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arXiv 2108.00844math.NT
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Ramanujan型级数的截断误差的渐近展开

Asymptotic expansions for the truncation error in Ramanujan-type series

Lorenz Milla

摘要:

许多已知的计算$π$的最快算法涉及广义超几何级数,例如Ramanujan-Sato级数。本文研究了几个这类级数的收敛速度,并给出了有限逼近误差的渐近展开式。例如,当使用Chudnovskys级数的第一$n$项时,我们得到有限近似$π_n\approx π$。已知截断误差满足$|π_n-π|\approx 53360^{-3n}.$在本文中,我们证明了Chudnovskys级数中的截断误差的渐近展开式是$$\left|π_n-π\right|=53360^{-3n}\cdot\frac{{106720}\sqrt{{10005}π}}{{1672209}\sqrt{n}}\cdot\exp\left(\frac{A_1}{n}+\frac{A_2}{n^2}+\frac{δ_n}{n^3}\right),$$,具有${0.006907}<δ_n<{0.008429}$和$A_1$和$A_2$的精确有理数值:$$A_1= -\frac{1781843197433}{7456754505816},$$ $$A_2= -\frac{1080096011925710088395}{3475199235000451148614116}.$$因此,我们演示了如何建立精确的误差界,通过对$1/π$的Ramanujan样级数获得的$π$的近似。在附录中,我们还给出了所有已知的$1/π$有理超几何级数的渐近展开式。

英文摘要:

Many of the fastest known algorithms to compute $π$ involve generalized hypergeometric series, such as the Ramanujan-Sato series. In this paper, we investigate the rates of convergence for several such series and we give asymptotic expansions for the error of finite approximation. For example, when using the first $n$ terms of the Chudnovskys' series, we obtain the finite approximation $π_n\approx π$. It is known that the truncation error satisfies $|π_n-π|\approx 53360^{-3n}.$ In this paper, we prove that the asymptotic expansion for the truncation error in the Chudnovskys' series is $$\left|π_n-π\right|=53360^{-3n}\cdot\frac{{106720}\sqrt{{10005}π}}{{1672209}\sqrt{n}}\cdot\exp\left(\frac{A_1}{n}+\frac{A_2}{n^2}+\frac{δ_n}{n^3}\right),$$ with ${0.006907}<δ_n<{0.008429}$ and the exact rational values of $A_1$ and $A_2$: $$A_1= -\frac{1781843197433}{7456754505816},$$ $$A_2= -\frac{1080096011925710088395}{3475199235000451148614116}.$$ Thus we demonstrate how to establish precise error bounds for the approximations for $π$ obtained through Ramanujan-like series for $1/π$. We also give asymptotic expansions for all known rational hypergeometric series for $1/π$ in the appendix.

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