This is an attempt to present one aspect of the work of Jan-Erik Roos.

To every homogeneous ideal of a polynomial ring $S$ over a field $K$, Macaulay assigned an ideal generated by monomials in the indeterminates and with the same Hilbert function. Thus, from the point of view of Hilbert series residue rings modulo monomial ideals display the most general behavior. The homological perspective reveals a very different picture. Two aspects are particularly relevant to this paper:

If $I$ is generated by monomials, then the Poincaré series of the residue field $k$ of $S/I$ is rational by Backelin [7], and the homotopy Lie algebra of $S/I$ is finitely generated by Backelin and Roos [8]. Constructions of Anick [1] Roos [15], respectively, show that these properties may fail for general homogeneous ideals.

Recently, Gasharov, Peeva, and Welker [12] showed that some homological properties of $S/I$, such as being Golod, depend only on combinatorial data gathered from a minimal set of monomial generators.

Here we prove that these data determine the Poincaré series of $k$ over $S/I$, along with most of its homotopy Lie algebra. As a consequence, we obtain the surprising result that if the number of generators of the ideal $I$ is fixed, then the number of such Poincaré series is finite, even when $K$ ranges over all fields.

We introduce and study various properties of the secondary cohomology of a space. Certain Steenrod squares are shown to be related to the action of the symmetric groups on secondary cohomology.

We introduce the notion of crossed $n$-fold extensions of an algebra $B$ by a bimodule $M$ and prove that such extensions represent classes in the Hochschild cohomology of $B$ with coefficients in $M$. Moreover we consider this way characteristic classes of chain (resp. cochain) algebras in Hochschild cohomology.

In this paper a definition of a category of modules over the ring of differential operators on a smooth variety of finite type in positive characteristics is given. It has some of the good properties of holonomic D-modules in zero characteristic. We prove that it is a Serre category and that it is closed under the usual D-module functors, as defined by Haastert. The relation to the similar concept of F-finite modules, introduced by Lyubeznik, is elucidated, and several examples, such as etale algebras, are given.

Let $F$ be the homotopy fiber of a continuous map $f:X@>>>Y$, and let $R$ be a commutative, unitary ring. Given a morphism of chain Hopf algebras that models $(\Omega f)_{\sharp}:C_{*}(\Omega X;R)@>>>C_{*}(\Omega Y;R)$, we construct a cochain algebra that models $C^*(F;R)$. We explain how to simplify the model for certain large classes of maps $f$ and provide examples of the application of our model.

Second cohomology groups of irreducible representations of classical Lie algebras $A_2, B_2$ and $G_2$ over an algebraically closed field of characteristic $p>h$ are calculated. Here $h$ is the Coxeter number.

An algebra with the identity $x\circ (y\circ z-z\circ y)= (x\circ y)\circ z-(x\circ z)\circ y$ is called right-symmetric. A right-symmetric algebra with the identity $x\circ(y\circ z)= y\circ(x\circ z)$ is called Novikov. We describe bases of free right-symmetric algebras and free Novikov algebras and give realizations of them in terms of trees. The free Lie algebra is obtained as a Lie subalgebra of the free right-symmetric algebra. We use our methods to study identities of Witt algebras.

This paper takes its starting point in an idea of Grothendieck on the representation of homotopy types. We show that any locally finite nilpotent homotopy type can be represented by a simplicial set which is a finitely generated free group in all degrees and whose maps are given by polynomials with rational coefficients. Such a simplicial set is in some sense a universal localisation/completion as all localisations and completions of the homotopy is easily constructed from it. In particular relations with the Quillen and Sullivan approaches are presented. When the theory is applied to the Eilenberg-MacLane space of a torsion free finitely generated nilpotent group a close relation to the the theory of Passi polynomial maps is obtained.

Let $L$ be an infinite dimensional graded Lie algebra that is either the homotopy Lie algebra $\pi_*(\Omega X)\otimes {\mathbb Q}$ for a finite $n$-dimensional CW complex $X$, or else the homotopy Lie algebra for a local noetherian commutative ring $R $ ($UL = Ext_R(I\! k,I\! k)$) in which case put $n =$ (embdim $-$ depth)$(R)$.

Theorem: (i) The integers $\lambda_k = \displaystyle\sum_{q=k}^{k+n-2} \mbox{dim} L_i$ grow faster than any polynomial in $k$.

