A Ramsey theorem for trees

We prove a Ramsey theorem for trees. The infinite version of this theorem can be stated: if T is a rooted tree of infinite height with each node of T having at least one but finitely many immediate successors, if n is a positive integer, and if the collection of all strongly embedded, height-n subtrees of T is partitioned into finitely many classes, then there must exist a strongly embedded subtree S of T with S having infinite height and with all the strongly embedded, height-n subtrees of S in the same class.     

A limit theorem for projections

The main purpose of this paper is to establish a limit theorem for convex combinations of linear projections. We also include nonlinear extensions as well as several applications.     

A covering theorem for quasi-groups

It is shown that if Q is a quasi-group of order n and k is moderately large, there exists a subset A of Q of size k such that if t is the least number of left translates of A needed to cover Q, then $\text{t >}\text{c(n}\text{log}\text{n)}\text{k}$.     

A decomposition theorem for neutrices

Neutrices are convex additive subgroups of the nonstandard space R^k, most of them are external sets. Because of the convexity and the invariance under some translations and multiplications, external neutrices are models for orders of magnitude. One dimensional neutrices have been applied to asymptotics, singular perturbations, and statistics. This paper shows that in R^k, with standard k, every neutrix is the direct sum of k neutrices of R. These components may be chosen to be orthogonal.     

A factorization theorem for matrices

It is shown that a nonscalar invertible square matrix can be written as a product of two square matrices with prescribed eigenvalues subject only to the obvious determinant condition. As corollaries, we give short proofs of some known results such as Ballantine's characterization of products of four or five positive definite matrices, the commutator theorem of Shoda-Thompson for fields with sufficiently many elements and other results.     

A finiteness theorem for W-graphs

A W-graph for a Coxeter group W is a combinatorial structure that encodes a module for the group algebra of W, or more generally, a module for the associated Iwahori–Hecke algebra. Of special interest are the W-graphs that encode the action of the Hecke algebra on its Kazhdan–Lusztig basis, as well as the action on individual cells. In previous work, we isolated a few basic features common to the W-graphs in Kazhdan–Lusztig theory and used these to define the class of “admissible” W-graphs. The main result of this paper resolves one of the basic question about admissible W-graphs: there are only finitely many admissible W-cells (i.e., strongly connected admissible W-graphs) for each finite Coxeter group W. Ultimately, the finiteness depends only on the fact that admissible W-graphs have nonnegative integer edge weights. Indeed, we formulate a much more general finiteness theorem for “cells” in finite-dimensional algebras which in turn is fundamentally a finiteness theorem for nonnegative integer matrices satisfying a polynomial identity.     

A triangulation theorem for F-manifolds

An F-manifold is a manifold modelled on an infinite-dimensional Fréchet space F. The following theorem gives, in a sense, arbitrarily small triangulations of separable F-manifolds: Let M be a separable F-manifold and let G be an open cover of M. Then there exists a countable locally-finite simplicial complex K and a homeomorphism h: |K|×F→M such that for each simplex σϵK, h(|σ| × F) is contained in some element of G. Hopefully, this theorem can be generalized using locally-finite-dimensional complexes to non-separable F-manifolds.     

A symmetry theorem for condensers

We prove the radial symmetry of the solution of degenerate quasilinear capacity problems, when constant overdetermined Neumann data are assigned.