Witt kernels of bilinear forms for algebraic extensions in characteristic

Let be a field of characteristic and let be a purely inseparable extension of exponent . We determine the kernel of the natural restriction map between the Witt rings of bilinear forms of and , respectively. This complements a result by Laghribi who computed the kernel for the Witt groups of quadratic forms for such an extension . Based on this result, we will determine for a wide class of finite extensions which are not necessarily purely inseparable.

     

Functional calculus and -regularity of a class of Banach algebras

Suppose that is a -dynamical system such that is of polynomial growth. If is finite dimensional, we show that any element in has slow growth and that is -regular. Furthermore, if is discrete and is a ``nice representation' of , we define a new Banach -algebra which coincides with when is finite dimensional. We also show that any element in has slow growth and is -regular.

     

Relative Brauer groups and -torsion

Let be a field and its Brauer group. If is a field extension, then the relative Brauer group is the kernel of the restriction map . A subgroup of is called an algebraic relative Brauer group if it is of the form for some algebraic extension . In this paper, we consider the -torsion subgroup consisting of the elements of killed by , where is a positive integer, and ask whether it is an algebraic relative Brauer group. The case is already interesting: the answer is yes for squarefree, and we do not know the answer for arbitrary. A counterexample is given with a two-dimensional local field and .

     

Topological entropy and AF subalgebras of graph -algebras

Let be the canonical AF subalgebra of a graph -algebra associated with a locally finite directed graph . For Brown and Voiculescu's topological entropy of the canonical completely positive map on , is known to hold for a finite graph , where is the loop entropy of Gurevic and is the block entropy of Salama. For an irreducible infinite graph , the inequality has recently been known. It is shown in this paper that

where is the graph with the direction of the edges reversed. Some irreducible infinite graphs 1)$"> with are also examined.

     

Finite groups and the fixed points of coprime automorphisms

Let be a prime, and let be a finite -group acted on by an elementary abelian -group . The following results are proved:

1. If and is nilpotent of class at most for any , then the group is nilpotent of -bounded class.

2. If and is nilpotent of class at most for any , then the derived group is nilpotent of -bounded class.

     

Unimodular functions and interpolating Blaschke products

The result by Bourgain that every unimodular function on the unit circle can be factored as with and Blaschke products can be improved. We show that the same result holds with and interpolating Blaschke products. This will at the same time be a refinement of Jones's result that every unimodular function can be approximated in the -norm by a ratio of interpolating Blaschke products.

     

Proof of the prime power conjecture for projective planes of order

Let be an abelian collineation group of order of a projective plane of order . We show that must be a prime power, and that the -rank of is at least if for an odd prime .

     

A stability property for linear groups

For suitable rings of integers , we show that the mod group cohomology for comes from when restricted to the diagonal matrices for all ranks .

     

The algebraic closure of the power series field in positive characteristic

For an algebraically closed field, let denote the quotient field of the power series ring over . The ``Newton-Puiseux theorem' states that if has characteristic 0, the algebraic closure of is the union of the fields over . We answer a question of Abhyankar by constructing an algebraic closure of for any field of positive characteristic explicitly in terms of certain generalized power series.

     

Unique factorization in generalized power series rings

Let be a field of characteristic zero and let denote the ring of generalized power series (i.e., formal sums with well-ordered support) with coefficients in , and non-positive real exponents. Berarducci (2000) constructed an irreducible omnific integer, in the sense of Conway (2001), by first proving that an element of that is not divisible by a monomial and whose support has order type (or for some ordinal ) must be irreducible. In this paper, we consider elements of with support of order type . The irreducibility of these elements cannot be deduced solely from the order type of their support and, after developing new tools for studying these elements, we exhibit both reducible and irreducible elements of this type. We further prove that all elements whose support has order type and which are not divisible by a monomial factor uniquely into irreducibles. This provides, in the ring , a class of reducible elements for which we have unique factorization into irreducibles.

     

The extension of positive definite operator-valued functions defined on a symmetric interval of an ordered group

Let be an ordered abelian group and . Let be an abelian group and an operator-valued positive definite function on . We prove that admits a positive definite extension to , generalizing in this way existing results for the case when and is continuous.

     

On stable equivalences induced by exact functors

Let and be two Artin algebras with no semisimple summands. Suppose that there is a stable equivalence between and such that is induced by exact functors. We present a nice correspondence between indecomposable modules over and . As a consequence, we have the following: (1) If is a self-injective algebra, then so is ; (2) If and are finite dimensional algebras over an algebraically closed field , and if is of finite representation type such that the Auslander-Reiten quiver of has no oriented cycles, then and are Morita equivalent.

     

A dual graph construction for higher-rank graphs, and -theory for finite 2-graphs

Given a -graph and an element of , we define the dual -graph, . We show that when is row-finite and has no sources, the -algebras and coincide. We use this isomorphism to apply Robertson and Steger's results to calculate the -theory of when is finite and strongly connected and satisfies the aperiodicity condition.

     

Mapping spaces and homology isomorphisms

Let denote the space of pointed continuous maps from a finite cell complex to a space . Let be a generalized homology theory. We use Goodwillie calculus methods to prove that under suitable conditions on and , will send an -isomorphism in either variable to a map that is monic in homology. Interesting examples arise by letting be -theory, the finite complex be a sphere, and the map in the variable be an exotic unstable Adams map between Moore spaces.

     

-algebras and the tail-equivalence relation on Bratteli diagrams

We show that the -algebra associated to the tail-equivalence relation on a Bratteli diagram, according to a procedure recently introduced by the first-named author and A. Lopes, is isomorphic to the -algebra of the diagram. More generally we consider an approximately proper equivalence relation on a compact space for which the quotient maps are local homeomorphisms. We show that the algebra associated to under the above-mentioned procedure is isomorphic to the groupoid -algebra .

     

Remarks on spectra and multipliers for convolution operators

We prove that the spectrum of a convolution operator on a locally compact group by a self-adjoint -function is the same on and and consequently on all spaces, if and only if a Beurling algebra contains non-analytic functions on operating on into .

     

Eigenvalues of the Laplacian acting on -forms and metric conformal deformations

Let be a compact connected orientable Riemannian manifold of dimension and let be the -th positive eigenvalue of the Laplacian acting on differential forms of degree on . We prove that the metric can be conformally deformed to a metric , having the same volume as , with arbitrarily large for all .

Note that for the other values of , that is and , one can deduce from the literature that, 0$">, the -th eigenvalue is uniformly bounded on any conformal class of metrics of fixed volume on .

For , we show that, for any positive integer , there exists a metric conformal to such that, , , that is, the first eigenforms of are all exact forms.

     

The minimal spanning tree and the upper box dimension

We show that the -weight of an MST over points in a metric space with upper box dimension has a bound independent of if d$"> and does not have one if .

     

Subgroup growth in some pro- groups

For a group let be the number of subgroups of index and let be the number of normal subgroups of index . We show that for 2$">. If and does not divide or if and or , we show that for all sufficiently large . On the other hand if and divides , then is not even bounded as a function of .

     

Multiplication and division by inner functions in the space of Bloch functions

A subspace of the Hardy space is said to have the -property if whenever and is an inner function with . We let denote the space of Bloch functions and the little Bloch space. Anderson proved in 1979 that the space does not have the -property. However, the question of whether or not ( ) has the -property was open. We prove that for every the space does not have the -property.

We also prove that if is any infinite Blaschke product with positive zeros and is a Bloch function with , as , then the product is not a Bloch function.