In recent years, the problem of embedding the projective spaces in Euclidean spaces was studied very much, by different methods. Usually, the negative results on the embedding problem are proved by using suitable homotopy invariants. The best known example of such homotopy invariants is given by the Stiefel–Whitney classes.

To establish our notation N will always denote a (left) near-ring without any type of multiplicative identity (unless the contrary is stated) satisfying On = 0 for each n ∈ N where 0 is the additive identity of N. A group M, written additively, which admits N as a set of right multipliers is a (right) N-module if a ∈ M, n1, n2 ∈ N implies a(n1 + n2) = an1 + an2 and a(n1n2) = (an1)n2. When N has a two-sided identity, 1, we suppose that a 1 = a for each a ∈ M. A subgroup X of M is an N-subgroup of M if it is an N-module; X is a submodule of M if it is a normal subgroup of M and a ∈ M, x ∈ X. n ∈ N implies (a + x)n – an ∈ X. We denote by SL(M) the set of N-subgroups and by L(M) the set of submodules of M. Since N may be regarded as an N-module we can talk about N-subgroups and submodules of N although we usually call the submodules of N right ideals of N. Other definitions can be found in (6).

We firstly introduce some notation. Let a(t) ∈ Rn then we denote by α(t) a rearrangement of a(t) in non-decreasing order. We write a(1) ≺ a(2) if

and

Let E be a nuclear space provided with a topology different from the weak topology. Let {Ai: i ∈ I} be a fundamental system of equicontinuous subsets of the topological dual E' of E. If {Fi: i ∈ I} is a family of infinite dimensional Banach spaces with separable predual, there is a fundamental system {Bi: i ∈ I} of weakly closed absolutely convex equicontinuous subsets of E'such that is norm-isomorphic to Fi, for each i ∈ I. Other results related with the one above are also given.

Let w = w(a0, a1, an–1) be a word in the free group freely generated by a0, a1, …, an–1; let wi, denote the word w(ai, ai+1, …, ai+n–1), where the subscriptsj in aj are reduced modulo n; and let

We show that the class of Banach algebras A isomorphic with normclosed (non-self-adjoint) subalgebras of is characterized by the condition that the norms of polynomials in A be dominated by the norms of the same polynomials in .

Let G be a finite group, p be a prime and K be a field of characteristic p. Let

be a decomposition of the group ring of G over K as a sum of indecomposable two-sided ideals. An irreducible K(G)-module is said to be in the block Bi if it occurs as a composition factor of Bi. The block containing the trivial K(G)-module is called the principal block of G.

Let G be a finite group and let S be a subgroup of G with core

We say that (G, S) has property (*) if there exists x ∈ G such that S ∩ x−1

In an earlier paper (2) we considered the following question “If S is a cyclic subgroup of a finite group G and S ∩ F(G) = 1, where F(G) is the Fitting subgroup of G, does there necessarily exist a conjugate Sx of S in G with S ∩ Sx = l?” and we gave an affirmative answer for G simple or soluble. In this paper we answer the question affirmatively in general (in fact we prove a somewhat stronger result (Theorem 3)). We give an example of a group G with a cyclic subgroup S such that (i) no nontrivial subgroup of S is normal in G and (ii) no x exists for which S ∩ Sx = 1.

The theory of representing continuous linear operators on function spaces in terms of integrals has had a long and fruitful history, beginning with the Riesz representation theorem in 1909. If T is such an operator, then the standard representation is T (f) = ∫f dμ, where the integral is denned in diverse ways, depending on the nature of the set of functions and the nature of T.

A number of papers have been written concerning the properties of triangles which circumscribe convex sets, see for example (1), (2), (3).

In this note we shall characterise two set functionals, one of which has already been introduced in (6). This will enable us to produce some new results in the spirit of those obtained in the papers mentioned above. These two functionals are interesting in that although they are defined in a rather abstruse manner, they turn out to have an intuitive geometric meaning, namely that they are the minimal widths of certain circumscribing triangles.

The inverse problem we will consider in this paper has its origins in the following problem connected with the scattering of acoustic waves in a nonhomogeneous medium. Let an incoming plane acoustic wave of frequency ω moving in the direction of the z axis be scattered off a “soft” sphere Ω of radius one which is surrounded by a pocket of rarefied or condensed air in which the local speed of sound is given by c(r) where r = |x| for x ∈ R3. Let us(x)eiωt be the velocity potential of the scattered wave and let r, θ, φ be spherical coordinates in R3. Then from a knowledge of the far field pattern f(θ, φ, λ) for λ = ω/c0 contained in some finite interval 0 < λ0 ≤ λ ≤ λ1,, we would like to determine the unknown function c(r).