover, in the above formula we now replace by with . In order to estimate we use Stirling's formula Then, it follows that and similarly we have By using the definition of in Theorem 2.4, the formula (1.1) for , and the identities (3.8), (3.9), it follows that where is specified to and to . Moreover, we know from [4, Lemma 2] that By setting , the above formula for the series transformation of simplifies to where , and . Here, we have used the results from Corollary 1.7, (3.13), and (3.14). The sum vanishes, since for every real number we have where on the right-hand side for an integer with one term in the numerator equals to zero.
The inequality holds for all integers . Now, using Lemma 3.1, we estimate the right-hand side in (3.17) for and as follows: The last but one estimate holds for all integers , , and is a suitable positive real constant depending on . This completes the proof of Theorem 2.4.
4. On the Denominators of
In this section we will investigate the size of the denominators of our series transformations for tending to infinity, where and are coprime integers.
Theorem 4.1. For every there is an integer with , , and
Proof. We will need some basic facts on the arithmetical functions and . Let
where is restricted on primes. Moreover, let for positive integers . Then,
where (4.5) follows from [5, Theorem 420] and the prime number theorem. By [5, Theorem 118] (von Staudt's theorem) we know how to obtain the prime divisors of the denominators of Bernoulli numbers : The denominators of are squarefree, and they are divisible exactly by those primes with . Hence,
Next, let ( are the subscripts of in Theorem 2.4). First, we consider the following terms from the series transformation in :
For every there is a rational defined by
where , , , and
Similarly, we define rationals by
where , and . We have
Therefore, using the conclusion (4.6) from von Staudt's theorem, we get
Note that , since every integer with divides at least one integer with .
From (4.10) and (4.13) we conclude on Hence we have from (4.4) and (4.5) that The theorem is proved.
Remark 4.2. On the one side we have shown that and . On the other side, every prime dividing satisfies and therefore divides . Conversely, all primes with divide , but not . That means: is much bigger than , but is formed by powers of small primes, whereas is divisible by many big primes.
5. Simplification of the Transformed Series
Let such that In Theorem 2.4 the sequence is transformed. In view of a simplified process we now investigate the transformation of the series . Therefore we have to estimate the contribution of to the series transformation in Theorem 2.4. For this purpose, we define A major step in estimating is to expressthe sums on the right-hand side by integrals.
Lemma 5.1. For positive integers and one has
Proof. For integers and a real number with the identity holds, which we apply with and to substitute the fraction . Introducing the new variable , we then get The sum inside the brackets of the integrand can be expressed by using the equation in which we put and . This gives the identity stated in the lemma.
The following result deals with the case , in which we express the finite sum by a double integral on a rational function.
Corollary. For every positive integer one has
Proof. Set in Lemma 5.1, and note that Hence, Let be any positive integer. Then we have the following decomposition of a rational function, in which is considered as variable and as parameter: We additionally assume that . Then, differentiating this identity -times with respect to , the polynomial in on the right-hand side vanishes identically: Therefore, we get from (5.10) by iterated integrations by parts: The corollary is proved by noting that
In this section we estimate defined in (5.3). Substituting for into the integral in Lemma 5.1 and applying iterated integration by parts, we get Set where and are kept fixed. We have . For an integer we use Cauchy's formula