English

Rogers-Ramanujan continued fraction and approximations to $\mathbf{2\pi}$

Number Theory 2023-08-22 v1

Abstract

We observe that certain famous evaluations of the Rogers-Ramanujan continued fraction R(q)R(q) are close to 2π62\pi-6 and (2π6)/2π(2\pi-6)/2\pi, and that 2π62\pi-6 can be expressed by a Rogers-Ramanujan continued fraction in which qq is very nearly equal to R5(e2π)R^5(e^{-2\pi}). The value of 5αlnR(e2απ)-{5\over \alpha}\ln R(e^{-2\alpha \pi}) converges to 2π2\pi as α\alpha increases. For α=5n\alpha=5^n, a modular equation by Ramanujan provides recursive closed-form expressions that approximate the value of 2π2\pi, the number of correct digits increasing by a factor of five each time nn increases by one. If we forgo closed-form expressions, a modular equation by Rogers allows numerical iterations that converge still faster to 2π2\pi, each iteration increasing the number of correct digits by a multiple of eleven.

Keywords

Cite

@article{arxiv.2308.09774,
  title  = {Rogers-Ramanujan continued fraction and approximations to $\mathbf{2\pi}$},
  author = {Rajeev Kohli},
  journal= {arXiv preprint arXiv:2308.09774},
  year   = {2023}
}