Let K be any finite (possibly trivial) extension of ℚ, the field of rational numbers. Let denote the ring of integers of K, and let M ⊆ be a full module in K thus a free ℤ-module of rank [K : ℚ] contained in ; ℤ denoting the ring of rational integers. Regarding as an abelian group, the index (: M) is finite. Suppose that m1, …, mk is a ℤ-basis for M and let a ∊ Then the polynomial

(the xi; being indeterminates) will be called a full-norm polynomial; here NK/ℚ denotes the norm mapping from K to ℚ. Apart from constant factors, such a polynomial f(x) is necessarily irreducible in ℤ[x].

When K is a convex body in d-dimensional euclidean space E and 0 ≤ s ≤ d, the s-skeleton of K, denoted skel(s)K, consists of those points of K which are not centres of (s + l)-dimensional balls contained in K. The s-skeleton is thus the union of the extreme faces of K having dimension at most s. The s-skeleton is a -set [see 6] and it is therefore measurable with respect to the s-dimensional Hausdorff measure ℋ(s) [see 7]; here we normalize ℋ(s) so that it assigns unit measure to the s-dimensional unit cube.

A natural and welcomed decomposition theorem for elements in the positive cone of the tensor product of Archimedean vector lattices leads to substantial simplifications in the theory of tensor products of Archimedean vector lattices.

G. Higman [5] first considered conditions on a group G sufficient to ensure that for any ring R with no zero-divisors the group-ring RG contains no zero-divisors. It has been shown by various authors that if G belongs to one of the classes of locally indicible groups [5], right-ordered groups [6], polycyclic groups [4] or positive one-relator groups [1] then it is enough that G should be torsionfree. The proofs rely heavily on the special properties of the classes of groups involved but it may be conjectured that it is a sufficient condition in general that G should be torsionfree and no counterexamples are known.

If G is a topological group then we can think of G acting on itself by multiplying on the left. We would like to know when this action has the property that whenever g and h are distinct elements of G, then the element xg does not get arbitrarily close to xh as x varies in G. It is natural to say that this is the case if {(xg, xh): x∈G} is separated from the diagonal of G × G by a uniform neighbourhood of the diagonal.

Unfortunately the contours used in the proof of Lemma 3 of my recent paper «this volume pp. 62–71» have not all been specified in the appropriate way. The contours used at the foot of page 66 should be

and middle contour used near the middle of page 67 should be

Lyndon's axiomatic methods are used in [1] to show, among other things, that a group G with an integer valued length function satisfying certain conditions is free. At the end of his paper [2] Lyndon gives a method of embedding such a group in a free group whose natural length function extends the function on G. We construct here a simpler embedding with the same property.

The Picard group P(ZG) of the integral group ring ZG is defined as the class group of two-sided invertible ZG-ideals of QG modulo those principal ideals generated by an invertible central element. The basic properties of Picard groups have been established by A. Fröhlich, I. Reiner and S. Ullom [1], [2], [3]. In this note we settle an outstanding question by exhibiting a class of finite p-groups G whose Picard groups contain nontrivial elements which are represented by principal ideals; these elements remain nontrivial in P(ZpG) also. We obtain these ideals from outer automorphisms of the groups.

We say a motion g brings a mobile convex body K into inner contact with a fixed body K0 if the image gK lies in K0 and shares a boundary point with K0; we speak of the inner contact being at the common boundary point. The mobile body K is said to roll freely in K0 if, corresponding to each boundary point x of K0 and each rotation R, there is a translation t such that RK + t = gK has inner contact with K0 at x.

In an article generalising work of Roquette and Zassenhaus, Connell and Sussman [2] have demonstrated the importance of certain prime ideals in a number field k0 for estimating the l-rank of the class group of an extension k. These ideals have a power prime to l which is principal and all their prime factors in k have ramification index divisible by l. The products of the prime divisors of these ideals in the normal closure K of k/k0 are invariant under Gal (k/k0). Thus certain roots in k of the ideals in k0 are in some sense fixed by the Galois group. This leads to the concept of ambiguous ideals in an extension k/k0 which is not necessarily normal.

A Borel isomorphism that, together with its inverse, maps ℱσ-sets to ℱσ-sets will be said to be a Borel isomorphism at the first level. Such a Borel isomorphism will be called a first level isomorphism, for short. We study such first level isomorphisms between Polish spaces and between their Borel and analytic subsets.