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Publications and source records attributed to Victor Guba.
This is a survey of our recent results on the amenability problem for Thompson's group $F$. They mostly concern esimating the density of finite subgraphs in Cayley graphs of $F$ for various systems of generators, and also equations in the group ring of $F$. We also discuss possible approaches to solve the problem in both directions.
A (discrete) group is called amenable whenever there exists a finitely additive right invariant probablity measure on it. For Thompson's group $F$ the problem whether it is amenable is a long-standing open question. We consider presentation of $F$ in terms of non-spherical semigroup diagrams. There is a natural partition of $F$ into 7 parts in terms of these diagrams. We show that for any measure with the above properties on $F$, all but one of these sets have zero measure. This helps to clarify the structure of Folner sets in $F$ provided the group is amenable.
We improve some known estimates for the density of the Cayley graph of Thompson's group F in standard generators.
Let $R=K[G]$ be a group ring of a group $G$ over a field $K$. It is known that if $G$ is amenable then $R$ satisfies the Ore condition: for any $a,b\in R$ there exist $u,v\in R$ such that $au=bv$, where $u\ne0$ or $v\ne0$. It is also true for amenable groups that a non-zero solution exists for any finite system of linear equations over $R$, where the number of unknowns exceeds the number of equations. Recently Bartholdi proved the converse. As a consequence of this theorem, Kielak proved that R.\,Thompson's group $F$ is amenable if and only if it satisfies the Ore condition. The amenability problem for $F$ is a long-standing open question. In this paper we prove that some equations or their systems have non-zero solutions in the group rings of $F$. We improve some results by Donnelly showing that there exist finite sets $Y\subset F$ with the property $|AY| < \frac43|Y|$, where $A=\{x_0,x_1,x_2\}$. This implies some result on the systems of equations. We show that for any element $b$ in the group ring of $F$, the equation $(1-x_0)u=bv$ has a non-zero solution. The corresponding fact for $1-x_1$ instead of $1-x_0$ remains open. We deduce that for any $m\ge1$ the system $(1-x_0)u_0=(1-x_1)u_1=\cdots=(1-x_m)u_m$ has nonzero solutions in the group ring of $F$. We also analyze the equation $(1-x_0)u=(1-x_1)v$ giving a precise explicit description of all its solutions in $K[F]$. This is important since to any group relation between $x_0$, $x_1$ in $F$ one can naturally assign such a solution. So this can help to estimate the number of relations of a given length between generators.
In this paper we introduce a concept of an evacuation scheme on the Cayley graph of an infinite finitely generated group. This is a collection of infinite simple paths bringing all vertices to infinity. We impose a restriction that every edge can be used a uniformly bounded number of times in this scheme. An easy observation shows that existing of such a scheme is equivalent to non-amenability of the group. A special case happens if every edge can be used only once. These scheme are called pure. We obtain a criterion for existing of such a scheme in terms of isoperimetric constant of the graph. We analyze R.\,Thompson's group $F$, for which the amenability property is a famous open problem. We show that pure evacuation schemes do not exist for the set of generators $\{x_0,x_1,\bar{x}_1\}$, where $\bar{x}_1=x_1x_0^{-1}$. However, the question becomes open if edges with labels $x_0^{\pm1}$ can be used twice. Existing of pure evacuation scheme for this version is implied by some natural conjectures.
