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Alireza Abdollahi

Publications and source records attributed to Alireza Abdollahi.

At least 37 records · Page 2Linked to original sources
arXiv2017

Zero divisor and unit elements with support of size 4 in group algebras of torsion free groups

Kaplansky Zero Divisor Conjecture states that if $G $ is a torsion free group and $ \mathbb{F} $ is a field, then the group ring $\mathbb{F}[G]$ contains no zero divisor and Kaplansky Unit Conjecture states that if $G $ is a torsion free group and $ \mathbb{F} $ is a field, then $\mathbb{F}[G]$ contains no non-trivial units. The support of an element $ α= \sum_{x\in G}α_xx$ in $\mathbb{F}[G] $, denoted by $supp(α)$, is the set $ \{x \in G|α_x\neq 0\} $. In this paper we study possible zero divisors and units with supports of size $ 4 $ in $\mathbb{F}[G]$. We prove that if $ α, β$ are non-zero elements in $ \mathbb{F}[G] $ for a possible torsion free group $ G $ and an arbitrary field $ \mathbb{F} $ such that $ |supp(α)|=4 $ and $ αβ=0 $, then $|supp(β)|\geq 7 $. In [J. Group Theory, $16$ $ (2013),$ no. $5$, $667$-$693$], it is proved that if $ \mathbb{F}=\mathbb{F}_2 $ is the field with two elements, $ G $ is a torsion free group and $ α,β\in \mathbb{F}_2[G]\setminus \{0\}$ such that $|supp(α)|=4 $ and $ αβ=0 $, then $|supp(β)|\geq 8$. We improve the latter result to $|supp(β)|\geq 9$. Also, concerning the Unit Conjecture, we prove that if $\mathsf{a}\mathsf{b}=1$ for some $\mathsf{a},\mathsf{b}\in \mathbb{F}[G]$ and $|supp(\mathsf{a})|=4$, then $|supp(\mathsf{b})|\geq 6$.

math.GR↗
arXiv2016

Kaplansky's zero divisor and unit conjectures on elements with supports of size $3$

Kaplansky's zero divisor conjecture (unit conjecture, respectively) states that for a torsion-free group $G$ and a field $\mathbb{F}$, the group ring $\mathbb{F}[G]$ has no zero divisors (has no unit with support of size greater than $1$). In this paper, we study possible zero divisors and units in $\mathbb{F}[G]$ whose supports have size $3$. For any field $\mathbb{F}$ and all torsion-free groups $G$, we prove that if $αβ=0$ for some non-zero $α, β\in \mathbb{F}[G]$ such that $|supp(α)|=3$, then $|supp(β)|\geq 10$. If $\mathbb{F}=\mathbb{F}_2$ is the field with 2 elements, the latter result can be improved so that $|supp(β)|\geq 20$. This improves a result in [J. Group Theory, 16 (2013), no. 5, 667-693]. Concerning the unit conjecture, we prove that if $αβ=1$ for some $α, β\in \mathbb{F}[G]$ such that $|supp(α)|=3$, then $|supp(β)|\geq 9$. The latter improves a part of a result in [Exp. Math., 24 (2015), 326-338] to arbitrary fields.

math.GR↗
arXiv2016

Frankl's Conjecture for subgroup lattices

We show that the subgroup lattice of any finite group satisfies Frankl's Union-Closed Conjecture. We show the same for all lattices with a modular coatom, a family which includes all supersolvable and dually semimodular lattices. A common technical result used to prove both may be of some independent interest.

math.CO↗
arXiv2016

Distance-regular Cayley graphs with least eigenvalue $-2$

We classify the distance-regular Cayley graphs with least eigenvalue $-2$ and diameter at most three. Besides sporadic examples, these comprise of the lattice graphs, certain triangular graphs, and line graphs of incidence graphs of certain projective planes. In addition, we classify the possible connection sets for the lattice graphs and obtain some results on the structure of distance-regular Cayley line graphs of incidence graphs of generalized polygons.

