On the finite presentation of subdirect products and the nature of residually free groups

Martin R. Bridson*, James Howie, Charles F. Miller, Hamish Short

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

47 Citations (Scopus)


We establish virtual surjection to pairs (VSP) as a general criterion for the finite presentability of subdirect products of groups: if, Gamma(1),..., Gamma(n) are finitely presented and S <Gamma(1) x ... x Gamma(n) projects to a subgroup of finite index in each Gamma(i) x Gamma(j), then S is finitely presentable, indeed there is an algorithm that will construct a finite presentation for S.

We use the VSP criterion to characterize the finitely presented residually free groups. We prove that the class of such groups is recursively enumerable. We describe an algorithm that, given a finite presentation of a residually free group, constructs a canonical embedding into a direct product of finitely many limit groups. We solve the (multiple) conjugacy problem and membership problem for finitely presentable subgroups of residually free groups. We also prove that there is an algorithm that, given a finite generating set for such a subgroup, will construct a finite presentation.

New families of subdirect products of free groups are constructed, including the first examples of finitely presented subgroups that are neither FP infinity nor of Stallings-Bieri type.

Original languageEnglish
Pages (from-to)891-933
Number of pages43
JournalAmerican Journal of Mathematics
Issue number4
Publication statusPublished - 1 Aug 2013




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