Let $R=K[G]$ be a group ring of a group $G$ over a field $K$. The Ore condition says that for any $a,b\in R$ there exist $u,v\in R$ such that $au=bv$, where $u\ne0$ or $v\ne0$. It always holds whenever $G$ is amenable. Recently it was shown that for R.\,Thompson's group $F$ the converse is also true. So the famous amenability problem for $F$ is equivalent to the question on the Ore condition for the group ring of the same group. It is easy to see that the problem on the Ore condition for $K[F]$ is equivalent to the same property for the monoid ring $K[M]$, where $M$ is the monoid of positive elements of $F$. In this paper we reduce the problem to the case when $a$, $b$ are homogeneous elements of the same degree in the monoid ring. We study the case of degree $1$ and find solutions of the Ore equation. For the case of degree $2$, we study the case of linear combinations of monomials from $S=\{x_0^2,x_0x_1,x_0x_2,x_1^2,x_1x_2\}$. This set is not doubling, that is, there are nonempty finite subsets $X\subset M\subset F$ such that $|SX| < 2|X|$. As a consequence, the Ore condition holds for linear combinations of these monomials. We give an estimate for the degree of $u$, $v$ in the above equation. The case of monomials of higher degree is open as well as the case of degree $2$ for monomials on $x_0,x_1,...,x_m$, where $m\ge3$. Recall that negative answer to any of these questions will immediately imply non-amenability of $F$.
Let $M$ be a cancellative monoid. It is known~\cite{Ta54} that if $M$ is left amenable then the monoid ring $K[M]$ satisfies Ore condition, that is, there exist nontrivial common right multiples for the elements of this ring. In~\cite{Don10} Donnelly shows that a partial converse to this statement is true. Namely, if the monoid $\mathbb Z^{+}[M]$ of all elements of $\mathbb Z[M]$ with positive coefficients has nonzero common right multiples, then $M$ is left amenable. He asks whether the converse is true for this particular statement. We show that the converse is false even for the case of groups. If $M$ is a free metabelian group, then $M$ is amenable but the Ore condition fails for $\mathbb Z^{+}[M]$. Besides, we study the case of the monoid $M$ of positive elements of R.\,Thompson's group $F$. The amenability problem for it is a famous open question. It is equivalent to left amenability of the monoid $M$. We show that for this case the monoid $\mathbb Z^{+}[M]$ does not satisfy Ore condition. That is, even if $F$ is amenable, this cannot be shown using the above sufficient condition.
We answer the question by Matt Brin on the structure of diagram groups over semigroup presentation ${\mathcal P}=\langle a,b,c\mid a=bc,b=ca,c=ab\rangle$. In the talk on Oberwolfach workshop, Brin conjectured that the diagram group over $\mathcal P$ with base $a$ is isomorphic to the generalized Thompson's group $F_9$. We confirm this conjecture and consider some generalizations of this fact.
By the density of a finite graph we mean its average vertex degree. For an $m$-generated group, the density of its Cayley graph in a given set of generators, is the supremum of densities taken over all its finite subgraphs. It is known that a group with $m$ generators is amenable iff the density of the corresponding Cayley graph equals $2m$. A famous problem on the amenability of R.\,Thompson's group $F$ is still open. What is known due to the result by Belk and Brown, is that the density of its Cayley graph in the standard set of group generators $\{x_0,x_1\}$, is at least $3.5$. This estimate has not been exceeded so far. For the set of symmetric generators $S=\{x_1,\bar{x}_1\}$, where $\bar{x}_1=x_1x_0^{-1}$, the same example gave the estimate only $3$. There was a conjecture that for this generating set the equality holds. If so, $F$ would be non-amenable, and the symmetric generating set had doubling property. This means that for any finite set $X\subset F$, the inequality $|S^{\pm1}X|\ge2|X|$ holds. In this paper we disprove this conjecture showing that the density of the Cayley graph of $F$ in symmetric generators $S$ strictly exceeds $3$. Moreover, we show that even larger generating set $S_0=\{x_0,x_1,\bar{x}_1\}$ does not have doubling property.
To every finitely generated group one can assign the conjugacy growth function that counts the number of conjugacy classes intersecting a ball of radius $n$. Results of Ivanov and Osin show that the conjugacy growth function may be constant even if the (ordinary) growth function is exponential. The aim of this paper is to provide conjectures, examples and statements that show that in "normal" cases, groups with exponential growth functions also have exponential conjugacy growth functions.
