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For questions requesting examples of a certain structure or phenomenon

8 votes

Applications of logic to group theory?

In recent years, surely the most important and influential (and, unfortunately, sometimes controversial) work on the first-order theory of groups has been Sela's work on free and hyperbolic groups, an …
HJRW's user avatar
  • 25.2k
8 votes

What can you do with a compact moduli space?

Thurston's compactification of Teichmueller space gives an elegant proof of the Nielsen--Thurston classification of surface automorphisms. The compactified Teichmueller space of the surface of genus …
HJRW's user avatar
  • 25.2k
17 votes

Applications of the notion of of Gromov-Hausdorff distance

Examples include: Bestvina's construction of a boundary for the set of hyperbolic structures on a manifold; Paulin's Theorem: if $\Gamma$ is word-hyperbolic and $\mathrm{Out}(\Gamma)$ is infinite then …
11 votes

Torsion-free virtually free-by-cyclic groups

Many more examples, including ones where the free kernel is finitely generated, arise by looking at knot complements. …
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11 votes

Examples of residually-finite groups

Martino--Minasyan used this construction to give examples of conjugacy separable finitely presented groups with non-conjugacy separable subgroups of finite index. …
37 votes

What are your favorite instructional counterexamples?

The Baumslag--Solitar groups have presentations $BS(p,q)=\langle a,b\mid a^p=b^{-1}a^q b\rangle$. They have the following nice properties: they're two generator, one relator groups; they can be wr …