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| In the mathematical subject of [[group theory]], '''small cancellation theory''' studies groups given by [[Presentation of a group|group presentations]] satisfying '''small cancellation conditions''', that is where defining relations have "small overlaps" with each other. Small cancellation conditions imply algebraic, geometric and algorithmic properties of the group. [[Finitely presented group]]s satisfying sufficiently strong small cancellation conditions are [[word-hyperbolic group|word hyperbolic]] and have [[Word problem for groups|word problem]] solvable by '''Dehn's algorithm'''. Small cancellation methods are also used for constructing [[Tarski monster group|Tarski monsters]], and for solutions of [[Burnside's problem]].
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| ==History==
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| Some ideas underlying the small cancellation theory go back to the work of [[Max Dehn]] in 1910s.<ref>Bruce Chandler and [[Wilhelm Magnus]], ''The history of combinatorial group theory. A case study in the history of ideas.'' Studies in the History of Mathematics and Physical Sciences, 9. Springer-Verlag, New York, 1982. ISBN 0-387-90749-1.</ref> Dehn proved that fundamental groups of closed orientable surfaces of genus at least two have [[Word problem for groups|word problem]] solvable by what is now called [[Dehn's algorithm]]. His proof involved drawing the [[Cayley graph]] of such a group in the [[Hyperbolic manifold|hyperbolic plane]] and performing curvature estimates via the [[Gauss-Bonnet theorem]] for a closed loop in the Cayley graph to conclude that such a loop must contain a large portion (more than a half) of a defining relation.
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| A 1949 paper of Tartakovskii<ref>V. A. Tartakovskii,
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| ''Solution of the word problem for groups with a k-reduced basis for k>6''. (Russian)
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| Izvestiya Akad. Nauk SSSR. Ser. Mat., vol. 13, (1949), pp. 483–494.</ref> was an immediate precursor for small cancellation theory: this paper provided a solution of the word problem for a class of groups satisfying a complicated set of combinatorial conditions, where small cancellation type assumptions played a key role. The standard version of small cancellation theory, as it is used today, was developed by Martin Greendlinger in a series of papers in early 1960s,<ref name="G1">Martin Greendlinger, [http://www3.interscience.wiley.com/journal/113397463/abstract?CRETRY=1&SRETRY=0 ''Dehn's algorithm for the word problem.''] Communications in Pure and Applied Mathematics, vol. 13 (1960), pp. 67–83.</ref><ref name="G2">Martin Greendlinger, [http://www3.interscience.wiley.com/journal/113397594/abstract ''On Dehn's algorithms for the conjugacy and word problems, with applications''.] Communications in Pure and Applied Mathematics, vol. 13 (1960), pp. 641–677.</ref><ref name="G3">Martin Greendlinger, [http://www.springerlink.com/content/m5856445r1131422/ ''An analogue of a theorem of Magnus.''] Archiv der Mathematik, vol 12 (1961), pp. 94–96.</ref> who primarily dealt with the "metric" small cancellation conditions. In particular, Greendlinger proved that [[finitely presented group]]s satisfying the C'(1/6) small cancellation condition have word problem solvable by Dehn's algorithm. The theory was further refined and formalized in the subsequent work of Lyndon,<ref>[[Roger Lyndon|Roger C. Lyndon]],
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| [http://www.springerlink.com/content/m32r66835135170m/?p=2e1f25b8a8ed4d00b8f6f67ce7198f9b&pi=1 ''On Dehn's algorithm.''] [[Mathematische Annalen]], vol. 166 (1966), pp. 208–228.</ref> Schupp<ref>Paul E. Schupp, [http://www.springerlink.com/content/v6147177706mp654/ ''On Dehn's algorithm and the conjugacy problem.''] [[Mathematische Annalen]], vol 178 (1968), pp. 119–130.</ref> and Lyndon-Schupp,<ref name="LS"/> who also treated the case of non-metric small cancellation conditions and developed a version of small cancellation theory for [[free product with amalgamation|amalgamated free product]]s and [[HNN-extension]]s.
