Geometry of Infinitely Presented Small Cancellation Groups and Quasi-homomorphisms

We study the geometry of infinitely presented groups satisfying the small cancellation condition $C^{\prime }(1/8)$, and introduce a standard decomposition (called the criss-cross decomposition) for the elements of such groups. Our method yields a direct construction of a linearly independent set of power continuum in the kernel of the comparison map between the bounded and the usual group cohomology in degree 2, without the use of free subgroups and extensions.

Random Subgroups of Acylindrically Hyperbolic Groups and Hyperbolic Embeddings

Let $G$ be an acylindrically hyperbolic group. We consider a random subgroup $H$ in $G$, generated by a finite collection of independent random walks. We show that, with asymptotic probability one, such a random subgroup $H$ of $G$ is a free group, and the semidirect product of $H$ acting on $E(G)$ is hyperbolically embedded in $G$, where $E(G)$ is the unique maximal finite normal subgroup of $G$. Furthermore, with control on the lengths of the generators, we show that $H$ satisfies a small cancellation condition with asymptotic probability one.

Infinitely presented small cancellation groups have the Haagerup property

We prove the Haagerup property (= Gromov's a-T-menability) for finitely generated groups defined by infinite presentations satisfying the C'(1/6)-small cancellation condition. We deduce that these groups are coarsely embeddable into a Hilbert space and that the strong Baum–Connes conjecture holds for them. The result is a first nontrivial advancement in understanding groups with such properties among infinitely presented non-amenable direct limits of hyperbolic groups. The proof uses the structure of a space with walls introduced by Wise. As the main step we show that C'(1/6)-complexes satisfy the linear separation property.

GRAPH SMALL CANCELLATION THEORY APPLIED TO ALTERNATING LINK GROUPS

We show that the Wirtinger presentation of a prime alternating link group satisfies a generalized small cancellation condition. This new version of Weinbaum's solution to the word and conjugacy problems for these groups easily extends to finite sums of alternating links.

SOLUTION OF THE MEMBERSHIP PROBLEM FOR CERTAIN RATIONAL SUBSETS OF ONE-RELATOR GROUPS WITH A SMALL CANCELLATION CONDITION

Let n ≥ 2 be a natural number, X = { x1,…, xn} and let F be the free group, freely generated by X. Let R be a cyclically reduced word in F such that its symmetric closure [Formula: see text] in F satisfies the small cancellation condition C′(1/5) & T(4). Let G be the group presented by [Formula: see text]. A Magnus subsemigroup of G is any subsemigroup of G generated by at most 2n - 1 elements of [Formula: see text]. In this paper we solve the Membership Problem for rational subsets of G which are contained in a Magnus subsemigroup of G, provided that [Formula: see text] satisfies certain combinatorial conditions. We use small cancellation theory with word combinatorics.

Generic properties of random subgroups of a free group for general distributions

International audience We consider a generalization of the uniform word-based distribution for finitely generated subgroups of a free group. In our setting, the number of generators is not fixed, the length of each generator is determined by a random variable with some simple constraints and the distribution of words of a fixed length is specified by a Markov process. We show by probabilistic arguments that under rather relaxed assumptions, the good properties of the uniform word-based distribution are preserved: generically (but maybe not exponentially generically), the tuple we pick is a basis of the subgroup it generates, this subgroup is malnormal and the group presentation defined by this tuple satisfies a small cancellation condition.

SOME SMALL CANCELLATION PROPERTIES OF RANDOM GROUPS

We work in the density model of random groups. We prove that they satisfy an isoperimetric inequality with sharp constant 1-2d depending upon the density parameter d. This implies in particular a property generalizing the ordinary C′ small cancellation condition, which could be termed "macroscopic cancellation". This also sharpens the evaluation of the hyperbolicity constant δ. As a consequence we get that the standard presentation of a random group at density d < 1/5 satisfies the Dehn algorithm and Greendlinger's lemma, and that it does not for d > 1/5. For this we establish a version of the local-global principle for hyperbolic spaces (Cartan–Hadamard–Gromov theorem) involving arbitrarily small loss in the isoperimetric constant.

On T(6)-groups

This paper is concerned with group presentations satisfying the small cancellation condition T(6). The definition of this condition is given in §1·2, together with some examples. Before giving the definition, however, we describe (in §1·1) some material which, to a certain extent, motivated our paper. In § 1·3 we state our main theorem, which provides new solutions to the word and conjugacy problems for T(6)-groups.