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Continued fractions of cubic Laurent series

Number Theory 2024-11-15 v5

Abstract

We construct continued fraction expansions for several families of the Laurent series in Q[[t1]]\mathbb{Q}[[t^{-1}]]. To the best of the author's knowledge, this is the first result of this kind since Gauss derived the continued fraction expansion for (1+t)r(1+t)^r, rQr\in\mathbb{Q} in 1813. As an application, we apply an analogue of the hypergeometric method to one of those families and derive non-trivial lower bounds on the distance xpq|x - \frac{p}{q}| between one of the real roots of 3x33tx23ax+at3x^3 - 3tx^2-3ax+at, a,tZa,t\in\mathbb{Z} and any rational number, under relatively mild conditions on the parameters aa and tt. We also show that every cubic irrational xRx\in\mathbb{R} admits a (generalised) continued fraction expansion in a closed form that can be explicitly computed. Finally, we provide an infinite series of cubic irrationals xx that have arbitrarily (but finitely) many better-than-expected rational approximations. That is, they are such that for any τ<3+15ln2243.4332...\tau< 3+\frac{15\ln 2}{24}\approx 3.4332... the inequality qx<(H(x)τqeclnq)1||qx|| < (H(x)^{\tau} qe^{c\sqrt{\ln q}})^{-1} has many solutions in integer qq.

Keywords

Cite

@article{arxiv.2211.08663,
  title  = {Continued fractions of cubic Laurent series},
  author = {Dmitry Badziahin},
  journal= {arXiv preprint arXiv:2211.08663},
  year   = {2024}
}

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39 pages