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| Small cancellation theory was further generalized by Alexander Ol'shanskii who developed<ref name="Ol">Alexander Yu. Olʹshanskii, ''Geometry of defining relations in groups''. Translated from the 1989 Russian original by Yu. A. Bakhturin. Mathematics and its Applications (Soviet Series), 70. Kluwer Academic Publishers Group, Dordrecht, 1991. ISBN 0-7923-1394-1.</ref> a "graded" version of the theory where the set of defining relations comes equipped with a filtration and where a defining relator of a particular grade is allowed to have a large overlap with a defining relator of a higher grade. Olshaskii used graded small cancellation theory to construct various "monster" groups, including the [[Tarski monster group|Tarski monster]]<ref>A. Yu. Olshanskii, ''An infinite group with subgroups of prime orders'', Math. USSR Izv. 16 (1981), 279–289; translation of Izvestia Akad. Nauk SSSR Ser. Matem. 44 (1980), 309–321.</ref> and also to give a new proof<ref>A. Yu. Olshanskii, ''Groups of bounded period with subgroups of prime order'', Algebra and Logic 21 (1983), 369-418; translation of Algebra i Logika 21 (1982), 553-618.</ref> that [[Burnside's problem|free Burnside groups]] of large odd exponent are infinite (this result was originally proved by [[Sergei Adian|Adian]] and [[Pyotr Novikov|Novikov]] in 1968 using more combinatorial methods).<ref>P. S. Novikov, S. I. Adian, [http://www.mathnet.ru/php/archive.phtml?wshow=paper&jrnid=im&paperid=2699&option_lang=eng ''Infinite periodic groups. I''.] Izvestia Akademii Nauk SSSR. Ser. Mat., vol. 32 (1968), no. 1, pp. 212–244.</ref><ref>P. S. Novikov, S. I. Adian, ''Infinite periodic groups. II''. Izvestia Akademii Nauk SSSR. Ser. Mat., vol. 32 (1968), no. 2, pp. 251–524.</ref><ref>P. S. Novikov, S. I. Adian. ''Infinite periodic groups. III''. Izvestia Akademii Nauk SSSR. Ser. Mat., vol. 32 (1968), no. 3, pp. 709–731.</ref>
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| Small cancellation theory supplied a basic set of examples and ideas for the theory of [[word-hyperbolic group]]s that was put forward by [[Mikhail Gromov (mathematician)|Gromov]] in a seminal 1987 monograph "Hyperbolic groups".<ref>M. Gromov, ''Hyperbolic Groups'', in "Essays in Group Theory" (G. M. Gersten, ed.), MSRI Publ. 8, 1987, pp. 75-263.</ref>
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| ==Main definitions==
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| The exposition below largely follows Ch. V of the book of Lyndon and Schupp.<ref name="LS"/>
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| ===Pieces===
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| Let
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| :<math> G=\langle X|R\rangle\qquad (*)</math>
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| be a [[presentation of a group|group presentation]] where ''R'' ⊆ ''F''(''X'') is a set of freely reduced and cyclically reduced words in the [[free group]] ''F''(''X'') such that ''R'' is ''symmetrized'', that is, closed under taking cyclic permutations and inverses.
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| A nontrivial freely reduced word ''u'' in ''F''(''X'') is called a ''piece'' with respect to (∗) if there exist two distinct elements ''r''<sub>1</sub>, ''r''<sub>2</sub> in ''R'' that both have ''u'' as maximal initial segment.
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| Note that if <math>G=\langle X|S\rangle</math> is a group presentation where the set of defining relators ''S'' is not symmetrized, we can always take the ''symmetrized closure'' ''R'' of ''S'', where ''R'' consists of all cyclic permutations of elements of ''S'' and ''S''<sup>−1</sup>. Then ''R'' is symmetrized and <math>G=\langle X|R\rangle</math> is also a presentation of ''G''.
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| ===Metric small cancellation conditions===
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| Let 0 < ''λ'' < 1. Presentation (∗) as above is said to satisfy the C'(''λ'') ''small cancellation condition'' if whenever ''u'' is a piece with respect to (∗) and ''u'' is a subword of some ''r'' ∈ ''R'', then |''u''| < ''λ''|''r''|. Here |''v''| is the length of a word ''v''.
