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Improving Known Bounds to the p-adic Littlewood Conjecture (NB: NEW TIME AND VENUE)

John Blackman (University of Liverpool)
Thursday 10 November 2022 16:00–17:00 Aud. D4 (1531-219)
Mathematics Seminar

One of the main themes of Diophantine approximation is to study how well a real number x can be approximated by a rational number p/q accounting for the size of the denominator q. One can measure how well a real number can be rationally approximated by computing the Markov constant M(x):=lim inf q2|xp/q|. If the Markov constant is 0 then x is well approximable. For M(x)>0 the number is badly approximable -- with larger values of M(x) representing worse rates of approximation.

In a similar vein, the p-adic Littlewood Conjecture asks if, given a prime p and a badly approximable number x, one can always find a subsequence of pkx such that the Markov constant of this sequence tends to 0, i.e. if mp(x):=lim inf M(pkx)=0. Whilst this Conjecture has gathered a significant amount of interest, it has proven to be very difficult to solve directly. Instead, one can try and upper bounds for mp(x) for a fixed prime p. This general idea was explored by Badziahin, who produced an algorithm that showed mp(x) is bounded above by 1/9 for p=2. This algorithm took 3 seconds to compute this bound. However, when the same algorithm tried to compute a bound of 1/10, it ran for over 60 hours without a conclusive result.

In this talk, I will outline a brief history of the p-adic Littlewood Conjecture and discuss how hyperbolic geometry can be used to help understand the problem further. Finally, I will describe an alternative algorithm for obtaining upper bounds on mp(x) and discuss the results of this algorithm: most notably, that we were able to show that mp(x) is bounded above by 1/18 for p=2.

This talk is based on joint work with Dr M.J. Northey.

Organised by: ADA
Contact: Simon Kristensen Revised: 25.05.2023