Let M be a real hypersurface of the complex projective space Pn(C). The Ricci tensor S of M is recurrent if there exists a 1-form $\alpha$ such that $\nablaS=S\otimes\alpha;$. In this paper we show that there are no real hypersurfaces with recurrent Ricci tensor of Pn(C) under the condition that $\xi$ is a principal curvature vector.

1991 Mathematics Subject Classification 53C40 (53C25).

Consider a group presentation: $$\hat{[Pscr ]}\tfrm{=<\tfbf{x};}\tfbf{r}\tfrm{>}$$. Here x is a set and r is a set of non-empty, cyclically reduced words on the alphabet x ∪ x−1 (where x−1 is a set in one-to-one correspondence x[harr ]x−1 with x). We assume throughout that $\hat{[Pscr ]}$ is finite. Let $\hat{F}$ be the free group on x (thus $\hat{F}$ consists of free equivalence classes [W] of word on x∪x−1), and let N be the normal closure of {[R] : R∈r} in $\hat{F}$. Then the group G=G($\hat{[Pscr ]}$) defined by $\hat{[Pscr ]}$ is $\hat{F}\tfrm{/}N$. We will write W1 =GW2 if [W1]N=[W2]N.

Let n∈ℕ and let Fn be the free group on n generators. Let w be an arbitrary word in Fn, and let σ be an n-cycle in Sn. We consider groups of the type Γ(n,w)=Fn/N, where N is the normal closure in Fn of the “cycled words’’ w, σ(w), σ2(w),…,σn−1(w), and solve, by means of classical algebraic number theory, the following problems.

A. When is Γ(n,w)ab infinite?

B. When is Γ(n,w) a perfect group?

In this paper G denotes a non-identity finite soluble group. If A is an irreducible G-module, EndGA is a division ring by Schur's Lemma, actually a field, since G finite forces A to be finite. Moreover A is a vector space over EndGA with respect to $\alphaa:=\alpha(a),\alpha\in\rm{End}_GA,a\inA$. We let $\varphi_G(A):=\rm{dim}_{\rm{End}_GA}A$. Any chief factor of G is an irreducible G-module via the conjugation action, and it is central precisely when it is a trivial G-module. By a refined version of the Theorem of Jordan-Hölder [1, p. 33] the number $\delta_G(A)$ of complemented chief factors of G, which are G-isomorphic to a given A, is constant for any chief series of G. We say that A is complemented, as aG-module, if $\delta_G(A)>0$.

Dedicated to the memory of Professor Katsutoshi Takahashi

In this paper, we introduce p-hyponormal tuples in the sense of D. Xia. Furthermore we extend Putnam’s inequality to these tuples and show an equivalence relation of two spectra.

Rings in which each right ideal is quasi-continuous (right π-rings) are shown to be a direct sum of semisimple artinian square full ring and a right square free ring. Among other results it is also shown that (i) a nonlocal right continuous indecomposable right π-ring is either simple artinian or a ring of matrices of a certain type, and (ii) an indecomposable non-local right continuous ring is both a right and a left π-ring if and only if it is a right q-ring. In particular, a non local indecomposable right q-ring is a left q-ring.

It is known that the class of mils generalizes that of pramarts and martingales in the limit. Also every Banach space-valued mil (Xn) with lim infnE(‖Xn‖)<∞ can be written in a unique form: $X_n=M_n+P_n(n\in\rm{N})$, where $(M_n)$ is a uniformly integrable martingale and $(P_n)$ converges to zero a.s. in norm. We shall show that this result still holds for a class which essentially generalizes that of mils. Another class of Banach space-valued martingale-like sequences, still containing all pramarts is defined and shown to have the decomposition above under the following much weaker condition: $\rm{lim inf}_{r\inT}E(\VertX_{\tau}\Vert)<\infty$, where T denotes the set of all bounded stopping times.

1991 Mathematics Subject Classification. 60G48, 60B11.

Let A={a1,…,an,a1−1,…,an−1} and iteration of A denoted by A[starf ] to be the set of words in A (including the empty word). Let S⊆A[starf ]; then the growth function of the set S is the function Γ(l)=number of words in S of length l. For m≤n let $\vec {a}$=(ai1,…,aim), where ik∈{1,…,n} are different; then the relative growth function with respect to $\vec {a}$ is the function Γ$\vec {a}$(l,l1,…,lm)=number of words in S of length l+l1+…+lm having (for each k) lk total occurrences of aik and aik−1.

