Characterization of the finite simple groups by spectrum and order

We give an affirmative answer to Question 12.39 in the Kourovka Notebook. Namely, it is proved that a finite simple group and a finite group having equal orders and same sets of element orders are isomorphic.     

Recognition of some simple groups by their noncommuting graphs

Let G be a nonabelian group and associate a noncommuting graph ∇(G) with G as follows: The vertex set of ∇(G) is G\Z(G) with two vertices x and y joined by an edge whenever the commutator of x and y is not the identity. Abdollahi et al. (J Algebra 298(2):468–492, 2006) put forward a conjecture called AAM’s Conjecture in as follows: If M is a finite nonabelian simple group and G is a group such that ∇(G) ≅ ∇(M), then G ≅ M. Even though this conjecture is well known to hold for all simple groups with nonconnected prime graphs and the alternating group A ₁₀ [see Darafsheh (Groups with the same non-commuting graph. Discrete Appl Math (2008) doi:), Wang and Shi (Commun Algebra 36(2):523–528, 2008)], it is still unknown for all simple groups with connected prime graphs except A ₁₀. In the present paper, we prove that this conjecture is also true for the projective special linear simple group L ₄(9). The new method used in this paper also works well in the cases L ₄(4), L ₄(7), U ₄(7), etc.     

Recognition of some simple groups by their noncommuting graphs

Let G be a nonabelian group and associate a noncommuting graph ?(G) with G as follows: The vertex set of ?(G) is G\Z(G) with two vertices x and y joined by an edge whenever the commutator of x and y is not the identity. Abdollahi et al. (J Algebra 298(2):468–492, 2006) put forward a conjecture called AAM’s Conjecture in as follows: If M is a finite nonabelian simple group and G is a group such that ?(G) ? ?(M), then G ? M. Even though this conjecture is well known to hold for all simple groups with nonconnected prime graphs and the alternating group A ₁₀ [see Darafsheh (Groups with the same non-commuting graph. Discrete Appl Math (2008) doi:10.1016/j.dam.2008.06.010), Wang and Shi (Commun Algebra 36(2):523–528, 2008)], it is still unknown for all simple groups with connected prime graphs except A ₁₀. In the present paper, we prove that this conjecture is also true for the projective special linear simple group L ₄(9). The new method used in this paper also works well in the cases L ₄(4), L ₄(7), U ₄(7), etc.     

Characterization of automorphism groups of sporadic simple groups

It is proved that all automorphism groups of the sporadic simple groups are characterized by their element orders and the group orders.     

On recognizability of some finite simple orthogonal groups by spectrum

It is proved that, if G is a finite group that has the same set of element orders as the simple group D _p (q), where p is prime, p ≥ 5 and q ∈ {2, 3, 5}, then the commutator group of G/F(G) is isomorphic to D _p (q), the subgroup F(G) is equal to 1 for q = 5 and to O _q (G) for q ∈ {2, 3}, F(G) ≤ G′, and |G/G′| ≤ 2.     

On recognizability of some finite simple orthogonal groups by spectrum

It is proved that, if G is a finite group that has the same set of element orders as the simple group D _p (q), where p is prime, p ≥ 5 and q ∈ {2, 3, 5}, then the commutator group of G/F(G) is isomorphic to D _p (q), the subgroup F(G) is equal to 1 for q = 5 and to O _q (G) for q ∈ {2, 3}, F(G) ≤ G′, and |G/G′| ≤ 2.     

Torsion units for some almost simple groups

We investigate the Zassenhaus conjecture regarding rational conjugacy of torsion units in integral group rings for certain automorphism groups of simple groups. Recently, many new restrictions on partial augmentations for torsion units of integral group rings have improved the effectiveness of the Luther-Passi method for verifying the Zassenhaus conjecture for certain groups. We prove that the Zassenhaus conjecture is true for the automorphism group of the simple group PSL(2, 11). Additionally we prove that the Prime graph question is true for the automorphism group of the simple group PSL(2, 13).     

On the existence of models in some sporadic simple groups

This work has been supported by the Deutsche Forschungsgemeinschaft and Carleton University, Ottawa.     

The products of conjugacy classes in some infinite simple groups

LetH _ν =S/S ^ν , whereS is the group of all permutations of a set of cardinality ? _ν andS ^v is its subgroup of permutations moving less than ? _ν elements. The infinite simple groupsH _ν ,ν>0, have covering number two; that is,C ²=H ^ν holds for each nonunit conjugacy classC[M]. Janko’s small groupJ ₁, the only finite simple group with covering number two, satisfies also: 1 $$C_{^1 } \subseteq C_{^2 } \cdot C_{^3 } for any nonunit classes C_{^1 } ,C_{^2 } ,C_{^3 } $$ . In fact,H _ν (ν>0) are the only groups of covering number two where (*) is known to fail. In this paper we determine arbitrary products of classes inH _ν (ν>0).     

Characterization of some finite groups by order and length of one conjugacy class

We study the possibility of characterizing S ∈ {²D_n(2), ²D_n+1(2)} by simple conditions when 2ⁿ⁺¹ > 5 is a prime. Furthermore, we will show that Thompson’s conjecture is valid under some weak condition for these groups.     

Spectral multiplicity of some stochastic processes

In this paper we consider the connection between the canonical and the weak-canonical representations for the given second-order stochastic process in a separable Hilbert space and we extend a well-known theorem of H. Cramer concerning sufficient conditions for a process to be of multiplicity one.

     

On the periodic groups saturated by direct products of finite simple groups

We prove the local finiteness of a periodic group G saturated by direct products of an elementary abelian 2-group of fixed order and the simple groups L ₂(q) under condition that G contains an element of order 4.     

The periodic groups saturated by finitely many finite simple groups

Denote by $\mathfrak{M}$ the set whose elements are the simple 3-dimensional unitary groups U ₃(q) and the linear groups L ₃(q) over finite fields. We prove that every periodic group, saturated by the groups of a finite subset of $\mathfrak{M}$ , is finite.