Robust HGCD with no backup steps

Abstract

Subquadratic divide-and-conquer algorithms for computing the greatest common divisor have been studied for a couple of decades. The integer case has been notoriously difficult, with the need for "backup steps" in various forms. This paper explains why backup steps are necessary for algorithms based directly on the quotient sequence, and proposes a robustness criterion that can be used to construct a "half-gcd" algorithm without any backup steps. © Springer-Verlag Berlin Heidelberg 2006.