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| The condition C'(''λ'') is sometimes called a ''metric small cancellation condition''.
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| ===Non-metric small cancellation conditions===
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| Let ''p'' ≥ 3 be an integer. A group presentation (∗) as above is said to satisfy the C(''p'') ''small cancellation condition'' if whenever ''r'' ∈ ''R'' and
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| :<math>r=u_1\dots u_m</math>
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| where ''u''<sub>''i''</sub> are pieces and where the above product is freely reduced as written, then ''m'' ≥ ''p''. That is, no defining relator can be written as a reduced product of fewer than ''p'' pieces.
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| Let ''q'' ≥ 3 be an integer. A group presentation (∗) as above is said to satisfy the T(''q'') ''small cancellation condition'' if whenever 3 ≤ t < ''q'' and ''r''<sub>1</sub>,...,''r''<sub>t</sub> in ''R'' are such that ''r''<sub>1</sub> ≠ ''r''<sub>2</sub><sup>−1</sup>,...,
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| ''r''<sub>t</sub> ≠ ''r''<sub>1</sub><sup>−1</sup> then at least one of the products ''r''<sub>1</sub>''r''<sub>2</sub>,...,''r''<sub>t−1</sub>''r''<sub>t</sub>, ''r''<sub>t</sub>''r''<sub>1</sub> is freely reduced as written.
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| Geometrically, condition T(''q'') essentially means that if ''D'' is a reduced [[van Kampen diagram]] over (∗) then every interior vertex of ''D'' of degree at least three actually has degree at least ''q''.
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| ===Examples===
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| *Let <math>G=\langle a,b| aba^{-1}b^{-1}\rangle</math> be the standard presentation of the [[free abelian group]] of rank two. Then for the symmetrized closure of this presentation the only pieces are words of length 1. This symmetrized form satisfies the C(4)-T(4) small cancellation conditions and the C'(''λ'') condition for any 1 > ''λ'' > 1/4.
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| *Let <math>G=\langle a_1,b_1,\dots, a_k,b_k| [a_1,b_1]\cdot\dots\cdot [a_k,b_k]\rangle</math>, where ''k'' ≥ 2, be the standard presentation of the [[fundamental group]] of a closed orientable surface of genus ''k''. Then for the symmetrization of this presentation the only pieces are words of length 1 and this symmetrization satisfies the C'(1/7) and C(8) small cancellation conditions.
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| *Let <math>G=\langle a,b| abab^2ab^3\dots ab^{100}\rangle</math>. Then, up to inversion, every piece for the symmetrized version of this presentation, has the form ''b<sup>i</sup>ab<sup>j</sup>'' or ''b<sup>i</sup>'', where 0 ≤ ''i'',''j'' ≤ 100. This symmetrization satisfies the C'(1/20) small cancellation condition.
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| *If a symmetrized presentation satisfies the C'(1/''m'') condition then it also satisfies the C(''m'') condition.
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| *Let ''r'' ∈ ''F''(''X'') be a nontrivial cyclically reduced word which is not a proper power in ''F''(''X'') and let ''n'' ≥ 2. Then the symmetrized closure of the presentation <math> G=\langle X| r^n\rangle</math> satisfies the C(2n)<ref>Stephen J. Pride. [http://www.springerlink.com/content/xut2111215321213/ ''Small cancellation conditions satisfied by one-relator groups''.] [[Mathematische Zeitschrift]], vol. 184 (1983), no. 2, pp. 283–286.</ref> and C'(1/n) small cancellation conditions.
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| ==Basic results of small cancellation theory==
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| ===Greendlinger's lemma===
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| The main result regarding the metric small cancellation condition is the following statement (see Theorem 4.4 in Ch. V of)<ref name="LS"/> which is usually called
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| '''Greendlinger's lemma''':
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| Let (∗) be a group presentation as above satisfying the C'(''λ'') small cancellation condition where 0 ≤ ''λ'' ≤ 1/6. Let ''w'' ∈ ''F''(''X'') be a nontrivial freely reduced word such that ''w'' = 1 in ''G''. Then there is a subword ''v'' of ''w'' and a defining relator ''r'' ∈ ''R'' such that ''v'' is also a subword of ''r'' and such that
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| : |''v''| > (1 − 3''λ'')|''r''|.
