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1 Introduction A Method to Construct Continued-Fraction Approximations and Its Applications Chao-Ping Chen1;∗, H. M. Srivastava2;3 and Qin Wang4 1School of Mathematics and Informatics, Henan Polytechnic University, Jiaozuo 454000, Henan, People’s Republic of China E-Mail: [email protected] ∗Corresponding Author 2Department of Mathematics and Statistics, University of Victoria, Victoria, British Columbia V8W 3R4, Canada 3Department of Medical Research, China Medical University Hospital, China Medical University, Taichung 40402, Taiwan, Republic of China E-Mail: [email protected] 4School of Mathematics and Informatics, Henan Polytechnic University, Jiaozuo 454000, Henan, People’s Republic of China E-Mail: [email protected] Abstract In this paper, we provide a method to construct a continued-fraction approx- imation based upon a given asymptotic expansion. As applications of the method developed here, we establish several continued-fraction approximations for the gam- ma and the digamma (or psi) functions. Finally, some closely-related open problems are also presented. 2010 Mathematics Subject Classification. Primary 11A55, 33B15; Secondary 11Y60. Key Words and Phrases. Gamma function; Digamma (or psi) function; Euler-Mascheroni constant; Asymptotic expansions; Continued-fraction approximations. 1 Introduction The gamma function Γ(x) given by Z 1 Γ(x) = tx−1 e−t dt (x > 0) 0 A Method to Construct Continued-Fraction Approximations and Its Applications 2 is one of the most important functions in mathematical analysis and has applications in many diverse areas. The logarithmic derivative (x) of the gamma function Γ(x) given by Γ0(x) Z x (x) = or ln Γ(x) = (x) dt Γ(x) 1 is known as the psi (or digamma) function. The psi function (x) is connected to the Euler- Mascheroni constant γ and the harmonic numbers Hn by means of the following well-known relation (see [1, p. 258, Eq. (6.3.2)]): (n + 1) = −γ + Hn (n 2 N := f1; 2; 3; · · · g); (1.1) where n X 1 H := (n 2 ) n k N k=1 is the nth harmonic number and γ is the Euler-Mascheroni constant defined by γ = lim Dn = 0:577215664 ··· ; n!1 where n X 1 D = − ln n: (1.2) n k k=1 Various approximations of the psi function (x) are used in the relation (1.1) and interpreted as approximation for the harmonic number Hn or as approximations of the Euler-Mascheroni constant γ. There has been significant interest and research on γ as evidenced by survey papers (see, for details, [14]) and expository books (see, for example, [19]), which reveal its essential properties and surprising connections with other areas of the mathematical sciences. The following two-sided inequality for the difference Dn − γ was established in [28, 33]: 1 1 < D − γ < (n 2 ): 2(n + 1) n 2n N The convergence of the sequence Dn to γ is very slow. By changing the logarithmic term in (1.2), DeTemple [15, 16] presented the following inequality: 1 1 < R − γ < (n 2 ); (1.3) 24(n + 1)2 n 24n2 N where 1 R = H − ln n + : (1.4) n n 2 A Method to Construct Continued-Fraction Approximations and Its Applications 3 On the other hand, Negoi [26] proved that the sequence fTngn2N given by n X 1 1 1 T = − ln n + + (1.5) n k 2 24n k=1 is strictly increasing and convergent to γ. Moreover, Negoi [26] proved that 1 1 < γ − T < (n 2 ): (1.6) 48(n + 1)3 n 48n3 N The following faster approximation formulas can be found in [10, 12]: 1 1 1 X := H − ln n + + − = γ + O n−4 (n ! 1) (1.7) n n 2 24n 48n2 and 1 1 1 23 Y := H − ln n + + − + = γ + O n−5 (n ! 1): (1.8) n n 2 24n 48n2 5760n3 In view of (1.3), (1.6), (1.7) and (1.8), Chen and Mortici [10] posed the following open prob- lem: Open Problem 1. For a given m 2 N; find the constants pj (j = 1; 2; 3; ··· ; m) such that m ! X pj H − ln n 1 + (1.9) n nj j=1 is the fastest sequence which would converge to γ. Yang [31] first presented the solution of Open Problem 1 by using the Bell polynomials of a logarithmic type. Subsequently, other proofs of Open Problem 1 (1.9) were published by Gavrea and Ivan [17, 18], Lin [20], Chen et al. [8], and Chen and Choi [6]. The following familiar Stirling’s formula: p nn n! ∼ 2πn (n ! 1) (1.10) e has many applications in statistical physics, probability theory and number theory. Actually, it was first discovered in 1733 by the French mathematician, Abraham de Moivre (1667–1754), in the form given by p nn n! ∼ constant · n (n ! 