This is an introductory survey of the passage from groups to groupoids and their higher dimensional versions, with most emphasis on calculations with crossed modules and the construction and use of homotopy double groupoids.

In this paper we give an elementary derivation of a 2-groupoid from a fibration. This extends a previous result for pointed fibrations due to Loday. Discussion is included as to the translation between 2-groupoids and cat$^1$-groupoids.

This is a survey of the author`s results about chain functors, tensor products of chain functors, (co-)localizations of chain functors and their relationship with the associated concepts for spectra in the Boardman category. Some introductory remarks about quotient categories, stable homotopy theory and application of the theory to strong homology theories are included.

We describe a category htTop whose objects are pointed continuous maps and whose morphisms are generated under composition by the tracks (relative homotopy classes) of homotopies. For example, if $m_t:hk\to *$ is a nullhomotopy then its track is a morphism from $k$ to $h$. The composition of tracks in htTop amounts to a sharpening of the classical secondary composition operation (Toda bracket). Standard properties of the Toda bracket can be derived in this setting. Moreover we show that htTop is itself the homotopy category of a bicategory htTop and so admits also a secondary composition operation.

This is an updated and expanded version of my pre-print #68 in the $K$-theory server at Urbana (which was an abstract of my talk at Vechta conference on commutative algebra, 1994). In $\S2$ two conjectures on nilpontency of the `monoid Frobenius action' on the $K$-theory of toric cones and on stabilizations of the corresponding $K$-groups are stated. Both of these conjectures are higher analogues of Anderson's conjecture and their proof would bring a rather complete understanding of $K$-theory of toric varieties/semigroup rings.

Based on a rather arbitrary class of morphisms in a category, which play the role of "closed maps", we present a general approach to separation and compactness, both at the object and the morphism levels. It covers essential parts of the classical topological theory, generalizes various previous categorical treatments of the theme, and allows for a number of less expected applications outside topology.

The paper describes a simple procedure for computing the second cohomology $H^2(G,A)$ of a finitely presented finite group $G$ with coefficients in a finitely presented finite abelian group $A$. An accompanying text-file contains a MAGMA implementation of the procedure.

We survey recent developments introducing quantum algebraic methods into the study of cohomology operations in complex oriented cohomology theory. In particular, we discuss geometrical and homotopy theoretical aspects of the quantum double of the Landweber-Novikov algebra, as represented by a subalgebra of operations in double complex cobordism. We work in the context of Boardman's eightfold way, which offers an important framework for clarifying the relationship between quantum doubles and the standard machinery of Hopf algebroids of homology cooperations. These considerations give rise to novel structures in double cohomology theory, and we explore the twist operation and extensions of the quantum antipode by way of example.

The notions of tensor end exterior products modulo $q$ of two crossed $P$-modules, where $q$ is a positive integer and $P$ is a Lie algebra, are introduced and some properties are established. The condition for the existence of a universal $q$-central relative extension of a Lie epimorphism is given and this extension is described as an exterior product modulo $q$.

Let $X$ be a connected CW complex and $[X]$ be its homotopy type. As usual, $\mbox{SNT}(X)$ denotes the pointed set of homotopy types of CW complexes $Y$ such that their $n^{th}$-Postnikov approximations $X^{(n)}$ and $Y^{(n)}$ are homotopy equivalent for all $n$. In this paper we study a particularly interesting subset of \mbox{SNT}$(X)$, denoted SNT$ _{\pi } (X)$, of strong $n$ type; the $n^{th}$-Postnikov approximations $X^{(n)}$ and $Y^{(n)}$ are homotopy equivalent by homotopy equivalences satisfying an extra condition at the level of homotopy groups. First, we construct a CW complex $X$ such that $\mbox{SNT}_\pi(X) \neq \{ [X] \}$ and we establishe a connection between the pointed set $\mbox{SNT}_\pi (X)$ and sub-groups of homotopy classes of self-equivalences via a certain $\displaystyle\lim_{\leftarrow}{}^1 $ set. Secondly, we prove a conjecture of Arkowitz and Maruyama concerning subgroups of the group of self equivalences of a finite CW complex and we use this result to establish a characterization of simply connected CW complexes with finite dimensional rational cohomology such that $\mbox{SNT}_\pi(X) = \{[X]\}$.