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| Note that the assumption ''λ'' ≤ 1/6 implies that (1-3''λ'') ≥ 1/2, so that ''w'' contains a subword more than a half of some defining relator.
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| Greendlinger's lemma is obtained as a corollary of the following geometric statement:
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| Under the assumptions of Greendlinger's lemma, let ''D'' be a reduced [[van Kampen diagram]] over (∗) with a cyclically reduced boundary label such that ''D'' contains at least two regions. Then there exist two distinct regions ''D''<sub>1</sub> and ''D''<sub>2</sub> in ''D'' such that for ''j'' = 1,2 the region ''D''<sub>''j''</sub> intersects the boundary cycle ∂''D'' of ''D'' in a simple arc whose length is bigger than (1-3''λ'')|∂''D''<sub>''j''</sub>|.
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| This result in turn is proved by considering a dual diagram for ''D''. There one defines a combinatorial notion of curvature (which, by the small cancellation assumptions, is negative at every interior vertex), and one then obtains a combinatorial version of the [[Gauss-Bonnet theorem]]. Greendlinger's lemma is proved as a consequence of this analysis and in this way the proof evokes the ideas of the original proof of Dehn for the case of surface groups.
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| ===Dehn's algorithm===
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| For any symmetrized group presentation (∗), the following abstract procedure is called '''Dehn's algorithm''':
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| *Given a freely reduced word ''w'' on ''X''<sup>±1</sup>, construct a sequence of freely reduced words ''w'' = ''w''<sub>0</sub>, ''w''<sub>1</sub>, ''w''<sub>2</sub>,..., as follows.
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| *Suppose ''w''<sub>''j''</sub> is already constructed. If it is the empty word, terminate the algorithm. Otherwise check if ''w''<sub>''j''</sub> contains a subword ''v'' such that ''v'' is also a subword of some defining relator ''r'' = vu ∈ ''R'' such that |''v''| > |''r''|/2. If no, terminate the algorithm with output ''w''<sub>''j''</sub>. If yes, replace ''v'' by ''u''<sup>−1</sup> in ''w''<sub>''j''</sub>, then freely reduce, denote the resulting freely reduced word by ''w''<sub>''j''+1</sub>and go to the next step of the algorithm.
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| Note that we always have
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| :|''w''<sub>0</sub>| > |''w''<sub>1</sub>| > |''w''<sub>2</sub>| >...
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| which implies that the process must terminate in at most |''w''| steps. Moreover all the words ''w''<sub>''j''</sub> represent the same element of ''G'' as does ''w'' and hence if the process terminates with the empty word, then ''w'' represents the identity element of ''G''.
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| One says that for a symmetrized presentation (∗) ''Dehn's algorithm solves the [[Word problem for groups|word problem]] in'' ''G'' if the converse is also true, that is if for any freely reduced word ''w'' in ''F''(''X'') this word represents the identity element of ''G'' '''if and only if''' Dehn's algorithm, starting from ''w'', terminates in the empty word.
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| Greendlinger's lemma implies that for a C'(1/6) presentation Dehn's algorithm solves the word problem.
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| If a C'(1/6) presentation (∗) is finite (that is both ''X'' and ''R'' are finite), then Dehn's algorithm is an actual [[Nondeterministic algorithm|non-deterministic]] [[algorithm]] in the sense of [[recursion theory]]. However, even if (∗) is an infinite C'(1/6) presentation, Dehn's algorithm, understood as an abstract procedure, still correctly decides whether or not a word in the generators ''X''<sup>±1</sup> represents the identity element of ''G''.