Let $F(p)$, $p\ge2$ be the family of generalized Thompson's groups. Here F(2) is the famous Richard Thompson's group usually denoted by $F$. We find the growth rate of the monoid of positive words in $F(p)$ and show that it does not exceed $p+1/2$. Also we describe new normal forms for elements of $F(p)$ and, using these forms, we find a lower bound for the growth rate of $F(p)$ in its natural generators. This lower bound asymptotically equals $(p-1/2)\log_2 e+1/2$ for large values of $p$.
Let $G$ be a group generated by a finite set $A$. An element $g\in G$ is a strict dead end of depth $k$ (with respect to $A$) if $|g|>|ga_1|>|ga_1a_2|>...>|ga_1a_2... a_k|$ for any $a_1,a_2, ..., a_k\in A^{\pm1}$ such that the word $a_1a_2... a_k$ is freely irreducible. (Here $|g|$ is the distance from $g$ to the identity in the Cayley graph of $G$.) We show that in finitely generated free soluble groups of degree $d\ge2$ there exist strict dead elements of depth $k=k(d)$, which grows exponentially with respect to $d$.
Recently Akhmedov introduced a property of finitely generated groups called the Traveller Salesman Property (TSP). It was shown by Thurston that TSP implies non-amenability. Akhmedov conjectured that R. Thompson's group $F$ has TSP. The aim of this article is to disprove this conjecture.
We give first examples of finitely generated groups having an intermediate, with values in (0,1), Hilbert space compression (which is a numerical parameter measuring the distortion required to embed a metric space into Hilbert space). These groups include certain diagram groups. In particular, we show that the Hilbert space compression of Richard Thompson's group $F$ is equal to 1/2, the Hilbert space compression of the restricted wreath product $Z\wr Z$ is between 1/2 and 3/4, and the Hilbert space compression of $Z\wr (Z\wr Z)$ is between 0 and 1/2. In general, we find a relationship between the growth of $H$ and the Hilbert space compression of $Z\wr H$.
Let $F_m$ be a free group with $m$ generators and let $R$ be its normal subgroup such that $F_m/R$ projects onto $\zz$. We give a lower bound for the growth rate of the group $F_m/R'$ (where $R'$ is the derived subgroup of $R$) in terms of the length $ρ=ρ(R)$ of the shortest nontrivial relation in $R$. It follows that the growth rate of $F_m/R'$ approaches $2m-1$ as $ρ$ approaches infinity. This implies that the growth rate of an $m$-generated amenable group can be arbitrarily close to the maximum value $2m-1$. This answers an open question by P. de la Harpe. In fact we prove that such groups can be found already in the class of abelian-by-nilpotent groups as well as in the class of finite extensions of metabelian groups.
In this paper, we introduce the concept of the independence graph of a directed 2-complex. We show that the class of diagram groups is closed under graph products over independence graphs of rooted 2-trees. This allows us to show that a diagram group containing all countable diagram groups is a semi-direct product of a partially commutative group and R. Thompson's group $F$. As a result, we prove that all diagram groups are totally orderable.
We prove that the Dehn function (that is, the smallest isoperimetric function) of the Richard Thompson's group F is quadratic.
We study some properties of the Cayley graph of the R.Thompson's group F in generators $x_0$, $x_1$. We show that the density of this graph, that is, the least upper bound of the average vertex degree of its finite subgraphs is at least 3. It is known that a 2-generated group is not amenable if and only if the density of the corresponding Cayley graph is strictly less than 4. It is well known this is also equivalent to the existence of a doubling function on the Cayley graph. This means there exists a mapping from the set of vertices into itself such that for some constant $K>0$, each vertex moves into the distance at most K and each vertex has at least two preimages. We show that the density of the Cayley graph of a 2-generated graph does not exceed 3 if and only if the group satisfies the above condition with K=1. Besides, we give a very easy formula to find the length (norm) of a given element of F in generators $x_0$, $x_1$. This simplifies the algorithm by Fordham. The length formula may be useful to find the general growth function of F in generators $x_0$, $x_1$ and the growth rate of this function. In this paper we show that the lower bound for the growth rate of F is $(3+\sqrt5)/2$.