# Commensurability (group theory)

In mathematics, specifically in group theory, two groups are **commensurable** if they differ only by a finite amount, in a precise sense. The **commensurator** of a subgroup is another subgroup, related to the normalizer.

## Commensurability in group theory

Two groups *G*_{1} and *G*_{2} are said to be (**abstractly**) **commensurable** if there are subgroups *H*_{1} ⊂ *G*_{1} and *H*_{2} ⊂ *G*_{2} of finite index such that *H*_{1} is isomorphic to *H*_{2}.[1] For example:

- A group is finite if and only if it is commensurable with the trivial group.
- Any two finitely generated free groups on at least 2 generators are commensurable with each other.[2] The group
*SL*(2,**Z**) is also commensurable with these free groups. - Any two surface groups of genus at least 2 are commensurable with each other.

A different but related notion is used for subgroups of a given group. Namely, two subgroups Γ_{1} and Γ_{2} of a group *G* are said to be **commensurable** if the intersection Γ_{1} ∩ Γ_{2} is of finite index in both Γ_{1} and Γ_{2}. Clearly this implies that Γ_{1} and Γ_{2} are abstractly commensurable.

Example: for nonzero real numbers *a* and *b*, the subgroup of **R** generated by *a* is commensurable with the subgroup generated by *b* if and only if the real numbers *a* and *b* are commensurable, meaning that *a*/*b* belongs to the rational numbers **Q**.

In geometric group theory, a finitely generated group is viewed as a metric space using the word metric. If two groups are (abstractly) commensurable, then they are quasi-isometric.[3] It has been fruitful to ask when the converse holds.

There is an analogous notion in linear algebra: two linear subspaces *S* and *T* of a vector space *V* are **commensurable** if the intersection *S* ∩ *T* has finite codimension in both *S* and *T*.

## In topology

Two path-connected topological spaces are sometimes called *commensurable* if they have homeomorphic finite-sheeted covering spaces. Depending on the type of space under consideration, one might want to use homotopy equivalences or diffeomorphisms instead of homeomorphisms in the definition. By the relation between covering spaces and the fundamental group, commensurable spaces have commensurable fundamental groups.

Example: the Gieseking manifold is commensurable with the complement of the figure-eight knot; these are both noncompact hyperbolic 3-manifolds of finite volume. On the other hand, there are infinitely many different commensurability classes of compact hyperbolic 3-manifolds, and also of noncompact hyperbolic 3-manifolds of finite volume.[4]

## The commensurator

The **commensurator** of a subgroup Γ of a group *G*, denoted Comm_{G}(Γ), is the set of elements *g* of *G* that such that the conjugate subgroup *g*Γ*g*^{−1} is commensurable with Γ.[5] In other words,

This is a subgroup of *G* that contains the normalizer N_{G}(Γ) (and hence contains Γ).

For example, the commensurator of the special linear group *SL*(*n*,**Z**) in *SL*(*n*,**R**) contains *SL*(*n*,**Q**). In particular, the commensurator of *SL*(*n*,**Z**) in *SL*(*n*,**R**) is dense in *SL*(*n*,**R**). More generally, Grigory Margulis showed that the commensurator of a lattice Γ in a semisimple Lie group *G* is dense in *G* if and only if Γ is an arithmetic subgroup of *G*.[6]

## The abstract commensurator

The **abstract commensurator** of a group , denoted Comm, is the group of equivalence classes of isomorphisms , where and are finite index subgroups of , under composition.[7] Elements of are called **commensurators** of .

If is a connected semisimple Lie group not isomorphic to , with trivial center and no compact factors, then by the Mostow rigidity theorem, the abstract commensurator of any irreducible lattice is linear. Moreover, if is arithmetic, then Comm is virtually isomorphic to a dense subgroup of , otherwise Comm is virtually isomorphic to .

## Notes

- Druțu & Kapovich (2018), Definition 5.13.
- Druțu & Kapovich (2018), Proposition 7.80.
- Druțu & Kapovich (2018), Corollary 8.47.
- Maclachlan & Reid (2003), Corollary 8.4.2.
- Druțu & Kapovich (2018), Definition 5.17.
- Margulis (1991), Chapter IX, Theorem B.
- Druțu & Kapovich (2018), Section 5.2.

## References

- Druțu, Cornelia; Kapovich, Michael (2018),
*Geometric Group Theory*, American Mathematical Society, ISBN 9781470411046, MR 3753580 - Maclachlan, Colin; Reid, Alan W. (2003),
*The Arithmetic of Hyperbolic 3-Manifolds*, Springer Nature, ISBN 0-387-98386-4, MR 1937957 - Margulis, Grigory (1991),
*Discrete Subgroups of Semisimple Lie Groups*, Springer Nature, ISBN 3-540-12179-X, MR 1090825