Supernatural numbers

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In the [[Mathematics|mathematical]] field of [[group theory]], the '''Kurosh subgroup theorem''' describes the algebraic structure of [[subgroup]]s of [[free product]]s of [[group (mathematics)|groups]]. The theorem was obtained by [[Aleksandr Gennadievich Kurosh|Alexander Kurosh]], a Russian mathematician, in 1934.<ref>A. G. Kurosh, ''Die Untergruppen der freien Produkte von beliebigen Gruppen.'' [[Mathematische Annalen]], vol. 109 (1934), pp. 647–660.</ref> Informally, the theorem says that every subgroup of a free product is itself a free product of a [[free group]] and of its intersections with the [[Conjugate (group theory)|conjugates]] of the factors of the original free product.

==History and generalizations==
After the original 1934 proof of Kurosh, there were many subsequent proofs of the Kurosh subgroup theorem, including proofs of Kuhn (1952),<ref>H. W. Kuhn. ''Subgroup theorems for groups presented by generators and relations.'' [[Annals of Mathematics]] (2), vol. 56, (1952), pp. 22–46</ref> [[Saunders Mac Lane|Mac Lane]] (1958)<ref>S. Mac Lane.
''A proof of the subgroup theorem for free products.'' Mathematika, vol. 5 (1958), pp. 13–19</ref> and others. The theorem was also generalized for describing subgroups of [[free product with amalgamation|amalgamated free product]]s and [[HNN extension]]s.<ref>A. Karrass, and D. Solitar. ''The subgroups of a free product of two groups with an amalgamated subgroup.''
[[Transactions of the American Mathematical Society]], vol. 150 (1970), pp. 227–255.</ref><ref>A. Karrass, and D. Solitar. ''Subgroups of HNN groups and groups with one defining relation''. [[Canadian Journal of Mathematics]], vol. 23 (1971), pp. 627–643.</ref> Other generalizations include considering subgroups of free [[pro-finite group|pro-finite]] products<ref>{{cite journal| last=Zalesskii | first=Pavel Aleksandrovich | year=1990 | title=[Open subgroups of free profinite products over a profinite space of indices] | language=Russian | journal=[[Doklady Akademii Nauk SSSR]] | volume=34 | issue=1 | pages=17–20}}</ref> and a version of the Kurosh subgroup theorem for [[topological group]]s.<ref>P. Nickolas. ''A Kurosh subgroup theorem for topological groups.'' [[Proceedings of the London Mathematical Society]] (3), vol. 42 (1981), no. 3, pp. 461–477</ref>

In modern terms, the Kurosh subgroup theorem is a straightforward corollary of the basic structural results of [[Bass–Serre theory]] about groups [[group action|acting]] on [[Tree (graph theory)|trees]].<ref name="cohen">Daniel Cohen. ''Combinatorial group theory: a topological approach.'' [[London Mathematical Society]] Student Texts, 14. [[Cambridge University Press]], Cambridge, 1989. ISBN 0-521-34133-7; 0-521-34936-2</ref>

==Statement of the theorem==

Let ''G'' = ''A''∗''B'' be the [[free product]] of groups ''A'' and ''B'' and let ''H'' ≤ ''G'' be a [[subgroup]] of ''G''. Then there exist a family (''A''''i'')''i'' ∈ ''I'' of subgroups ''A''''i'' ≤ ''A'', a family (''B''''j'')''j'' ∈ ''J'' of subgroups ''B''''j'' ≤ ''B'', families ''g''''i'', ''i'' ∈ ''I'' and ''f''''j'', ''j'' ∈ ''J'' of elements of ''G'', and a subset ''X'' ⊆ ''G'' such that

:<math>H=F(X)*(*_{i\in I} g_i A_ig_i^{-1})* (*_{j\in J} f_jB_jf_j^{-1}).</math>

This means that ''X'' ''freely [[Generating set of a group|generates]]'' a subgroup of ''G'' isomorphic to the [[free group]] ''F''(''X'') with free basis ''X'' and that, moreover, ''g''''i''''A''''i''''g''''i''−1, ''f''''j''''B''''j''''f''''j''−1 and ''X'' [[Generating set of a group|generate]] ''H'' in ''G'' as a free product of the above form.

There is a generalization of this to the case of free products with arbitrarily many factors.<ref>William S. Massey, [http://books.google.com/books?id=IX0dhDDHezgC&pg=PA218&dq=%22Kurosh+subgroup+theorem%22&as_brr=3&ei=dQ10S8zsKKasNaSNgJsE&cd=1#v=onepage&q=%22Kurosh%20subgroup%20theorem%22&f=false Algebraic topology: an introduction.] Graduate Texts in Mathematics, [[Springer-Verlag]], New York, 1977, ISBN 0-387-90271-6; pp. 218–225</ref> Its formulation is:

If ''H'' is a subgroup of ∗i∈I''G''i = ''G'', then
:<math>H=F(X)*(*_{j\in J} g_jH_jg_j^{-1}),</math>
where ''X'' ⊆ ''G'' and ''J'' is some index set and ''g''j ∈ ''G'' and each ''H''j is a subgroup of some ''G''i.

==Proof using Bass–Serre theory==

The Kurosh subgroup theorem easily follows from the basic structural results in [[Bass–Serre theory]], as explained, for example in the book of Cohen (1987):<ref name="cohen"/>

Let ''G'' = ''A''∗''B'' and consider ''G'' as the fundamental group of a [[graph of groups]] '''Y''' consisting of a single non-loop edge with the vertex groups ''A'' and ''B'' and with the trivial edge group. Let ''X'' be the Bass–Serre universal covering tree for the graph of groups '''Y'''. Since ''H'' ≤ ''G'' also acts on ''X'', consider the quotient graph of groups '''Z''' for the action of ''H'' on ''X''. The vertex groups of '''Z''' are subgroups of ''G''-stabilizers of vertices of ''X'', that is, they are conjugate in ''G'' to subgroups of ''A'' and ''B''. The edge groups of '''Z''' are trivial since the ''G''-stabilizers of edges of ''X'' were trivial. By the fundamental theorem of Bass–Serre theory, ''H'' is canonically [[Group isomorphism|isomorphic]] to the fundamental group of the [[graph of groups]] '''Z'''. Since the edge groups of '''Z''' are trivial, it follows that ''H'' is equal to the free product of the vertex groups of '''Z''' and the free group ''F''(''X'') which is the [[fundamental group]] (in the standard topological sense) of the underlying graph ''Z'' of '''Z'''. This implies the conclusion of the Kurosh subgroup theorem.

==Extension==
The result extends to the case that ''G'' is the [[amalgamated product]] along a common subgroup ''A'', under the condition that ''H'' meets every conjugate of ''A'' only in the identity element.<ref>{{cite book | title=Trees | first=Jean-Pierre | last=Serre | publisher=Springer | year=2003 | isbn=3-540-44237-5 | pages=56–57 }}</ref>

==See also==
*[[HNN extension]]
*[[Geometric group theory]]

==Notes==
{{reflist}}

[[Category:Geometric group theory]]
[[Category:Theorems in group theory]]

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