On Multiplicative Invariants of Finite Reflection Groups

This article focuses on two recent results on multiplicative invariants of finite reflection groups: Lorenz (2001 Lorenz , M. ( 2001 ). Multiplicative invariants and semigroup algebras . Alg. and Rep. Theory 4 : 293 – 304 . [Google Scholar]) showed that such invariants are affine normal semigroup algebras, and Reichstein (2003 Reichstein , Z. ( 2003 ). SAGBI bases in rings of multiplicative invariants . Commentarii Math. Helvetici 78 ( 1 ): 185 – 202 .[Web of Science ®] , [Google Scholar]) proved that the invariants have a finite SAGBI basis. Reichstein (2003 Reichstein , Z. ( 2003 ). SAGBI bases in rings of multiplicative invariants . Commentarii Math. Helvetici 78 ( 1 ): 185 – 202 .[Web of Science ®] , [Google Scholar]) also showed that, conversely, if the multiplicative invariant algebra of a finite group G has a SAGBI basis, then G acts as a reflection group. There is no obvious connection between these two results. We will show that multiplicative invariants of finite reflection groups have a certain embedding property that implies both results simultaneously.     

Free Resolution Invariants for Finite Groups

Given a finite group G and a G-free resolution F _* of Z, then d _G (Im(F _m+1F _m ))–(–1) ^m–i d _G (F _i ) is almost always an invariant of G.     

Invariants of Algebras and Finite Groups

Group theoretical constructions usually terminate in the problem to decide whether two groups are isomorphic. In the case of arbitrary finite groups the calculation of ordinary group characters is not sufficient to decide about it. R. Brauer posed the problem, to find suitable additional group invariants. Applying the theory of norm-type forms of associative algebras, specialized to group algebras, we found, that the 1-, 2-, and 3-characters in the nonmodular case (with some restrictions on the characteristic), especially over the field of complex numbers, are necessary and sufficient for the finite groups. This sharpens a recent result of E. Formanek and D. Sibley on group determinants. Detailed proofs will be given elsewhere. Here we give an overview on recent related results and add a remark concerning calculations in the modular group case.     

Free Modules of Relative Invariants of Finite Groups

This paper addresses questions about when modules of relative invariants of a finite group G acting on a polynomial ring R are free over the ring of invariant polynomials R^G. A converse (first obtained by Shchvartsman) is proven of a result asserting that these modules are always free when the group is generated by pseudoreflections. We also re-prove the characterization given by Shchvartsman of which characters χ of degree one have the above property, and deduce from this a characterization of which G have the above property for all their degree one characters.     

Words and Polynomial Invariants of Finite Groups in Non-Commutative Variables

Let V be a complex vector space with basis {x ₁, x ₂, . . . , x _n } and G be a finite subgroup of GL(V). The tensor algebra T(V) over the complex is isomorphic to the polynomials in the non-commutative variables x ₁, x ₂, . . . , x _n with complex coefficients. We want to give a combinatorial interpretation for the decomposition of T(V) into simple G-modules. In particular, we want to study the graded space of invariants in T(V) with respect to the action of G. We give a general method for decomposing the space T(V) into simple modules in terms of words in a Cayley graph of the group G. To apply the method to a particular group, we require a homomorphism from a subalgebra of the group algebra into the character algebra. In the case of G as the symmetric group, we give an example of this homomorphism from the descent algebra. When G is the dihedral group, we have a realization of the character algebra as a subalgebra of the group algebra. In those two cases, we have an interpretation for the graded dimensions and the number of free generators of the algebras of invariants in terms of those words.     

On Reflection Subgroups of Finite Coxeter Groups

Let W be a finite Coxeter group. We classify the reflection subgroups of W up to conjugacy and give necessary and sufficient conditions for the map that assigns to a reflection subgroup R of W the conjugacy class of its Coxeter elements to be injective, up to conjugacy.     

On the Lp Minkowski Problem for Polytopes

Two new approaches are presented to establish the existence of polytopal solutions to the discrete-data L_p Minkowski problem for all p > 1.     

On the Orlicz Minkowski Problem for Polytopes

Quite recently, an Orlicz Minkowski problem has been posed and the existence part of this problem for even measures has been presented. In this paper, the existence part of the Orlicz Minkowski problem for polytopes is demonstrated. Furthermore, we obtain a solution of the Orlicz Minkowski problem for general (not necessarily even) measures.     

关于Asgeirsson均值等式的反问题

对于常系数线性偏微分算子(?),方程L_xu(x,y)=L_yu(x,y)的所有C^m解满足Asgcirsson均值等式的充分必要条件是L_x=c+a△_x,这里a(≠0),c为常数,△_x为Laplace算子.     

