A brief guide to reversing and extended symmetries of dynamical systems

The reversing symmetry group is a well-studied extension of the symmetry group of a dynamical system, the latter being defined by the action of a single homeomorphism on a topological space. While it is traditionally considered in nonlinear dynamics, where the space is simple but the map is complicated, it has an interesting counterpart in symbolic dynamics, where the map is simple but the space is not. Moreover, there is an interesting extension to the case of higher-dimensional shifts, where a similar concept can be introduced via the centraliser and the normaliser of the acting group in the full automorphism group of the shift space. We recall the basic notions and review some of the known results, in a fairly informal manner, to give a first impression of the phenomena that can show up in the extension from the centraliser to the normaliser, with some emphasis on recent developments.Comment: 18 pages, 2 figures; informal introduction and overvie

Discrete symmetries as automorphisms of the proper Poincare group

We present the consistent approach to finding the discrete transformations in the representation spaces of the proper Poincar\'e group. To this end we use the possibility to establish a correspondence between involutory automorphisms of the proper Poincar\'e group and the discrete transformations. As a result, we derive rules of the discrete transformations for arbitrary spin-tensor fields without the use of relativistic wave equations. Besides, we construct explicitly fields carrying representations of the extended Poincar\'e group, which includes the discrete transformations as well.Comment: 33 pages, LaTe

Mackey's theory of τ\tau-conjugate representations for finite groups. APPENDIX: On Some Gelfand Pairs and Commutative Association Schemes

The aim of the present paper is to expose two contributions of Mackey, together with a more recent result of Kawanaka and Matsuyama, generalized by Bump and Ginzburg, on the representation theory of a finite group equipped with an involutory anti-automorphism (e.g. the anti-automorphism $g\mapsto g^{-1}$). Mackey's first contribution is a detailed version of the so-called Gelfand criterion for weakly symmetric Gelfand pairs. Mackey's second contribution is a characterization of simply reducible groups (a notion introduced by Wigner). The other result is a twisted version of the Frobenius-Schur theorem, where "twisted" refers to the above-mentioned involutory anti-automorphism. APPENDIX: We consider a special condition related to Gelfand pairs. Namely, we call a finite group $G$ and its automorphism $\sigma$ satisfy Condition ($\bigstar$) if the following condition is satisfied: if for $x,y\in G$, $x\cdot x^{-\sigma}$ and $y\cdot y^{-\sigma}$ are conjugate in $G$, then they are conjugate in $K=C_G(\sigma)$. We study the meanings of this condition, as well as showing many examples of $G$ and $\sigma$ which do (or do not) satisfy Condition ($\bigstar$).Comment: This consists of a 38 pages paper and a 7 pages APPENDIX. The original version of the appendix appeared in the unofficial proceedings, "Combinatorial Number Theory and Algebraic Combinatorics", November 18--21, 2002, Yamagata University, Yamagata, Japan, pp. 1--

Discrete symmetries as automorphisms of the proper Poincaré group

We present the consistent approach to finding the discrete transformations in the representation spaces of the proper Poincar\'e group. To this end we use the possibility to establish a correspondence between involutory automorphisms of the proper Poincar\'e group and the discrete transformations. As a result, we derive rules of the discrete transformations for arbitrary spin-tensor fields without the use of relativistic wave equations. Besides, we construct explicitly fields carrying representations of the extended Poincar\'e group, which includes the discrete transformations as well

Transitive Group Actions: (IM)PRIMITIVITY and Semiregular Subgroups

The following problem is considered: if $H$ is a semiregular abelian subgroup of a transitive permutation group $G$ acting on a finite set $X$, find conditions for (non) existence of $G$-invariant partitions of $X$. Conditions presented in this paper are derived by studying spectral properties of associated $G$-invariant digraphs. As an essential tool, irreducible complex characters of $H$ are used. Questions of this kind arise naturally when classifying combinatorial objects which enjoy a certain degree of symmetry. As an illustration, a new and short proof of an old result of Frucht, Graver and Watkins ({\it Proc. Camb. Phil. Soc.}, {\bf 70} (1971), 211-218) classifying edge-transitive generalized Petersen graphs, is given.Comment: 18 pages, 0 figure

On von Neumann's Examples of Types

The paper introduces in a new although maybe unusual form the examples of types provided by J. von Neumann and F.J. Murray in their outstanding papers on algebraic factorization (1936-1943)pursuing three main aims: speculating about the physical reasons and motivations that are likely to have been at the origin of von Neumann's investigation; describing the examples of factors provided by those authors with the purpose of clarifying the general concepts standing at the base of the classification of factors into three general types; outlining the perspective of extending the theory to non--separable Hilbert spaces with the purpose of suggesting a novel approach to the representation of infinite systems controlled by external gauge fields.Comment: 24 pages, no figure

Homotopy approach to quantum gravity

I construct a finite-dimensional quantum theory from general relativity by a homotopy method. Its quantum history is made up of at least two levels of fermionic elements. Its unitary group has the diffeomorphism group as singular limit. Its gravitational metrical form is the algebraic square. Its spinors are multivectors.Comment: For International Journal of Theoretical Physics, Oberwolfach 2006 issu

Reflections and spinors on manifolds

This paper reviews some recent work on (s)pin structures and the Dirac operator on hypersurfaces (in particular, on spheres), on real projective spaces and quadrics. Two approaches to spinor fields on manifolds are compared. The action of space and time reflections on spinors is discussed, also for two-component (chiral) spinors.Comment: 10 pages, 1 figure, AMS-LaTe

Subrings of invariants for actions of finite dimensional Hopf algebras

This is a survey article on the invariant rings of Hopf actionsComment: plain Te

Introduction to Sporadic Groups for physicists

We describe the collection of finite simple groups, with a view on physical applications. We recall first the prime cyclic groups $Z_p$, and the alternating groups $Alt_{n>4}$. After a quick revision of finite fields $\mathbb{F}_q$, $q = p^f$, with $p$ prime, we consider the 16 families of finite simple groups of Lie type. There are also 26 \emph{extra} "sporadic" groups, which gather in three interconnected "generations" (with 5+7+8 groups) plus the Pariah groups (6). We point out a couple of physical applications, including constructing the biggest sporadic group, the "Monster" group, with close to $10^{54}$ elements from arguments of physics, and also the relation of some Mathieu groups with compactification in string and M-theory.Comment: This paper is published in: Journal of Physics A, (vol.) 46, (2013), as Topical Revie