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| ===Asphericity===
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| Let (∗) be a C'(1/6) or, more generally, C(6) presentation where every ''r'' ∈ ''R'' is not a proper power in ''F''(''X'') then ''G'' is [[Aspherical space|aspherical]] in the following sense. Consider a minimal subset ''S'' of ''R'' such that the symmetrized closure of ''S'' is equal to ''R''. Thus if ''r'' and ''s'' are distinct elements of ''S'' then ''r'' is not a cyclic permutation of ''s''<sup>±1</sup> and <math>G=\langle X|S\rangle </math> is another presentation for ''G''. Let ''Y'' be the [[presentation complex]] for this presentation. Then (see <ref>Ian M. Chiswell, Donald J. Collins, Johannes Huebschmann, ''Aspherical group presentations''.
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| [[Mathematische Zeitschrift]], vol. 178 (1981), no. 1, pp. 1–36.</ref> and Theorem 13.3 in <ref name="Ol"/>), under the above assumptions on (∗), ''Y'' is a [[classifying space]] for ''G'', that is ''G'' = ''π''<sub>1</sub>(''Y'') and the [[universal cover]] of ''Y'' is [[contractible space|contractible]]. In particular, this implies that ''G'' is torsion-free and has [[cohomological dimension]] two.
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| ===More general curvature===
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| More generally, it is possible to define various sorts of local "curvature" on any van Kampen diagram to be - very roughly - the average excess of vertices + faces - edges (which, by Euler's formula, must total 2) and, by showing, in a particular group, that this is always non-positive (or - even better - negative) internally, show that the curvature must all be on or near the boundary and thereby try to obtain a solution of the word problem. Furthermore one can restrict attention to diagrams that do not contain any of a set of "regions" such that there is a "smaller" region with the same boundary.
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| ===Other basic properties of small cancellation groups===
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| *Let (∗) be a C'(1/6) presentation. Then an element ''g'' in ''G'' has order ''n'' > 1 if and only if there is a relator ''r'' in ''R'' of the form ''r'' = ''s''<sup>''n''</sup> in ''F''(''X'') such that ''g'' is [[conjugacy class|conjugate]] to ''s'' in ''G''. In particular, if all elements of ''R'' are not proper powers in ''F''(''X'') then ''G'' is torsion-free.
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| *If (∗) is a finite C'(1/6) presentation, the group ''G'' is [[word-hyperbolic group|word-hyperbolic]].
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| *If ''R'' and ''S'' are finite symmetrized subsets of ''F''(''X'') with equal [[Normal closure (group theory)|normal closures]] in ''F''(''X'') such that both presentations <math>\langle X|R\rangle </math> and <math>\langle X|S\rangle </math> satisfy the C'(1/6) condition then ''R'' = ''S''.
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| *If a finite presentation (∗) satisfies one of C'(1/6), C'(1/4)–T(4), C(6), C(4)–T(4), C(3)–T(6) then the group ''G'' has solvable [[Word problem for groups|word problem]] and solvable [[conjugacy problem]]
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| ==Applications==
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| Examples of applications of small cancellation theory include:
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| *Solution of the [[conjugacy problem]] for groups of [[alternating knot]]s (see<ref>C. M. Weinbaum, ''The word and conjugacy problems for the knot group of any tame, prime, alternating knot.'' [[Proceedings of the American Mathematical Society]], vol. 30 (1971), pp. 22–26.</ref><ref>K. I. Appel, P. E. Schupp, ''The conjugacy problem for the group of any tame alternating knot is solvable.'' [[Proceedings of the American Mathematical Society]], vol. 33 (1972), pp. 329–336.</ref> and Chapter V, Theorem 8.5 in <ref name="LS">[[Roger Lyndon|Roger C. Lyndon]] and Paul Schupp, [http://books.google.com/books?id=aiPVBygHi_oC&pg=PR3&dq=Roger+Lyndon+and+Paul+Schupp,+Combinatorial+group+theory.+Reprint+of+the+1977+edition ''Combinatorial group theory''.] Reprint of the 1977 edition. Classics in Mathematics. [[Springer-Verlag]], Berlin, 2001. ISBN 3-540-41158-5.</ref>), via showing that for such knots augmented knot groups admit C(T)–T(4) presentations.
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| *Finitely presented C'(1/6) small cancellation groups are basic examples of [[word-hyperbolic group]]s. One of the equivalent characterizations of word-hyperbolic groups is as those admitting finite presentations where Dehn's algorithm solves the [[Word problem for groups|word problem]].
