REAL QUADRATIC FIELDS, CONTINUED FRACTIONS, AND A CONSTRUCTION OF PRIMARY SYMMETRIC PARTS OF ELE TYPE

Abstract

For a non-square positive integer d with 4 ∤ d, put ω(d) :=(1 + √d)/2 if d is congruent to 1 modulo 4 and ω(d) := √d otherwise. Let a₁, a₂, . . . , a_ℓ-1 be the symmetric part of the simple continued fraction expansion of ω(d). We say that the sequence a₁, a₂, . . . , a_[ℓ/2] is the primary symmetric part of the simple continued fraction expansion of ω(d). A notion of ‘ELE type' for a finite sequence was introduced in Kawamoto et al (Comment. Math. Univ. St. Pauli 64(2) (2015), 131-155). The aims of this paper are to introduce a notion of ‘pre-ELE type' for a finite sequence and to give a way of constructing primary symmetric parts of ELE type. As a byproduct, we show that there exist infinitely many real quadratic fields with period ℓ of minimal type for each even ℓ ≥ 6.