Foundations of Computational Mathematics

, Volume 9, Issue 5, pp 599–609 | Cite as

Real Computational Universality: The Word Problem for a Class of Groups with Infinite Presentation



The word problem for discrete groups is well known to be undecidable by a Turing Machine; more precisely, it is reducible both to and from and thus equivalent to the discrete Halting Problem. The present work introduces and studies a real extension of the word problem for a certain class of groups which are presented as quotient groups of a free group and a normal subgroup. As a main difference to discrete groups these groups may be generated by uncountably many generators with index running over certain sets of real numbers. We study the word problem for such groups within the Blum–Shub–Smale (BSS) model of real number computation. The main result establishes the word problem to be computationally equivalent to the Halting Problem for such machines. It thus gives the first non-trivial example of a problem complete, that is, computationally universal for this model.


Word problem for groups Computational universality Blum–Shub–Smale model Real halting problem 

Mathematics Subject Classification (2000)

20F10 68Q17 68Q10 


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Copyright information

© SFoCM 2009

Authors and Affiliations

  1. 1.Theoretical Computer ScienceBTU CottbusCottbusGermany
  2. 2.Faculty of Computer Science, Electrical Engineering and MathematicsUniversity of PaderbornPaderbornGermany

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