English

Creepers: Real quadratic orders with large class number

Number Theory 2007-05-23 v1

Abstract

Shanks's sequence of quadratic fields \Q(Sn)\Q(\sqrt{S_{n}}) where Sn=(2n+1)2+2n+2S_{n}=(2^n+1)^2 + 2^{n+2} instances a class of quadratic fields for which the class number is large and, therefore, the continued fraction period is relatively short. Indeed, that period length increases linearly with nn, that is: in arithmetic progression. The fields have regulator O(n2)O(n^2). In the late nineties, these matters intrigued Irving Kaplansky, and led him to compute period length of the square root of sequences a2x2n+bxn+ca^2x^{2n}+bx^{n}+c for integers aa, bb, cc, and xx. In brief, Kap found unsurprisingly that, generically, triples (a,b,c)(a,b,c) are `leapers': they yield sequences with period length increasing at exponential rate. But there are triples yielding sequences with constant period length, Kap's `sleepers'. Finally, there are triples, as exemplified by the Shanks's sequence, for which the period lengths increase in arithmetic progression. Felicitously, Kaplansky called these `creepers'. It seems that the sleepers and creepers are precisely those for which one is able to detail the explicit continued fraction expansion for all nn. Inter alia, this thesis noticeably extends the known classes of creepers and finds that not all are `kreepers' (of the shape identified by Kaplansky) and therefore not of the shape of examples studied by earlier authors looking for families of quadratic number fields with explicitly computable unit and of relatively large regulator. The work of this thesis includes the discovery of old and new families of hyperelliptic curves of increasing genus gg and torsion divisor of order O(g2)O(g^2). It follows that the apparent trichotomy leaper/sleeper/creeper coincides with the folk belief that the just-mentioned torsion is maximum possible.

Keywords

Cite

@article{arxiv.math/0703519,
  title  = {Creepers: Real quadratic orders with large class number},
  author = {Roger Patterson},
  journal= {arXiv preprint arXiv:math/0703519},
  year   = {2007}
}

Comments

PhD Thesis, Macquarie University, Sydney, November 2003

R2 v1 2026-07-22T17:52:50.242Z