These are expanded notes of four introductory talks on $A_\infty$-algebras, their modules and their derived categories.

Every chain functor ${\bf K}_{*}$ determines a homology theory on a given category of topological spaces resp. of spectra $H_{*}(\bf K_{*})(\cdot)$ cf. $\S$ 4. If $\bf K_{*}$, ${\bf L}_{*}$ are chain functors such that $H_{*}({\bf K}_{*})(\cdot) \approx H_{*}({\bf L}_{*})(\cdot)$ then there exists a third chain functor ${\bf C}_{*}$ and transformations of chain functors ${}^{K}\gamma :{\bf K}_{*} \longrightarrow {\bf C}_{*}$, ${}^{L}\gamma:\ {\bf L}_{*} \longrightarrow {\bf C}_{*}$ inducing isomorphisms of the associated homology theories (theorem 1.1.). Moreover the distinction between regular and irregular chain functors is introduced.

1) The tensor product of two spectra, different from the $\wedge$-product, is introduced in such a way that a Künneth theorem holds. 2) Localizations of spectra are treated by using the more algebraic category of chain functors (instead of the category of CW-spectra). 3) The localization of a given chain functor ${\bf K}_*$ can up to chain homotopy be expressed by tensoring ${\bf K}_*$ with the localization of a fixed chain functor $\boldsymbol{\mathbb Z}_*$.

Teisi are certain higher-dimensional categorical structures proposed for doing non-abelian homotopical and homological algebra. I give a partial justification for this proposal by showing that in the abelian case teisi indeed reduce to chain complexes. I also show that the result holds for a much wider class of weaker higher-dimensional categorical structures, which however need to have strict identities. The main step in the proof is an elegant Eckmann-Hilton type argument.

Mitchell characterized all finite partially ordered sets with incidence ring of Hochschild dimension 0, 1, and 2. Cheng characterized all finite partially ordered sets of cohomological dimension one. There are no conjectures in other dimensions. This article contains the algorithms for calculating the dimensions of finite partially ordered sets by elementary operations over rows and columns of matrices with integer entries.

By considering the notion of action of a categorical group ${\mathbb G}$ on another categorical group ${\mathbb H}$ we define the semidirect product ${\mathbb H}\ltimes {\mathbb G}$ and classify the set of all split extensions of ${\mathbb G}$ by ${\mathbb H}$. Then, in an analogous way to the group case, we develop an obstruction theory that allows the classification of all split extensions of categorical groups inducing a given pair $(\varphi,\psi)$ (called a collective character of ${\mathbb G}$ in ${\mathbb H}$) where $\varphi:\pi_0({\mathbb G})\rightarrow \pi_0({\cal E}q({\mathbb H}))$ is a group homomorphism and $\psi:\pi_1({\mathbb G})\rightarrow \pi_1({\cal E}q({\mathbb H}))$ is a homomorphism of $\pi_0({\mathbb G})$-modules.

We define homotopy Lie-Rinehart pairs and the associated homotopy Rinehart algebra in the context of coalgebras. We define homotopy Lie-Rinehart resolutions and present conditions under which the associated homotopy Rinehart algebra is a cohomological model for the Rinehart algebra of the resolved Lie-Rinehart pair.

We use an interlaced inductive procedure reminiscent of the integration process from traditional deformation theory to construct a homotopy Lie-Rinehart resolution for the Lie-Rinehart pair which arises as an exercise in Poisson reduction in the context of the BFV construction of classical BRST cohomology. We show that the associated homotopy Rinehart algebra and the BRST algebra are isomorphic as graded commutative algebras. In the irreducible case, the two have the same cohomology.

Let $X$ be a simply connected space and $\Bbbk$ a commutative ring. Goodwillie, Burghelea and Fiedorowicz proved that the Hochschild cohomology of the singular chains on the space of pointed loops $HH^{*}S_*(\Omega X)$ is isomorphic to the free loop space cohomology $H^{*}(X^{S^{1}})$. We prove that this isomorphism is compatible with the usual cup product on $H^{*}(X^{S^{1}})$ and the cup product of Cartan and Eilenberg on $HH^{*}S_*(\Omega X)$. In particular, we make explicit the algebra $H^{*}(X^{S^{1}})$ when $X$ is a suspended space, a complex projective space or a finite CW-complex of dimension $p$ such that $\frac {1}{(p-1)!}\in {\Bbbk}$.