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| *Finitely presented groups given by finite C(4)–T(4) presentations where every piece has length one are basic examples of [[CAT(0) group]]s: for such a presentation the [[universal cover]] of the [[presentation complex]] is a [[CAT(k) space|CAT(0)]] square complex.
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| *Early applications of small cancellation theory involve obtaining various embeddability results. Examples include a 1974 paper<ref>George S. Sacerdote and Paul E. Schupp, ''SQ-universality in HNN groups and one relator groups.'' [[Journal of the London Mathematical Society]] (2), vol. 7 (1974), pp. 733–740.</ref> of Sacerdote and Schupp with a proof that every one-relator group with at least three generators is [[SQ universal group|SQ-universal]] and a 1976 paper of Schupp<ref>Paul E. Schupp, ''Embeddings into simple groups.'' [[Journal of the London Mathematical Society]] (2), vol. 13 (1976), no. 1, pp. 90–94.</ref> with a proof that every countable group can be embedded into a [[simple group]] generated by an element of order two and an element of order three.
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| *The so-called ''Rips construction'', due to [[Eliyahu Rips]],<ref>E. Rips, ''Subgroups of small cancellation groups''. [[Bulletin of the London Mathematical Society]], vol. 14 (1982), no. 1, pp. 45–47.</ref> provides a rich source of counter-examples regarding various [[subgroup]] properties of [[word-hyperbolic group]]s: Given an arbitrary finitely presented group ''Q'', the construction produces a [[short exact sequence]] <math>1\to K\to G\to Q\to 1</math> where ''K'' is two-generated and where ''G'' is torsion-free and given by a finite C'(1/6)-presentation (and thus ''G'' is word-hyperbolic). The construction yields proofs of unsolvability of several algorithmic problems for [[word-hyperbolic group]]s, including the subgroup membership problem, the generation problem and the [[rank of a group|rank problem]].<ref>G. Baumslag, C. F. Miller, H. Short, [http://blms.oxfordjournals.org/cgi/content/abstract/26/1/97 ''Unsolvable problems about small cancellation and word hyperbolic groups''.] [[Bulletin of the London Mathematical Society]], vol. 26 (1994), no. 1, pp. 97–101.</ref> Also, with a few exceptions, the group ''K'' in the Rips construction is not [[finitely presented group|finitely presentable]]. This implies that there exist word-hyperbolic groups that are not ''coherent'' that is which contain subgroups that are finitely generated but not finitely presentable.
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| *Small cancellation methods (for infinite presentations) were used by Ol'shanskii<ref name="Ol"/> to construct various "monster" groups, including the [[Tarski monster group|Tarski monster]] and also to give a proof that [[free Burnside group]]s of large odd exponent are infinite (a similar result was originally proved by Adian and Novikov in 1968 using more combinatorial methods). Some other "monster" groups constructed by Ol'shanskii using this methods include: an infinite [[simple group|simple]] [[Noetherian group]]; an infinite group in which every proper subgroup has prime order and any two subgroups of the same order are conjugate; a [[amenable group|nonamenable group]] where every proper subgroup is cyclic; and others.<ref>A. Yu. Olʹshanskii,
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| ''On a geometric method in the combinatorial group theory''. Proceedings of the International Congress of Mathematicians, Vol. 1, 2 (Warsaw, 1983), 415–424, PWN–Polish Scientific Publishers, Warsaw; North-Holland Publishing Co., Amsterdam, 1984. ISBN 83-01-05523-5.</ref>
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| *[[Brian Bowditch|Bowditch]]<ref>B. H. Bowditch, ''Continuously many quasi-isometry classes of 2-generator groups.'' [[Commentarii Mathematici Helvetici]], vol. 73 (1998), no. 2, pp. 232–236.</ref> used infinite small cancellation presentations to prove that there exist continuumly many [[quasi-isometry|quasi-isometry types]] of two-generator groups.
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| *Thomas and Velickovic used small cancellation theory to construct<ref>S. Thomas and B. Velickovic. ''Asymptotic cones of finitely generated groups''.
