Computer Science/Discrete Mathematics Seminar II | |

Topic: | Taming the hydra: the Word Problem, Dehn functions, and extreme integer compression |

Speaker: | Timothy Riley |

Affiliation: | Cornell University; Member, School of Mathematics |

Date: | Tuesday, December 2 |

Time/Room: | 10:30am - 12:30pm/S-101 |

Video Link: | http://video.ias.edu/csdm/2014/1202-TimothyRiley |

For a finitely presented group, the Word Problem asks for an algorithm which declares whether or not words on the generators represent the identity. The Dehn function is the time-complexity of a direct attack on the Word Problem by applying the defining relations. I will survey these topics and their interrelationships. A "hydra phenomenon" gives rise to novel groups with extremely fast growing (Ackermannian) Dehn functions. I will explain why, nevertheless, there are efficient (polynomial time) solutions to the Word Problems of these groups. The main innovation is a means of computing efficiently with compressed forms of enormous integers. This is joint work with Will Dison and Eduard Einstein.