Given $X$ a complex Banach space, $L$ a complex nilpotent finite dimensional Lie algebra, and $\rho\,{\colon}\, L\to L(X)$, a representation of $L$ in $X$ such that $\rho (l\,)\,{\in}\, K(X)$ for all $l\,{\in}\, L$, the Taylor, the Słodkowski, the Fredholm, the split and the Fredholm split joint spectra of the representation $\rho$ are computed.

In this article the following are proved: 1. Let $G$ be an infinite $p$-group of cardinality either ${\bf {\mathbb N}_{0}}$ or greater than $2^{\bf {\mathbb N}_{0}}$. If $G$ is center-by-finite and non-$\skew5\check{C}$ernikov, then it is non-co-Hopfian; that is, $G$ is isomorphic to a proper subgroup of itself. 2. Let $G$ be a nilpotent $p$-group of class $2$ with $G/G'$ a non-$\skew5\check{C}$ernikov group of cardinality ${\bf {\mathbb N}_{0}}$ or greater than $2^{{\bf {\mathbb N}_{0}}}$. If $G'$ is of order $p$, then $G$ is non-co-Hopfian.

We prove, under quite general assumptions, the global existence of classical solutions for quasilinear parabolic equations in bounded domains with homogeneous Dirichlet boundary conditions. The same results for weakly coupled reaction-diffusion systems are also given.

We consider the isomorphism problem for partial group rings $R_{\hbox{\scriptsize\it par}}G$ and show that, in the modular case, if $\textit{char}(R)\,{=}\,p$ and $R_{\hbox{\scriptsize\it par}}G_1\,{\cong}\, R_{\hbox{\scriptsize\it par}}G_2$ then the corresponding group rings of the Sylow $p$-subgroups are isomorphic. We use this to prove that finite abelian groups having isomorphic modular partial group algebras are isomorphic. Moreover, in the integral case, we show that the isomorphism of partial group rings of finite groups $G_1$ and $G_2$ implies $\Z G_1\,{\cong}\, \Z G_2$.

We prove that the lower radical, determined by a set of rings, is strong if and only if it is the lower radical determined by a ring with zero multiplication.

In this paper we show that the normal parts of quasisimilar log-hyponormal operators are unitarily equivalent. A Fuglede-Putnam type theorem for log-hyponormal operators is proved. Also, it is shown that a log-hyponormal operator that is quasisimilar to an isometry is unitary and that a log-hyponormal spectral operator is normal.

We prove that the norm $\Vert\,{\cdot}\,\Vert_{n}$ of the space $T[\mathcal{S}_{n},\theta]$ and the norm $\Vert\,{\cdot}\,\Vert_{n}^{M}$ of its modified version $T_{M}[\mathcal{S}_{n},\theta]$ are 3-equivalent. As a consequence, using the results of E. Odell and N. Tomczak-Jaegermann, we obtain that there exists a $K\,{<}\,\infty$ such that for all $n$, $\Vert\cdot\Vert_{n}^{M}$ does not $K-$ distort any subspace of Tsirelson's space $T$.

We define and study some properties of spectral maximal projections of a bounded operator on a complex Banach space. Then we apply these results to the new concepts of weakly projection-relative decomposable operators and projection-relative decomposable operators in the spirit of the works of C. Foias [6], A. Jafarian [7], I. Erdelyi and R. Lange [5].

In this paper we study a seemingly unnoticed property of bases in a Hilbert space that falls in the general area of constructing new bases from old, yet is quite atypical of others in this regard. Namely, if $\{x_n\}$ is any normalized basis for a Hilbert space $ H $ and $ \{\,f_n\}$ the associated basis of coefficient functionals, then the sequence $\{x_n+f_n\} $ is again a basis for $ H $. The unusual aspect of this observation is that the basis $ \{x_n+f_n\} $ obtained in this way from $\{x_n\} $ and $ \{\,f_n\} $ need not be equivalent to either, in contrast to the standard techniques of constructing new bases from given ones by means of an isomorphism on $ H $. In this paper we study bases of this form and their relation to the component bases $ \{x_n\} $ and $ \{\,f_n\}$.

A ring $R$ satisfies the dual of the isomorphism theorem if $R/Ra\cong \mathtt{l}(a)$ for every element $a\in R.$ We call these rings left morphic, and say that $R$ is left P-morphic if, in addition, every left ideal is principal. In this paper we characterize the left and right P-morphic rings and show that they form a Morita invariant class. We also characterize the semiperfect left P-morphic rings as the finite direct products of matrix rings over left uniserial rings of finite composition length. J. Clark has an example of a commutative, uniserial ring with exactly one non-principal ideal. We show that Clark's example is (left) morphic and obtain a non-commutative analogue.

We characterize norm closed subspaces $B$ of $\linf (\partial D)$ such that $C(\partial D) B \subset B$ and maximal ones in the family of proper closed subspaces $B$ of $L^\infty(\partial D)$ such that $A(D) B \subset B$, where $A(D)$ is the disk algebra. Analogously, we characterize closed subspaces of $H^\infty$ that are simultaneously invariant under $S$ and $S^\ast$, the forward and the backward shift operators, and maximal invariant subspaces of $H^\infty$.

The notion of a completely hyperexpansive operator has been introduced in [1] by Athavale. In this paper hyperexpansive operator valued unilateral weighted shifts are investigated. A unique semispectral measure is associated with a bounded completely hyperexpansive operator valued unilateral weighted shift with invertible weights. Examples of bounded and unbounded hyperexpansive operator valued unilateral weighted shifts with invariant domains are given.

An ${\cal L}$-isomorphism between inverse semigroups $S$ and $T$ is an isomorphism between their lattices ${\cal L}(S)$ and ${\cal L}(T)$ of inverse subsemigroups. The author and others have shown that if $S$ is aperiodic – has no nontrivial subgroups – then any such isomorphism $\Phi$ induces a bijection $\phi$ between $S$ and $T$. We first characterize the bijections that arise in this way and go on to prove that under relatively weak ‘archimedean’ hypotheses, if $\phi$ restricts to an isomorphism on the semilattice of idempotents of $S$, then it must be an isomorphism on $S$ itself, thus generating a result of Goberstein. The hypothesis on the restriction to idempotents is satisfied in many applications. We go on to prove theorems similar to the above for the class of completely semisimple inverse semigroups.

Let $A$ be a commutative noetherian local ring, $I$ an ideal of $A$, and $B\,{=}\,A/I$. Assume that the André-Quillen homology functors $H_n (A,B,-) = 0$ for all $n \,{\ge}\, 3$. Then $A$ is Gorenstein if and only if $B$ is.

Non-abelian homology of Lie algebras with coefficients in Lie algebras is constructed and studied, generalising the classical Chevalley-Eilenberg homology of Lie algebras. The relationship between cyclic homology and Milnor cyclic homology of non-commutative associative algebras is established in terms of the long exact non-abelian homology sequence of Lie algebras. Some explicit formulae for the second and the third non-abelian homology of Lie algebras are obtained.