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| [[Bulletin of the London Mathematical Society]], vol. 32 (2000), no. 2, pp. 203–208.</ref> a finitely generated group with two non-homeomorphic asymptotic cones, thus answering a question of [[Mikhail Gromov (mathematician)|Gromov]].
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| *McCammond and Wise showed how to overcome difficulties posed by the Rips construction and produce large classes of small cancellation groups that are ''coherent'' (that is where all finitely generated subgroups are finitely presented) and, moreover, locally quasiconvex (that is where all finitely generated subgroups are quasiconvex).<ref>Jonathan P. McCammond and Daniel T. Wise, ''Coherence, local quasiconvexity, and the perimeter of 2-complexes.'' [[Geometric and Functional Analysis]], vol. 15 (2005), no. 4, pp. 859–927.</ref><ref>Jonathan P. McCammond and Daniel T. Wise, ''Locally quasiconvex small-cancellation groups.'' [[Transactions of the American Mathematical Society]], vol. 360 (2008), no. 1, pp. 237–271.</ref>
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| *Small cancellation methods play a key role in the study of various models of "generic" or [[Random group|"random" finitely presented groups]] (see <ref>Yann Ollivier,
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| ''A January 2005 invitation to random groups.''
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| Ensaios Matemáticos [Mathematical Surveys], 10. Sociedade Brasileira de Matemática, Rio de Janeiro, 2005. ISBN 85-85818-30-1.</ref>). In particular, for a fixed number ''m'' ≥ 2 of generators and a fixed number ''t'' ≥ 1 of defining relations and for any ''λ'' < 1 a ''random'' ''m''-generator ''t''-relator group satisfies the C'(''λ'') small cancellation condition. Even if the number of defining relations ''t'' is not fixed but grows as (2''m''−1)<sup>''εn''</sup> (where ''ε'' ≥ 0 is the fixed ''density'' parameter in Gromov's density model of "random" groups, and where <math>n\to\infty</math> is the length of the defining relations), then an ''ε''-random group satisfies the C'(1/6) condition provided ''ε'' < 1/12.
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| *[[Mikhail Gromov (mathematician)|Gromov]]<ref name="Gromov"/> used a version of small cancellation theory with respect to a graph to prove the existence of a [[finitely presented group]] that "contains" (in the appropriate sense) an infinite sequence of [[expander graph|expanders]] and therefore does not admit a uniform embedding into a [[Hilbert space]]. This result provides a direction (the only one available so far) for looking for counter-examples to the [[Novikov conjecture]].
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| *Osin<ref name="Osin"/> used a generalization of small cancellation theory to obtain an analog of [[Hyperbolic Dehn surgery|Thurston's hyperbolic Dehn surgery theorem]] for [[relatively hyperbolic group]]s.
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| ==Generalizations==
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| *A version of small cancellation theory for quotient groups of [[free product|amalgamated free products]] and [[HNN extension]]s was developed in the paper of Sacerdote and Schupp and then in the book of Lyndon and Schupp.<ref name="LS"/>
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| *Ol'shanskii<ref name="Ol"/> developed a "stratified" version of small cancellation theory where the set of relators is filtered as an ascending union of stata (each stratum satisfying a small cancellation condition) and for a relator ''r'' from some statum and a relator ''s'' from a higher stratum their overlap is required to be small with respect to |''s''| but is allowed to have a large with respect to |''r''|. This theory allowed Ol'shanskii to construct various "moster" groups including the [[Tarski monster group|Tarski monster]] and to give a new proof that [[free Burnside group]]s of large odd exponent are infinite.
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| *Ol'shanskii<ref>A. Yu. Olʹshanskii,
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| ''On residualing homomorphisms and G-subgroups of hyperbolic groups.''
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| International Journal of Algebra and Computation, vol. 3 (1993), no. 4, pp. 365–409.</ref> and Delzant<ref>Thomas Delzant, ''Sous-groupes distingués et quotients des groupes hyperboliques.'' [Distinguished subgroups and quotients of hyperbolic groups] [[Duke Mathematical Journal]], vol. 83 (1996), no. 3, pp. 661–682.</ref> later on developed versions of small cancellation theory for quotients of [[word-hyperbolic group]]s.
