Symmetric Automorphisms Of Free Products

This memoir examines the automorphism group of a group $G$ with a fixed free product decomposition $G_1*\cdots *G_n$. An automorphism is called symmetric if it carries each factor $G_i$ to a conjugate of a (possibly different) factor $G_j$. The symmetric automorphisms form a group $\Sigma Aut(G)$ which contains the inner automorphism group $Inn(G)$. The quotient $\Sigma Aut(G)/Inn(G)$ is the symmetric outer automorphism group $\Sigma Out(G)$, a subgroup of $Out(G)$. It coincides with $Out(G)$ if the $G_i$ are indecomposable and none of them is infinite cyclic. To study $\Sigma Out(G)$, the authors construct an $(n-2)$-dimensional simplicial complex $K(G)$ which admits a simplicial action of $Out(G)$. The stabilizer of one of its components is $\Sigma Out(G)$, and the quotient is a finite complex. The authors prove that each component of $K(G)$ is contractible and describe the vertex stabilizers as elementary constructs involving the groups $G_i$ and $Aut(G_i)$. From this information, two new structural descriptions of $\Sigma Aut (G)$ are obtained. One identifies a normal subgroup in $\Sigma Aut(G)$ of cohomological dimension $(n-1)$ and describes its quotient group, and the other presents $\Sigma Aut (G)$ as an amalgam of some vertex stabilizers. Other applications concern torsion and homological finiteness properties of $\Sigma Out (G)$ and give information about finite groups of symmetric automorphisms. The complex $K(G)$ is shown to be equivariantly homotopy equivalent to a space of $G$-actions on $\mathbb R$-trees, although a simplicial topology rather than the Gromov topology must be used on the space of actions.

Free probability theory, introduced by Voiculescu, has developed very actively in the last few years and has had an increasing impact on quite different fields in mathematics and physics. Whereas the subject arose out of the field of von Neumann algebras, presented here is a quite different view of Voiculescu's amalgamated free product. This combinatorial description not only allows re-proving of most of Voiculescu's results in a concise and elegant way, but also opens the way for many new results. Unlike other approaches, this book emphasizes the combinatorial structure of the concept of ``freeness''. This gives an elegant and easily accessible description of freeness and leads to new results in unexpected directions. Specifically, a mathematical framework for otherwise quite ad hoc approximations in physics emerges.

This volume is about tree-like structures, namely semilinear ordering, general betweenness relations, $C$-relations and $D$-relations. It contains a systematic study of betweenness and introduces $C$ and $D$-relations to describe the behavior of points at infinity ('leaves' or 'ends' or 'directions') of trees. The focus is on structure theorems and on automorphism groups, with applications to the theory of infinite permutation groups. This work: offers the first systematic treatment of betweenness relations; introduces important new concepts of $C$-relations and $D$-relations; elucidates the close relationships between semilinear orderings, betweenness relations, $C$ and $D$-relations; and, considers their automorphism groups as important examples of Jordan permutation groups.

In this book, the author introduces and studies the construction of the crossed product of a von Neumann algebra $M = \int _X M(x)d\mu (x)$ by an equivalence relation on $X$ with countable cosets. This construction is the generalization of the construction of the crossed product of an abelian von Neumann algebra by an equivalence relation introduced by J. Feldman and C. C. Moore. Many properties of this construction are proved in the general case. In addition, the generalizations of the Spectral Theorem on Bimodules and of the theorem on dilations are proved.

The importance of separable continuous trace $C^*$-algebras arises from the following facts: Firstly, their stable isomorphism classes are completely classifiable by topological data and, secondly, continuous-trace $C^*$-algebras form the building blocks of the more general type I $C^*$-algebras. This memoir presents an extensive study of strongly continuous actions of abelian locally compact groups on $C^*$-algebras with continuous trace. Under some natural assumptions on the underlying system $(A,G,\alpha )$, necessary and sufficient conditions are given for the crossed product $A{\times }_{\alpha }G$ to have continuous trace, and some relations between the topological data of $A$ and $A{\times }_{\alpha }G$ are obtained. The results are applied to investigate the structure of group $C^*$-algebras of some two-step nilpotent groups and solvable Lie groups. For readers' convenience, expositions of the Mackey-Green-Rieffel machine of induced representations and the theory of Morita equivalent $C^*$-dynamical systems are included. There is also an extensive elaboration of the representation theory of crossed products by actions of abelian groups on type I $C^*$-algebras, resulting in a new description of actions leading to type I crossed products. Features: The most recent results on the theory of crossed products with continuous trace. Applications to the representation theory of locally compact groups and structure of group $C^*$-algebras. An exposition on the modern theory of induced representations. New results on type I crossed products.

Almost free divisors and complete intersections form a general class of nonisolated hypersurface and completer intersection singularities. They also include discriminants of mappings, bifurcation sets, and certain types of arrangements of hyperplanes such as Coxeter arrangements and generic arrangements. Associated to the singularities of this class is a "singular Milnor fibration" which has the same homotopy properties as the Milnor fibration for isolated singularities. This memoir deduces topological properties of singularities in a number of situations including: complements of hyperplane arrangements, various nonisolated complete intersections, nonlinear arrangements of hypersurfaces, functions on discriminants, singularities defined by compositions of functions, and bifurcation sets.

The class of cycle-free partial orders (CFPOs) is defined, and the CFPOs fulfilling a natural transitivity assumption, called $k$-connected set transitivity ($k$-$CS$-transitivity), are analyzed in some detail. Classification in many of the interesting cases is given. This work generalizes Droste's classification of the countable $k$-transitive trees ($k \geq 2$). In a CFPO, the structure can branch downwards as well as upwards, and can do so repeatedly (though it never returns to the starting point by a cycle). Mostly it is assumed that $k \geq 3$ and that all maximal chains are finite. The main classification splits into the sporadic and skeletal cases. The former is complete in all cardinalities. The latter is performed only in the countable case. The classification is considerably more complicated than for trees, and skeletal CFPOs exhibit rich, elaborate and rather surprising behavior. Features: Lucid exposition of an important generalization of Droste's work Extended introduction clearly explaining the scope of the memoir Visually attractive topic with copious illustrations Self-contained material, requiring few prerequisites