New 3-designs and 2-designs having U(3,3) as an automorphism group

In this paper we classify $2$-designs and $3$-designs with $28$ or $36$ points admitting a transitive action of the unitary group $U(3,3)$ on points and blocks. We also construct $2$-designs and $3$-designs with $56$ or $63$ points and strongly regular graphs on 36, 63 or 126 vertices having $U(3,3)$ as a transitive automorphism group. Further, we show that this completes the classification of $3$-designs admitting a transitive action of the group $U(3,3)$, in terms of parameters. A number of the $3$-designs and $2$-designs obtained in this paper have not been known before up to our best knowledge.     

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.     

New 5-designs with automorphism group PSL(2, 23)

Blocks of the unique Steiner system S(5, 8, 24) are called octads. The group PSL(2, 23) acts as an automorphism group of this Steiner system, permuting octads transitively. Inspired by the discovery of a 5-(24, 10, 36) design by Gulliver and Harada, we enumerate all 4- and 5-designs whose set of blocks are union of PSL(2, 23)-orbits on 10-subsets containing an octad. © 1999 John Wiley & Sons, Inc. J Combin Designs 7: 147–155, 1999     

Uniform and natural existence proofs for Janko‚s sporadic groups J2 and J3

In this article both sporadic Janko-groups J₂ and J₃ are constructed from their common involution centralizer H @ 2¹⁺⁴ H \cong 2^{1+4} : A₅ within one single run of Michler‘s deterministic algorithm described in [11]. At the beginning we choose the -- in some sense -- most natural point to start from, and in the end we realize that Michler‘s algorithm does not necessary lead us to a simple group but sometimes to a covering group of a simple group.     

Symmetric presentation of the Janko group J4

We demonstrate how readily a definition of the largest Jankogroup J₄ follows from a primitive action of the Mathieu groupM₂₄ by exhibiting J₄ as an image of the progenitor 2^*3795:M₂₄.This symmetric presentation is converted into an ordinary presentationon three generators.     

Representing the automorphism group of an almost crystallographic group

Let be an almost crystallographic (AC-) group, corresponding to the simply connected, connected, nilpotent Lie group and with holonomy group . If , there is a faithful representation . In case is crystallographic, this condition is known to be equivalent to or . We will show (Example 2.2) that, for AC-groups , this is no longer valid and should be adapted. A generalised equivalent algebraic (and easier to verify) condition is presented (Theorem 2.3). Corresponding to an AC-group and by factoring out subsequent centers we construct a series of AC-groups, which becomes constant after a finite number of terms. Under suitable conditions, this opens a way to represent faithfully in (Theorem 4.1). We show how this can be used to calculate . This is of importance, especially, when is almost Bieberbach and, hence, is known to have an interesting geometric meaning.

     

B2×B2的全纯自同构群

讨论B2×B2上的全纯自同构群,并计算它的Bergman核函数及其自同构的Jacobi行列式.     

圈的自同结构群的阶

n个顶点的圈Cn的自同结构群的阶是2n.     

Graded automorphism group of TKK algebra

The classification of extended affine Lie algebras of type A_1 depends on the Tits-Kantor- Koecher (TKK) algebras constructed from semilattices of Euclidean spaces.One can define a unitary Jordan algebra J(S) from a semilattice S of R~v (v≥1),and then construct an extended affine Lie algebra of type A_1 from the TKK algebra T(J(S)) which is obtained from the Jordan algebra J(S) by the so-called Tits-Kantor-Koecher construction.In R~2 there are only two non-similar semilattices S and S′,where S is a lattice and S′is a non-lattice semilattice.In this paper we study the Z~2-graded automorphisms of the TKK algebra T(J(S)).     

On the order of a group of even order

We present an upper bound for |G| of a group G of even order possessing a unique conjugacy class of involutions.     

Regular representations of finite groups as automorphism groups of k‐tournaments

It was shown by Babai and Imrich [2] that every finite group of odd order except and admits a regular representation as the automorphism group of a tournament. Here, we show that for k ≥ 3, every finite group whose order is relatively prime to and strictly larger than k admits a regular representation as the automorphism group of a k‐tournament. Our constructions are elementary, suggesting that the problem is significantly simpler for k‐tournaments than for binary tournaments. © 2002 Wiley Periodicals, Inc. J Graph Theory 41: 238–248, 2002     

A minimal permutation representation of the finite simple Janko group J4

Translated from Matematicheskie Zametki, Vol. 47, No. 1, pp. 137–146, January, 1990.