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| *McCammond provided a higher-dimensional version of small cancellation theory.<ref>Jonathan P. McCammond, ''A general small cancellation theory''. International Journal of Algebra and Computation, vol. 10 (2000), no. 1, pp. 1–172.</ref>
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| *McCammond and Wise pushed substantially further the basic results of the standard small cancellation theory (such as Greendlinger's lemma) regarding the geometry of [[van Kampen diagram]]s over small cancellation presentations.<ref>Jonathan P. McCammond and Daniel T. Wise, [http://plms.oxfordjournals.org/cgi/content/short/84/3/599 ''Fans and ladders in small cancellation theory.''] [[Proceedings of the London Mathematical Society]] (3), vol. 84 (2002), no. 3, pp. 599–644.</ref>
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| *[[Mikhail Gromov (mathematician)|Gromov]] used a version of ''small cancellation theory with respect to a graph'' to prove<ref name="Gromov">M. Gromov, [http://www.springerlink.com/content/b0jjpb1ld65dyxtd/ ''Random walk in random groups.''] [[Geometric and Functional Analysis]], vol. 13 (2003), no. 1, pp. 73–146.</ref> the existence of a finitely presented group that "contains" (in the appropriate sense) an infinite sequence of expanders and therefore does not admit a uniform embedding into a [[Hilbert space]]. See also <ref>Yann Ollivier, [http://projecteuclid.org/DPubS?service=UI&version=1.0&verb=Display&handle=euclid.bbms/1148059334 ''On a small cancellation theorem of Gromov''.] Bulletin of the Belgian Mathematical Society. Simon Stevin. vol. 13 (2006), no. 1, pp. 75–89.</ref> for more details on small cancellation theory with respect to a graph.
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| *Osin<ref name="Osin">Denis V. Osin, ''Peripheral fillings of relatively hyperbolic groups.'' [[Inventiones Mathematicae]], vol. 167 (2007), no. 2, pp. 295–326.</ref> gave a version of small cancellation theory for quotiens of [[relatively hyperbolic group]]s and used it to obtain a relatively hyperbolic generalization of [[Hyperbolic Dehn surgery|Thurston's hyperbolic Dehn surgery theorem]].
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| ==Basic references==
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| *Roger Lyndon and Paul Schupp, [http://books.google.com/books?id=aiPVBygHi_oC&printsec=frontcover&dq=lyndon+and+schupp ''Combinatorial group theory''.] Reprint of the 1977 edition. Classics in Mathematics. [[Springer-Verlag]], Berlin, 2001. ISBN 3-540-41158-5.
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| *Alexander Yu. Olʹshanskii, ''Geometry of defining relations in groups.'' Translated from the 1989 Russian original by Yu. A. Bakhturin. Mathematics and its Applications (Soviet Series), 70. Kluwer Academic Publishers Group, Dordrecht, 1991. ISBN 0-7923-1394-1.
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| *Ralph Strebel, ''Appendix. Small cancellation groups.'' Sur les groupes hyperboliques d'après Mikhael Gromov (Bern, 1988), pp. 227–273, Progress in Mathematics, 83, Birkhäuser Boston, Boston, MA, 1990. ISBN 0-8176-3508-4.
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| *Milé Krajčevski, [http://books.google.com/books?id=x6QfX8pLlSwC&pg=PP1&dq=Kraj%C4%8Devski,+%27%27Tilings+of+the+plane,+hyperbolic+groups+and+small+cancellation+conditions%27%27 ''Tilings of the plane, hyperbolic groups and small cancellation conditions.''] Memoirs of the American Mathematical Society, vol. 154 (2001), no. 733.
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| ==Notes==
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| {{reflist}}
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| ==See also==
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| *[[Geometric group theory]]
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| *[[Word-hyperbolic group]]
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| *[[Tarski monster group]]
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| *[[Burnside problem]]
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| *[[Finitely presented group]]
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| *[[Word problem for groups]]
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| *[[Van Kampen diagram]]
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| [[Category:Group theory]]
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| [[Category:Geometric group theory]]
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| [[Category:Combinatorics on words]]
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