(ii) For some finite sequence $x_1, \ldots , x_d$ of elements in $L$ and some $N$, any $y\in L_{\geq N}$ satisfies: some $[x_i,y] \neq 0$.

We study the Koszul dual for general superalgebras, and apply it to the Koszul homology of a graded algebra. We show that a part of the Koszul homology algebra is related to the homotopy Lie algebra by means of Koszul duality. This is used to study the "Minimal Resolution Conjecture" and the "Ideal Generating Conjecture" for sets of generic points in projective space, and for quotients of the polynomial ring (or exterior algebra) modulo generic quadratic forms.

The analogs of Chevalley generators are offered for simple (and close to them) $\Zee$-graded complex Lie algebras and Lie superalgebras of polynomial growth without Cartan matrix. We show how to derive the defining relations between these generators and explicitly write them for a "most natural" ("distinguished" in terms of Penkov and Serganova) system of simple roots. The results are given mainly for Lie superalgebras whose component of degree zero is a Lie algebra (other cases being left to the reader). Observe presentations presentations [sic!] of exceptional Lie superalgebras and Lie superalgebras of hamiltonian vector fields.

Resolutions that arise as iterated mapping cones are considered. Explicit resolutions are given for monomial ideals with linear quotients which admit decomposition functions, and it is studied under which conditions a mapping cone admits a DG algebra structure.

We give a version of the theorem of Lipman and Sathaye on Jacobian ideals but with substantially weaker hypotheses. Both their version and the result here are very useful in providing explicit test elements in tight closure theory. There are two separate ways in which the hypothesis in the theorem is weakened here: one is that the larger ring is not required to be a domain, although it will be reduced. Second, the regularity condition on the smaller ring is weakened to the point where one need not assume that it is Cohen-Macaulay. Instead, a condition on the ring homomorphism is imposed that may be viewed as a relative analogue of the Serre conditon S$_2$: a family of such conditions is introduced and studied here. The definition is made in terms of a presentation of an algebra, but is independent of the presentation.

Let $k$ be a field, and $A$ be a polynomial algebra over $k$. Let $I\subseteq A$ be an ideal. We present a novel method for computing resolutions of $A/I$ over $A$. The method is a synthesis of Gröbner basis techniques and homological perturbation theory. The examples in this paper were computed using computer algebra.

In this text we study classical Lie algebras. We prove that a periodisation of such Lie algebras without $\mathfrak{sl}$-component can be presented as a free graded Lie algebra modulo quadratic relations only. Our approach will be through a Chevalley basis and our method relies on elementary tools only.

The classical deformation theory for modules on a $k$-algebra, where $k$ is a field, is generalized. For any $k$-algebra, and for any finite family of $r$ modules, we consider a deformation functor defined on the category of Artinian $r$-pointed (not necessarily commutative) $k$-algebras, and prove that it has a prorepresenting hull, which can be computed using Massey-type products in the $Ext$-groups. This is first used to construct $k$-algebras with a preassigned set of simple modules, and to study the moduli space of iterated extensions of modules. In forthcoming papers we shall show that this noncommutative deformation theory is a useful tool in the study of k-algebras, and in establishing a noncommutative algebraic geometry.

For every nonholonomic manifold, i.e., manifold with nonintegrable distribution the analog of the Riemann tensor is introduced. It is calculated here for the contact and Engel structures: for the contact structure it vanishes (another proof of Darboux's canonical form); for the Engel distribution the target space of the tensor is of dimension 2. In particular, the Lie algebra preserving the Engel distribution is described.

The tensors introduced are interpreted as modifications of the Spencer cohomology and, as such, provide with a new way to solve partial differential equations. Goldschmidt's criteria for formal integrability (vanishing of certain Spencer cohomology) are only applicable to "one half" of all differential equations, the ones whose symmetries are induced by point transformations. Lie's theorem says that the "other half" consists of differential equations whose symmetries are induced by contact transformations. Therefore, we can now extend Goldschmidt's criteria for formal integrability to all differential equations.

We calculate the Hilbert series of a quotient of the exterior algebra by a generic form of even degree, and give conjectures about the Hilbert series of other generic quotients.

We discuss how lattices and posets can be used as tools to study minimal free resolutions of monomial or toric ideals.