关于有限群的一个组合问题

设ｐ是有限群Ｇ之阶ｎ的最小素因子，Ｇ之运算用“＋”来记（但不必可换），又设，本文证明了当Ｇ为幂零群及其它某些类型的群时，是满足下面条件的最小正整数：凡Ｇ的不含零元的元子集均使得Ｇ之每一个元ｇ都可表成ｇ＝ａ＿（ｉ１）＋…＋ａ＿（ｉ１），诸ｉ＿ｊ互异．     

有限伪反射群的相对不变式的 Poincar\'e级数

Let F be a field with characteristic 0, V = Fn the n-dimensional vector space over F and let G be a finite pseudo-reflection group which acts on V . Let χ ： G→ F＊ be a 1- dimensional representation of G. In this article we show that χ（g） = （detg）α（0 ≤ α ≤ r - 1）, where g ∈ G and r is the order of g. In addition, we characterize the relation between the relative invariants and the invariants of the group G, and then we use Molien’s Theorem of invariants to compute the Poincar′e series of relative invariants.     

Polynomial Invariants of Certain Pseudo-Symplectic Groups Over Finite Fields of Characteristic Two

Let F _q be a finite field of characteristic two, S be a nonsingular non-alternate symmetric matrix over F _q and Ps _n (F _q , S) be the associated pseudo-symplectic group. Let Ps _n (F _q , S) act linearly on the polynomial ring F _q [x ₁,…, x _n ]. In this note, we find an explicit set of generators of the ring of invariants of Ps _n (F _q , S) for n = 2, 4 and 2ν +1. In particular, the results assert that the ring of invariants of Ps ₄(F _q , S) is not a polynomial algebra but is an example of hypersurface and the ring of invariants of Ps _2ν+1(F _q , S) is a complete intersection.     

The Invariants of the Clifford Groups

The automorphism group of the Barnes-Wall lattice L _m in dimension 2 ^m (m ; 3) is a subgroup of index 2 in a certain Clifford group of structure 2 ₊ ^1+2m . O ⁺(2m,2). This group and its complex analogue of structure .Sp(2m, 2) have arisen in recent years in connection with the construction of orthogonal spreads, Kerdock sets, packings in Grassmannian spaces, quantum codes, Siegel modular forms and spherical designs. In this paper we give a simpler proof of Runge@apos;s 1996 result that the space of invariants for of degree 2k is spanned by the complete weight enumerators of the codes , where C ranges over all binary self-dual codes of length 2k; these are a basis if m k - 1. We also give new constructions for L _m and : let M be the -lattice with Gram matrix . Then L _m is the rational part of M ^m, and = Aut(M^m). Also, if C is a binary self-dual code not generated by vectors of weight 2, then is precisely the automorphism group of the complete weight enumerator of . There are analogues of all these results for the complex group , with doubly-even self-dual code instead of self-dual code.     

一些特殊变换群的联合不变量和联合微分不变量

The theory of moving frames developed by Peter J Olver and M Fels has importaut applications to geometry,classical invariant theory.We will use this theory to classify joint invariants and joint differential invariants of some transformation groups.     

Twisted Groups and Locally Toroidal Regular Polytopes

In recent years, much work has been done on the classification of abstract regular polytopes by their local and global topological type. Abstract regular polytopes are combinatorial structures which generalize the well-known classical geometric regular polytopes and tessellations. In this context, the classical theory is concerned with those which are of globally or locally spherical type. In a sequence of papers, the authors have studied the corresponding classification of abstract regular polytopes which are globally or locally toroidal. Here, this investigation of locally toroidal regular polytopes is continued, with a particular emphasis on polytopes of ranks and . For large classes of such polytopes, their groups are explicitly identified using twisting operations on quotients of Coxeter groups. In particular, this leads to new classification results which complement those obtained elsewhere. The method is also applied to describe certain regular polytopes with small facets and vertex-figures.

     

The Logarithmic Capacitary Minkowski Problem for Polytopes

The logarithmic capacitary Minkowski problem asks for necessary and sufficient conditions on a finite Borel measure on the unit sphere so that it is the logarithmic capacitary measure of a convex body. This article completely solves the case of discrete measures whose support sets are in general position.     

On the Isaacs Problem Concerning Finite p-Solvable Linear Groups

In this work a finite $\Pi$ -separable complex irreducible linear group G is studied. The conditions for its $S_\Pi$ -subgroup to be normal in G and Abelian are determined. The results provide a solution to the well-known Isaacs problem in some particular cases.