On the integral cohomology of a sylow subgroup of the symmetric group

Projectivity classes, which dualize injectivity classes (cf.[ll]), are introduced and some examples are given. Characterizations of hereditary rings, semisimple rings, Noetherian rings, (semi-)perfect rings, quasi-perfect rings, semiregular rings, semiregular modules and F- semiperfect modules using projectivity classes are given. Finally, for a projectivity class P, P-projective covers are defined and similar results with “quasi-projective cover” substituted by “T-projective cover” will still hold. Our results unify and generalize several well known results by Golan, Huynh and Smith, Rangaswamy and Vanaja, Tiwary and Pandeya, and Xue.     

On the length of subgroup chains in the symmetric group

We prove that for n≥2, the length of every subgroup chain in S_n is at most 2n-3. The proof rests on an upper bound for the order of primitive permutation groups, due to Praeger and Saxl. The result has applications to worst case complexity estimates for permutation group algorithms.     

Bispherical functions on the symmetric group associated with the hyperoctahedral subgroup

We consider an arbitrary irreducible representation of a symmetric group S_4m that has a B_2m-invariant and a B_2m-antiinvariant vector, where B_2m is a hyperoctahedral subgroup of S_4m. The main result is an expression for a matrix element corresponding to these two vectors in terms of an irreducible character of the symmetric group S_m.The author is deeply grateful to G. I. Olshansky for posing the problem and for constant interest in the work.Translated fromZapiski Nauchnykh Seminarov POMI, Vol. 240, 1997, pp. 96–114.Supported by the International Soros Educational Program, Grant 2093s.     

The quaternion group as a subgroup of the sphere braid groups

Let n 3. We prove that the quaternion group of order 8 is realisedas a subgroup of the sphere braid group B_n(²) if and only ifn is even. If n is divisible by 4, then the commutator subgroupof B_n(²) contains such a subgroup. Further, for all n 3, B_n(²)contains a subgroup isomorphic to the dicyclic group of order4n.     

A series of unitary irreducible representations induced from a symmetric subgroup of a semisimple Lie group

LetG/H be a semisimple symmetric space. Generalizing results of Flensted-Jensen we give a sufficient condition for the existence of irreducible closed invariant subspaces of the unitary representations ofG induced from unitary finite dimensional representations ofH. This provides a method of constructing unitary irreducible representations ofG, and we show by examples that for some irreducible admissible representations ofG, this method exhibits not previously known unitarity.This work was supported by the Danish Natural Science Research Council.     

Discriminant of symmetric matrices as a sum of squares and the orthogonal group

It is proved that the discriminant of n × n real symmetric matrices can be written as a sum of squares, where the number of summands equals the dimension of the space of n‐variable spherical harmonics of degree n. The representation theory of the orthogonal group is applied to express the discriminant of 3 × 3 real symmetric matrices as a sum of five squares and to show that it cannot be written as the sum of less than five squares. It is proved that the discriminant of 4 × 4 real symmetric matrices can be written as a sum of seven squares. These improve results of Kummer from 1843 and Borchardt from 1846. © 2010 Wiley Periodicals, Inc.     

Topologies on the subgroup lattice of a compact group

Let G be a compact topological group. The lattice ΣG of its closed subgroups is algebraic in the reversed order, hence is made a compact topological semilattice by its dual Lawson topology. A second natural order-compatible compact topology on ΣG arises from the usual topology on the set of closed subsets of G. These topologies are shown to coincide precisely if the identity component is central in G, but to be essentially different otherwise, since they also fail to satisfy natural weakenings of the equality condition. In the second part of the paper the groups G are determined in which one of the lattice operations of ΣG becomes continuous with respect to either one of these topologies; several different characterizations of these cases are also provided.     

On the commutator subgroup of a nonconnected central group

In this paperG denotes a central topologicalT ₂-group—G/Z(G) compact, whereZ(G) is the center. There are some results concerning compactness of the commutator subgroupG; in general (G)^– is compact ([3]), but not necessarilyG ([7]). If in additionG is a Lie group or ifG is connected,G is compact ([6], [5]). The purpose of this paper is to show, that if the componentG ₀ of the identity is open,G must be compact, and to give an example of a compact group with (G/G ₀) compact, whileG is not compact.Dedicated to Prof. R. Inzinger on his 70th birthday     

Modular irreducible representations of the symmetric group as linear codes

We describe a particularly easy way of evaluating the modular irreducible matrix representations of the symmetric group. It shows that Specht’s approach to the ordinary irreducible representations, along Specht polynomials, can be unified with Clausen’s approach to the modular irreducible representations using symmetrized standard bideterminants. The unified method, using symmetrized Specht polynomials, is very easy to explain, and it follows directly from Clausen’s theorem by replacing the indeterminate x_ij of the letter place algebra by x_jⁱ.Our approach is implemented in SYMMETRICA. It was used in order to obtain computational results on code theoretic properties of the p-modular irreducible representation [λ]_p corresponding to a p-regular partition λ via embedding it into representation spaces obtained from ordinary irreducible representations. The first embedding is into the permutation representation induced from the column group of a standard Young tableau of shape λ. The second embedding is the embedding of [λ]_p into the space of , the p-modular representation obtained from the ordinary irreducible representation [λ] by reducing the coefficients modulo p.We include a few tables with dimensions and minimum distances of these codes; others can be found via our home page.     

Extensions of group representations from a normal subgroup

We characterize principal nilpotent ideals in finite abelian group algebra F_q [G] (F_q a finite field of p^s elements, p prime), with greatest possible dimension. We prove also that the-se ideals are all isomorphic.     

Cumulants for random matrices as convolutions on the symmetric group

In this paper, we show that free cumulants can be naturally seen as the limiting value of ``cumulants of matrices'. We define these objects as functions on the symmetric group by some convolution relations involving the generalized moments. We state that some characteristic properties of the free cumulants already hold for these cumulants.     

Nilpotency of the derived subgroup of a finite tangled group

We study finite groups whose every subset containing the identity and closed with respect to the operation x ○ y = xy ^?1 x is a subgroup. We prove that the derived subgroup of such a group is nilpotent, which implies that the derived length of such a group is at most three.