Journée-séminaire de combinatoire

(équipe CALIN du LIPN, université Paris-Nord, Villetaneuse)

Le 03 juillet 2018 à 14h00 en B405, Pablo Rotondo nous parlera de : Efficient GCD computation: the continued logarithm algorithm and ergodic theory

Résumé : Introduced by Gosper in 1978, the Continued Logarithm Algorithm computes the gcd of two integers efficiently, performing only shifts and subtractions. Shallit has studied its worst-case complexity in 2016, showing it to be linear. Here, we perform the average-case analysis of the algorithm: we study its main parameters (number of iterations, total number of shifts) and obtain precise asymptotics for their mean values, with explicit constants. Our analysis involves the dynamical system underlying the algorithm, which produces continued fraction expansions whose quotients are powers of 2. Even though studied by Chan in 2005, this system from an Ergodic Theory perspective, the presence of powers of 2 in the quotients gives a dyadic flavour which cannot be analysed only with Chan's results. Thus we introduce a dyadic component and deal with a two-component dynamical system. With this new mixed system at hand, we provide a complete average-case analysis of the algorithm.

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