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§5.11 Asymptotic Expansions

Contents
  1. §5.11(i) Poincaré-Type Expansions
  2. §5.11(ii) Error Bounds and Exponential Improvement
  3. §5.11(iii) Ratios

§5.11(i) Poincaré-Type Expansions

As z\to\infty in the sector |\operatorname{ph}z|\leq\pi-\delta,

5.11.1 \operatorname{Ln}\Gamma\left(z\right)\sim\left(z-\tfrac{1}{2}\right)\ln z-z+%
\tfrac{1}{2}\ln\left(2\pi\right)+\sum_{k=1}^{\infty}\frac{B_{2k}}{2k(2k-1)z^{2%
k-1}}

and

For the Bernoulli numbers B_{2k}, see §24.2(i).

With the same conditions,

5.11.3 \Gamma\left(z\right)={\mathrm{e}}^{-z}z^{z}\left(\frac{2\pi}{z}\right)^{1/2}%
\Gamma^{*}\left(z\right)\sim{\mathrm{e}}^{-z}z^{z}\left(\frac{2\pi}{z}\right)^%
{1/2}\sum_{k=0}^{\infty}\frac{g_{k}}{z^{k}},

where

5.11.4
g_{0}=1,
g_{1}=\tfrac{1}{12},
g_{2}=\tfrac{1}{288},
g_{3}=-\tfrac{139}{51840},
g_{4}=-\tfrac{571}{24\;88320},
g_{5}=\tfrac{1\;63879}{2090\;18880},
g_{6}=\tfrac{52\;46819}{7\;52467\;96800}.

Also,

5.11.5 g_{k}=\sqrt{2}{\left(\tfrac{1}{2}\right)_{k}}a_{2k},

where a_{0}=\tfrac{1}{2}\sqrt{2} and

5.11.6 a_{0}a_{k}+\frac{1}{2}a_{1}a_{k-1}+\frac{1}{3}a_{2}a_{k-2}+\dots+\frac{1}{k+1}%
a_{k}a_{0}=\frac{1}{k}a_{k-1},k\geq 1.

The scaled gamma function \Gamma^{*}\left(z\right) is defined in (5.11.3) and its main property is \Gamma^{*}\left(z\right)\sim 1 as z\to\infty in the sector |\operatorname{ph}z|\leq\pi-\delta. Wrench (1968) gives exact values of g_{k} up to g_{20}. Spira (1971) corrects errors in Wrench’s results and also supplies exact and 45D values of g_{k} for k=21,22,\dots,30. For explicit formulas for g_{k} in terms of Stirling numbers see Nemes (2013a), and for asymptotic expansions of g_{k} as k\to\infty see Boyd (1994) and Nemes (2015a).

Terminology

The expansion (5.11.1) is called Stirling’s series (Whittaker and Watson (1927, §12.33)), whereas the expansion (5.11.3), or sometimes just its leading term, is known as Stirling’s formula (Abramowitz and Stegun (1964, §6.1), Olver (1997b, p. 88)).

Next, and again with the same conditions,

where a\;(>0) and b\;(\in\mathbb{C}) are both fixed, and

5.11.8 \operatorname{Ln}\Gamma\left(z+h\right)\sim\left(z+h-\tfrac{1}{2}\right)\ln z-%
z+\tfrac{1}{2}\ln\left(2\pi\right)+\sum_{k=2}^{\infty}\frac{(-1)^{k}B_{k}\left%
(h\right)}{k(k-1)z^{k-1}},

where h\;(\in\mathbb{C}) is fixed, and B_{k}\left(h\right) is the Bernoulli polynomial defined in §24.2(i). For similar results including a convergent factorial series see, Nemes (2013c).

Lastly, as y\to\pm\infty,

uniformly for bounded real values of x.

§5.11(ii) Error Bounds and Exponential Improvement

If the sums in the expansions (5.11.1) and (5.11.2) are terminated at k=n-1 (k\geq 0) and z is real and positive, then the remainder terms are bounded in magnitude by the first neglected terms and have the same sign. If z is complex, then the remainder terms are bounded in magnitude by {\sec}^{2n}\left(\tfrac{1}{2}\operatorname{ph}z\right) for (5.11.1), and {\sec}^{2n+1}\left(\tfrac{1}{2}\operatorname{ph}z\right) for (5.11.2), times the first neglected terms. For error bounds for (5.11.8) and an exponentially-improved extension, see Nemes (2013b).

For the remainder term in (5.11.3) write

Then

5.11.11 \left|R_{K}(z)\right|\leq\frac{(1+\zeta\left(K\right))\Gamma\left(K\right)}{2(%
2\pi)^{K+1}{\left|z\right|}^{K}}\*\left(1+\min(\sec\left(\operatorname{ph}z%
\right),2K^{\frac{1}{2}})\right),\left|\operatorname{ph}z\right|\leq\frac{1}{2}\pi,

where \zeta\left(K\right) is as in Chapter 25. In the case K=1 the factor 1+\zeta\left(K\right) is replaced with 4. For this result and a similar bound for the sector \frac{1}{2}\pi\leq\operatorname{ph}z\leq\pi see Boyd (1994).

For further information see Olver (1997b, pp. 293–295), and for other error bounds see Whittaker and Watson (1927, §12.33), Spira (1971), and Schäfke and Finsterer (1990).

For re-expansions of the remainder terms in (5.11.1) and (5.11.3) in series of incomplete gamma functions with exponential improvement (§2.11(iii)) in the asymptotic expansions, see Berry (1991), Boyd (1994), and Paris and Kaminski (2001, §6.4).

§5.11(iii) Ratios

In this subsection a, b, and c are real or complex constants.

If z\to\infty in the sector |\operatorname{ph}z|\leq\pi-\delta, then

5.11.14 \frac{\Gamma\left(z+a\right)}{\Gamma\left(z+b\right)}\sim\left(z+\frac{a+b-1}{%
2}\right)^{a-b}\sum_{k=0}^{\infty}\frac{H_{k}(a,b)}{\left(z+\tfrac{1}{2}(a+b-1%
)\right)^{2k}}.

Here

In terms of generalized Bernoulli polynomials B^{(\ell)}_{n}\left(x\right)24.16(i)), we have for k=0,1,\ldots,

5.11.17 G_{k}(a,b)=\genfrac{(}{)}{0.0pt}{}{a-b}{k}B^{(a-b+1)}_{k}\left(a\right),
5.11.18 H_{k}(a,b)=\genfrac{(}{)}{0.0pt}{}{a-b}{2k}B^{(a-b+1)}_{2k}\left(\frac{a-b+1}{%
2}\right).

For realistic error bounds in (5.11.14) see Frenzen (1987a, 1992). See also Burić and Elezović (2011).

Lastly, and again if z\to\infty in the sector |\operatorname{ph}z|\leq\pi-\delta, then

For the error term in (5.11.19) in the case z=x\;(>0) and c=1, see Olver (1995).