1) e A Method to Construct Continued-Fraction Approximations and Its Applications 4 when he was studying the Gaussian distribution and the central limit theorem. Afterwards,p the Scottish mathematician, James Stirling (1692–1770), found the missing constant 2π when he was attempting to give the normal approximation of the binomial distribution. Recently, Sandor´ and Debnath [29, Theorem 5] proved the following inequality for all posi- tive integers n = 2: 1 p nn p nn n 2 2πn < Γ(n + 1) < 2πn (1.11) e e n − 1 and proposed the approximation formula given below: 1 p nn n 2 Γ(n + 1) ∼ 2πn (n ! 1): (1.12) e n − 1 Motivated by the above-mentioned work by Sandor´ and Debnath (1.12), Mortici and Srivas- tava [25] introduced the following class of approximations for all real numbers a and b: ! p nn 1 −b Γ(n + 1) ∼ µ (a; b) n ! 1; µ (a; b) := 2πn 1 + : (1.13) n n e n + a We note that Stirling’s formula (1.10) is obtained from the Mortici-Srivastava result (1.13) in its special case when b = 0. Furthermore, the approximation formula (1.12) can be written as follows: 1 Γ(n + 1) ∼ µ −1; − (n ! 1): n 2 Indeed, in the aforecited work, Mortici and Srivastava [25] proved that 1 1 Γ(n + 1) ∼ µ − ; − (n ! 1) n 2 12 is the best approximation among all approximations given by (1.13). We choose to write this best approximation as follows: ! 1 p nn 1 12 Γ(n + 1) ∼ 2πn 1 + 1 (n ! 1): (1.14) e n − 2 Mortici and Srivastava [25] also developed the approximation formula (1.14) in order to produce a complete asymptotic expansion (see [25, Theorem 2]). In this paper, we provide a method to construct a continued-fraction approximation which is based upon a given asymptotic expansion (see Remarks 1 and 2). As applications of our A Method to Construct Continued-Fraction Approximations and Its Applications 5 continued-fraction approximation, we develop the approximation formula (1.14) to produce sev- eral further continued-fraction approximations (see Theorems 3, 4 and 5). We also establish continued-fraction approximation for the psi function (see Theorem 6). Finally, we present the higher-order estimate for the Euler-Mascheroni constant γ (see Theorem 7). The following lemmas will be useful in our present investigation. Lemma 1 (see [8]). Let 1 X −n g(x) ∼ bnx (x ! 1) n=1 be a given asymptotic expansion. Then the composition exp g(x) has asymptotic expansion of the following form: 1 X −n exp g(x) ∼ anx (x ! 1); n=0 where n 1 X a = 1 and a = kb a (n 2 ): (1.15) 0 n n k n−k N k=1 Lemma 2 (see [5, Theorem 9]). Let k = 1 and n = 0 be integers. Then; for all real numbers x > 0: k+1 (k) Sk(2n; x) < (−1) (x) < Sk(2n + 1; x); (1.16) where p " k−1 # (k − 1)! k! X Y 1 S (p; x) = + + B (2i + j) ; k xk 2xk+1 2i x2i+k i=1 j=1 fBngn2N0 (N0 = N [ f0g) are the Bernoulli numbers defined by 1 t X tn = B : et − 1 n n! n=0 It follows from (1.16) that 1 1 1 1 1 1 + + − + − < 0(x) x 2x2 6x3 30x5 42x7 30x9 1 1 1 1 1 1 5 < + + − + − + (x > 0): (1.17) x 2x2 6x3 30x5 42x7 30x9 66x11 By using the recurrence formula: 1 0(x + 1) = 0(x) − ; x2 A Method to Construct Continued-Fraction Approximations and Its Applications 6 we deduce from (1.17) that, for x > 0, 1 1 1 1 1 1 − + − + − < 0(x + 1) x 2x2 6x3 30x5 42x7 30x9 1 1 1 1 1 1 5 < − + − + − + : (1.18) x 2x2 6x3 30x5 42x7 30x9 66x11 The numerical calculations presented in this work were performed by using the Maple soft- ware for symbolic computations. 2 A Method for the Construction of Continued-Fraction Approxi- mations In this section, we present a method to construct a continued-fraction approximation based upon a given asymptotic expansion (see Remark 1 and Remark 2 below). Theorem 1 generalizes an earlier result [13, Lemma 1.1]. Theorem 1. Let a` 6= 0 (` 2 N) and 1 X aj A(x) ∼ (x ! 1) (2.1) xj j=` be a given asymptotic expansion. Define the function B(x) by a A(x) = ` : B(x) a` Then the function B(x) = A(x) has the following asymptotic expansion: 1 X bj B(x) ∼ x` + b x`−1 + b x`−2 + ··· + b x + b + (x ! 1); −(`−1) −(`−2) −1 0 xj j=1 A Method to Construct Continued-Fraction Approximations and Its Applications 7 where 8 a`+1 >b−(`−1) = − > a` > > a + a b > `+2 `+1 −(`−1) >b−(`−2) = − > a` > > > > .
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