Automatic group

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In mathematics, an automatic group is a finitely generated group equipped with several finite-state automata. These automata can tell if a given word representation of a group element is in a "canonical form" and can tell if two elements given in canonical words differ by a generator.

More precisely, let G be a group and A be a finite set of generators. Then an automatic structure of G with respect to A is a set of finite-state automata:

• the word-acceptor, which accepts for every element of G at least one word in A representing it
• multipliers, one for each , which accept a pair (w_1, w_2), for words w_i accepted by the word-acceptor, precisely when w₁a = w₂ in G.

The property of being automatic does not depend on the set of generators.

Contents 1 Properties 2 Examples of automatic groups 3 Examples of non-automatic groups 4 References

[edit] Properties

• Automatic groups have word problem solvable in quadratic time. A given word can actually be put into canonical form in quadratic time.

[edit] Examples of automatic groups

• Finite groups
• Polycyclic groups
• Negatively curved groups
• Euclidean groups
• Coxeter groups
• Braid groups
• Geometrically finite groups

[edit] Examples of non-automatic groups

• Baumslag-Solitar groups

[edit] References

• Epstein, David B. A.; Cannon, James W.; Holt, Derek F.; Levy, Silvio V. F.; Paterson, Michael S.; Thurston, William P. Word processing in groups. Jones and Bartlett Publishers, Boston, MA, 1992. xii+330 pp. ISBN 0-86720-244-0