On asymptotic approximations to the log-Gamma and Riemann-Siegel theta functions

Abstract: We give bounds on the error in the asymptotic approximation of the log-Gamma function $\ln\Gamma(z)$ for complex $z$ in the right half-plane. These improve on earlier bounds by Behnke and Sommer (1962), Spira (1971), and Hare (1997). We show that $|R_{k+1}(z)/T_k(z)| < \sqrt{\pi k}$ for nonzero $z$ in the right half-plane, where $T_k(z)$ is the $k$-th term in the asymptotic series, and $R_{k+1}(z)$ is the error incurred in truncating the series after $k$ terms. If $k \le |z|$, then the stronger bound $|R_{k+1}(z)/T_k(z)| < (k/|z|)^2/(\pi^2-1) < 0.113$ holds. Similarly for the asymptotic approximation of $\ln\Gamma(z+\frac{1}{2})$, except that a factor $\eta_k = 1/(1-2^{1-2k})$ multiplies some of the bounds. We deduce similar bounds for asymptotic approximation of the Riemann-Siegel theta function $\vartheta(t)$. We show that the accuracy of a well-known approximation to $\vartheta(t)$ can be improved by including an exponentially small term in the approximation. This improves the attainable accuracy for real $t>0$ from $O(\exp(-\pi t))$ to $O(\exp(-2\pi t))$. We discuss a similar example due to Olver (1964), and a connection with the Stokes phenomenon.