In a recent illuminating paper, June M. Parker [5] discussed Choquet integral representations of comonotonic additive functionals and related concepts. In our paper we provide a generalization of the Choquet integral and use this to obtain an integral representation for comonotonic additive operators.

Let the locally finite group G be the product of two locally nilpotent subgroups A and B, and assume that H is a subgroup of G belonging to a group classℱ . The question is considered whether there exists a subgroup X of G containing H which belongs to ℱ and satisfies $X=(X\capA)(X\capB)$. Under various assumptions on G and ℱ, necessary and sufficient conditions for the existence of such a subgroup X are obtained.

First we define the notion of k-Ricci curvature of a Riemannian n-manifold. Then we establish sharp relations between the k-Ricci curvature and the shape operator and also between the k-Ricci curvature and the squared mean curvature for a submanifold in a Riemannian space form with arbitrary codimension. Several applications of such relationships are also presented.

The primary purpose of this paper is to provide general sufficient conditions for any real quadratic order to have a cyclic subgroup of order n∈ℕ in its ideal class group. This generalizes results in the literature, including some seminal classical works. This is done with a simpler approach via the interplay between the maximal order and the non-maximal orders, using the underlying infrastructure via the continued fraction algorithm. Numerous examples and a concluding criterion for non-trivial class numbers are also provided. The latter links class number one criteria with new prime-producing quadratic polynomials.

The purpose of this note is to prove a results of Jain and López-Permouth under a weaker conditions replacing R-weak injectivity by R-tightness and even getting a simpler proof.

Let A be a UHF-algebra and K the C*-algebra of all compact operators on a countably infinite-dimensional Hilbert space. In this note we shall find all projections p in A with $pAp\congA$ and, using these projections, we shall determine the group of automorphisms of $K_0(A\otimes$K) induced by those of $A\otimes$K in some cases.

1991 Mathematics Subject Classification 46 L40.

An example of a non-topologizable algebra was given in [2]. In [4] Żelazko gave a simple proof of the fact that, if X is an infinite-dimensional vector space, then the algebra of all finite-rank linear operators on X is not topologizable as a topological algebra. In the following we use a similar idea to prove that, if E is a Fréchet space which is not normable, then each subalgebra A of the algebra of all bounded linear operators on E such that A contains the ideal of continuous, finite-rank operators, is non-topologizable as a topological algebra. This is a shorter proof and more general version of the result of [1].

The ‘canonical embedding approach’ was introduced by the second author and, subsequently, it has been applied several times to prove the embeddability of certain regular extensions by groups into semidirect products by groups. In the present paper this technique is generalized so that it is suitable to handle regular extensions by inverse semigroups. As an application, B. Billhardt's embedding theorem on regular extensions of semilattices by inverse semigroups is reproved.

For any permutation σ on N = {1,2,…,n}: (i) Green’s relations are characterized in the centralizer C(σ) of σ (relative to the semigroup PTn of partial transformations on N); and (ii) a criterion is given for C(σ) to be a regular semigroup (inverse semigroup, union of groups).

Let S be an (ideal) extension of a completely 0-simple semigroup S0 by a completely 0-simple semigroup S1. Congruences on S can be uniquely represented in terms of congruences on S0 and S1. In this representation, for a congruence ρ on S, we express ρK,ρT,ρK and ρT, where these denote the least (greatest) congruences with the same kernel (trace) as ρ. Let κ be the least completely 0-simple congruence on S. We provide necessary and sufficient conditions, in terms of the kernel of κ, in order that the relation K be a congruence, and also that [Cscr ](S)/K be a modular lattice, where [Cscr ](S) denotes the congruence lattice of S.

In this paper we give a characterization of the central elements of the algebra A** for a class of weakly sequentially complete Banach algebras A and present a detailed study of several questions related to Arens regularity of the algebra A.

1991 Mathematics Subject Classification Primary: 43A10, 46B20, 46J99.