For each rational number not less than 2, we provide an explicit family of continued fractions of algebraic power series in finite characteristic (together with the algebraic equations they satisfy) which has that rational number as its diophantine approximation exponent. We also provide some non-quadratic examples with bounded sequences of partial quotients.
Academic PressContinued fraction expansions of real numbers and laurent series over finite fields are well studied because, for example, of their close connection with best diophantine approximations. In both the cases, the expansion terminates exactly for rationals and is eventually periodic exactly for quadratic irrationals. But continued fraction expansion is not known even for a single algebraic real number of degree more than two. It is not even known whether the sequence of partial quotients is bounded or not for such a number. (Because of the numerical evidence and a belief that algebraic numbers are like most numbers in this respect, it is often conjectured that the sequence is unbounded.) It is hard to obtain such expansions for algebraic numbers, because the effect of basic algebraic operations (except for adding an integer or, more generally, an integral Mobius transformation of determinant \1), such as addition or multiplication or even multiple or power, is not at all transparent on the continued fraction expansions.In finite characteristic p, the algebraic operation of taking pth power has a very transparent effect: If :=[a 0 , a 1 , . a 0 , a 1 , ...] as a short form for the expansion a 0 +1Â(a 1 +1Â(a 2 + } } } )).