most recent 30 from http://mathoverflow.net 2013-06-19T18:38:37Z http://mathoverflow.net/feeds/question/55435 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/55435/need-a-good-reference-for-introduction-to-elementary-theory-of-groups dan 2011-02-14T19:23:33Z 2011-02-14T22:49:27Z

<p>I've recently become interested in the elementary theory of groups due to Sela and Myasnikov-Kharlampovich's work with free groups. I'd like a good introduction to the field of the elementary theory of groups, and in particular I'd like a reference to contain examples of group properties that cannot be read from a group's elementary theory. For example, it seems that the statements "G is vritually abelian" or "G is hopfian" could not be expressed with first-order sentences, but I don't have enough knowledge yet to determine if such things are true. Does anyone know such a reference? Or, specifically, does someone know a proof of the fact that being hopfian (or some other group property) can't be read from a group's elementary theory? Thanks!</p>

http://mathoverflow.net/questions/55435/need-a-good-reference-for-introduction-to-elementary-theory-of-groups/55440#55440 HW 2011-02-14T20:13:07Z 2011-02-14T21:55:51Z

<p>I learned a lot from reading Bestvina and Feighn's article <a href="http://arxiv.org/abs/0809.0467" rel="nofollow">Notes on Sela's work: Limit groups and Makanin-Razborov diagrams</a>. It's not a broad introduction to elementary theory, but it does express some of Sela's ideas quite succinctly. You may need a background in geometric group theory (specifically, in understanding laminations on 2-complexes or group actions on $\mathbb{R}$-trees) to get the most out of it.</p> <p>Regarding the question of whether or not the Hopf property is elementary: the answer is obviously `no' if you allow infinitely generated examples. Indeed, consider the free group on countably many generators, $F_\infty$. The elementary theory is completely determined by the set of finitely generated subgroups, so $F_\infty$ is elementary equivalent to $F_2$. But $F_2$ is Hopfian and $F_\infty$ is not.</p> <p><strong>EDIT:</strong> I'm getting a little nervous about the claim that the elementary theory is determined by the list of finitely generated subgroups. However, Sela and Kharlampovich--Miasnikov proved that the natural inclusions $F_n\subseteq F_{n+1}$ are elementary embeddings, from which it does indeed follow that $F_\infty$ is elementarily equivalent to $F_2$.</p> <p>I don't know a finitely generated example, although I agree that one must surely exist.</p>

http://mathoverflow.net/questions/55435/need-a-good-reference-for-introduction-to-elementary-theory-of-groups/55466#55466 Jeremy Macdonald 2011-02-14T22:49:27Z 2011-02-14T22:49:27Z

<p>You might try Champetier and Guirardel's <a href="http://arxiv.org/abs/math/0401042" rel="nofollow">Limit groups as limits of free groups</a>. It has a short section (section 5) on elementary and universal theory, though perhaps none of the "non-examples" you're looking for. It is, however, a pleasure to read and if you're interested in limit groups, Makanin-Razborov digrams, etc. I highly recommend it. </p>