math.CO↗
arXiv2015

Groups all of whose undirected Cayley graphs are determined by their spectra

Let $G$ be a finite group, and $S$ be a subset of $G\setminus\{1\}$ such that $S=S^{-1}$. Suppose that $Cay(G,S)$ is the Cayley graph on $G$ with respect to the set $S$ which is the graph whose vertex set is $G$ and two vertices $a,b\in G$ are adjacent if and only if $ab^{-1}\in S$. The adjacency spectrum $Spec(Γ)$ of a graph $Γ$ is the multiset of eigenvalues of its adjacency matrix. A graph $Γ$ is called "determined by its spectrum" (or for short DS) whenever if a graph $Γ'$ has the same spectrum as $Γ$, then $Γ\cong Γ'$. We say that the group $G$ is DS (Cay-DS, respectively) whenever if $Γ$ is a Cayley graph over $G$ and $Spec(Γ)=Spec(Γ')$ for some graph (Cayley graph, respectively) $Γ'$, then $Γ\cong Γ'$. In this paper, we study finite DS groups and finite Cay-DS groups. In particular we prove that all finite DS groups are solvable and all Sylow $p$-subgroups of a finite DS group is cyclic for all $p\geq 5$. We also give several infinite families of non Cay-DS solvable groups. In particular we prove that there exist two cospectral non-isomorphic $6$-regular Cayley graphs on the dihedral group of order $2p$ for any prime $p\geq 13$.

math.CO↗
arXiv2014

Non-solvable groups generated by involutions in which every involution is left $2$-Engel

The following problem is proposed as Problem 18.57 in [The Kourovka Notebook, No. 18, 2014] by D. V. Lytkina: Let $G$ be a finite $2$-group generated by involutions in which $[x, u, u] = 1$ for every $x \in G$ and every involution $u \in G$. Is the derived length of $G$ bounded? The question is asked of an upper bound on the solvability length of finite $2$-groups generated by involutions in which every involution (not only the generators) is also left $2$-Engel. We negatively answer the question.

math.GR↗
arXiv2014

Groups all of whose undirected Cayley graphs are integral

Let $G$ be a finite group, $S\subseteq G\setminus\{1\}$ be a set such that if $a\in S$, then $a^{-1}\in S$, where $1$ denotes the identity element of $G$. The undirected Cayley graph $Cay(G,S)$ of $G$ over the set $S$ is the graph whose vertex set is $G$ and two vertices $a$ and $b$ are adjacent whenever $ab^{-1}\in S$. The adjacency spectrum of a graph is the multiset of all eigenvalues of the adjacency matrix of the graph. A graph is called integral whenever all adjacency spectrum elements are integers. Following Klotz and Sander, we call a group $G$ Cayley integral whenever all undirected Cayley graphs over $G$ are integral. Finite abelian Cayley integral groups are classified by Klotz and Sander as finite abelian groups of exponent dividing $4$ or $6$. Klotz and Sander have proposed the determination of all non-abelian Cayley integral groups. In this paper we complete the classification of finite Cayley integral groups by proving that finite non-abelian Cayley integral groups are the symmetric group $S_{3}$ of degree $3$, $C_{3} \rtimes C_{4}$ and $Q_{8}\times C_{2}^{n}$ for some integer $n\geq 0$, where $Q_8$ is the quaternion group of order $8$.

math.CO↗
arXiv2014

Connected graphs cospectral with a Friendship graph

Let $n$ be any positive integer, the friendship graph $F_n$ consist of $n$ edge-disjoint triangles that all of them meeting in one vertex. A graph $G$ is called cospectral with a graph $H$ if their adjacency matrices have the same eigenvalues. Recently in [http://arxiv.org/pdf/1310.6529v1.pdf] it is proved that if $G$ is any graph cospectral with $F_n$ $(n\neq 16)$, then $G\cong F_n$. In this note, we give a proof of special case of the latter: Any connected graph cospectral with $F_n$ is isomorphic to $F_n$. Our proof is independent of ones given in [http://arxiv.org/pdf/1310.6529v1.pdf] and the proofs are based on our recent results given in [Trans. Com., 2 no. 4 (2013) 37-52.] Using an upper bound for the largest eigenvalue of a connected graph given in [J. Combinatorial Theory, Ser. B, 81 (2001) 177-183.].

math.CO↗
arXiv2013

Graphs cospectral with a friendship graph or its complement

Let $n$ be any positive integer and let $F_n$ be the friendship (or Dutch windmill) graph with $2n+1$ vertices and $3n$ edges. Here we study graphs with the same adjacency spectrum as the $F_n$. Two graphs are called cospectral if the eigenvalues multiset of their adjacency matrices are the same. Let $G$ be a graph cospectral with $F_n$. Here we prove that if $G$ has no cycle of length 4 or 5, then $G\cong F_n$. Moreover if $G$ is connected and planar then $G\cong F_n$. All but one of connected components of $G$ are isomorphic to $K_2$. The complement $\bar{F_n}$ of the friendship graph is determined by its adjacency eigenvalues, that is, if $\bar{F_n}$ is cospectral with a graph $H$, then $H\cong \bar{F_n}$.

