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On the Beta Expansion for Salem Numbers of degree 6

David W. Boyd
Department of Mathematics, University of British Columbia,
Vancouver, B.C., Canada V6T 1Z2

boyd@math.ubc.ca

Keywords Salem numbers, beta expansions, polynomials, computation
Subjclass Primary 11R06, 11K16; Secondary 11Y99

Abstract For a given beta > 1, the beta transformation T = T_beta is defined for x \in [0,1] by Tx := beta x (mod 1). The number beta is said to be a beta number if the orbit \{T^n(1)\}_{n \ge 1} is finite, hence eventually periodic. It is known that all Pisot numbers are beta numbers, and it is conjectured that this is true for Salem numbers, but this is known only for Salem numbers of degree 4. Here we consider some computational and heuristic evidence for the conjecture in the case of Salem numbers of degree 6, by considering the set of 11836 such numbers of trace at most 15. Although the orbit is small for the majority of these numbers, there are some examples for which the orbit size is shown to exceed 10^9 and for which the possibility remains that the orbit is infinite. There are also some very large orbits which have been shown to be finite: an example is given for which the preperiod length is 39420662 and the period length is 93218808. This is in contrast to Salem numbers of degree 4 where the orbit size is bounded by 2beta + 3. An heuristic probabilistic model is proposed which explains the difference between the degree-4 and degree-6 cases. The model predicts that all Salem numbers of degree 4 and 6 should be beta numbers but that degree-6 Salem numbers can have orbits which are arbitrarily large relative to the size of \beta. Furthermore, the model predicts that a positive proportion of Salem numbers of any fixed degree \ge 8 will not be beta numbers. This latter prediction is not tested here.

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