math.CO↗
arXiv2013

Non-commuting graphs of nilpotent groups

Let $G$ be a non-abelian group and $Z(G)$ be the center of $G$. The non-commuting graph $Γ_G$ associated to $G$ is the graph whose vertex set is $G\setminus Z(G)$ and two distinct elements $x,y$ are adjacent if and only if $xy\neq yx$. We prove that if $G$ and $H$ are non-abelian nilpotent groups with irregular isomorphic non-commuting graphs, then $|G|=|H|$.

math.GR↗
arXiv2011

Finite $p$-groups of class 3 with noninner automorphisms of order $p$

A longstanding conjecture asserts that every non-abelian finite $p$-group $G$ admits a non-inner automorphism of order $p$. The conjecture is valid for finite $p$-groups of class 2. Here, we prove every finite non-abelian $p$-group $G$ of class 3 with $p>2$ has a noninner automorphism of order $p$ leaving $Φ(G)$ elementwise fixed. We also prove that if $G$ is a finite 2-group of class 3 which cannot be generated by 4 elements, then $G$ has a non-inner automorphism of order 2 leaving $Φ(G)$ elementwise fixed. We also prove that the latter conclusion holds for finite 2-groups $G$ of class 3 such that the center of $G$ is not cyclic and the minimal number of generators of $G$ is 2 or 4 and it holds whenever the center of $G$ is {\em not} 2-generated and the minimal number of generators of $G$ is 3. Some results are also proved for the existence of non-inner automorphisms of order $p$ for a finite $p$-group $G$ under conditions in terms of the minimal number of generators of the center factor of $G$ and a certain function of the rank of $G$.

math.GR↗
arXiv2011

Groups generated by a finite Engel set

A subset $S$ of a group $G$ is called an Engel set if, for all $x,y\in S$, there is a non-negative integer $n=n(x,y)$ such that $[x,\,_n y]=1$. In this paper we are interested in finding conditions for a group generated by a finite Engel set to be nilpotent. In particular, we focus our investigation on groups generated by an Engel set of size two.

math.GR↗
arXiv2010

Engel Elements in Groups

We give a survey of results on the structure of right and left Engel elements of a group. We also present some new results in this topic.

math.GR↗
arXiv2009

Powerful $p$-groups have noninner automorphisms of order $p$ and some cohomology

In this paper we study the longstanding conjecture of whether there exists a noninner automorphism of order $p$ for a finite non-abelian $p$-group. We prove that if $G$ is a finite non-abelian $p$-group such that $G/Z(G)$ is powerful then $G$ has a noninner automorphism of order $p$ leaving either $Φ(G)$ or $Ω_1(Z(G))$ elementwise fixed. We also recall a connection between the conjecture and a cohomological problem and we give an alternative proof of the latter result for odd $p$, by showing that the Tate cohomology $H^n(G/N,Z(N))\not=0$ for all $n\geq 0$, where $G$ is a finite $p$-group, $p$ is odd, $G/Z(G)$ is $p$-central (i.e., elements of order $p$ are central) and $N\lhd G$ with $G/N$ non-cyclic.

math.GR↗
arXiv2009

Non-nilpotent graph of a group

We associate a graph $\mathcal{N}_{G}$ with a group $G$ (called the non-nilpotent graph of $G$) as follows: take $G$ as the vertex set and two vertices are adjacent if they generate a non-nilpotent subgroup. In this paper we study the graph theoretical properties of $\mathcal{N}_{G}$ and its induced subgraph on $G\backslash nil(G)$, where $nil(G)=\{x\in G | < x,y> \text{is nilpotent for all} y\in G\}$. For any finite group $G$, we prove that $\mathcal{N}_G$ has either $|Z^*(G)|$ or $|Z^*(G)|+1$ connected components, where $Z^*(G)$ is the hypercenter of $G$. We give a new characterization for finite nilpotent groups in terms of the non-nilpotent graph. In fact we prove that a finite group $G$ is nilpotent if and only if the set of vertex degrees of $\mathcal{N}_G$ has at most two elements.

math.GR↗