Abstract: Let $G$ be a finitely generated group, and let $\Bbbk{G}$ be its group algebrao ver a field of characteristic $0$. A Taylor expansion is a certain type of map from $G$ to the degree completion of the associated graded algebra of $\Bbbk{G}$ which generalizes the Magnus expansion of a free group. The group $G$ is said to be filtered-formal if its Malcev Lie algebra is isomorphic to the degree completion of its associated graded Lie algebra. We show that $G$ is filtered-formal if and only if it admits a Taylor expansion, and derive some consequences.

Abstract: The group of basis-conjugating automorphisms of the free group of rank $n$, also known as the McCool group or the welded braid group $P\Sigma_n$, contains a much-studied subgroup, called the upper McCool group $P\Sigma_n^+$. Starting from the cohomology ring of $P\Sigma_n^+$, we find, by means of a Gr\”obner basis computation, a simple presentation for the infinitesimal Alexander invariant of this group, from which we determine the resonance varieties and the Chen ranks of the upper McCool groups. These computations reveal that, unlike for the pure braid group $P_n$ and the full McCool group $P\Sigma_n$, the Chen ranks conjecture does not hold for $P\Sigma_n^+$, for any $n\ge 4$. Consequently, $P\Sigma_n^+$ is not isomorphic to $P_n$ in that range, thus answering a question of Cohen, Pakianathan, Vershinin, and Wu. We also determine the scheme structure of the resonance varieties $\mathcal{R}_1(P\Sigma_n^+)$, and show that these schemes are not reduced for $n\geq 4$.

Abstract: We explore the graded and filtered formality properties of finitely generated groups by studying the various Lie algebras over a field of characteristic 0 attached to such groups, including the Malcev Lie algebra, the associated graded Lie algebra, the holonomy Lie algebra, and the Chen Lie algebra. We explain how these notions behave with respect to split injections, coproducts, direct products, as well as field extensions, and how they are inherited by solvable and nilpotent quotients. A key tool in this analysis is the 1-minimal model of the group, and the way this model relates to the aforementioned Lie algebras. Another approach to formality is provided by Taylor expansions from the group to the completion of the associated graded algebra of the group ring. We illustrate our approach with examples drawn from a variety of group-theoretic and topological contexts, such as finitely generated torsion-free nilpotent groups, link groups, and fundamental groups of Seifert fibered manifolds.

Abstract: We generalize basic results relating the associated graded Lie algebra and the holonomy Lie algebra from finitely presented, commutator-relators groups to arbitrary finitely presented groups. In the process, we give an explicit formula for the cup-product in the cohomology of a finite 2-complex, and an algorithm for computing the corresponding holonomy Lie algebra, using a Magnus expansion method. We illustrate our approach with examples drawn from a variety of group-theoretic and topological contexts, such as link groups, one-relator groups, and fundamental groups of Seifert fibered manifolds.

Abstract: We investigate the resonance varieties, lower central series ranks, and Chen ranks of the pure virtual braid groups and their upper-triangular subgroups. As an application, we give a complete answer to the 1-formality question for this class of groups. In the process, we explore various connections between the Alexander-type invariants of a finitely generated group and several of the graded Lie algebras associated to it, and discuss possible extensions of the resonance-Chen ranks formula in this context.

Abstract: In this survey, we investigate the resonance varieties, the lower central series ranks, the Chen ranks, and the formality properties of several families of braid-like groups: the pure braid groups $P_n$, the welded pure braid groups $wP_n$, the virtual pure braid groups $vP_n$, as well as their `upper’ variants, $wP_n^+$ and $vP_n^+$. We also discuss several natural homomorphisms between these groups, and various ways to distinguish among the pure braid groups and their relatives.

Abstract: Let ($\Omega^{\ast}(M), d$) be the de Rham cochain complex for a smooth compact closed manifolds $M$ of dimension $n$. For an odd-degree closed form $H$, there are a twisted de Rham cochain complex $(\Omega^{\ast}(M), d+H_\wedge)$ and its associated twisted de Rham cohomology $H^*(M,H)$. We show that there exists a spectral sequence $\{E_r^{p, q}, d_r\}$ derived from the filtration $F_p(\Omega^{\ast}(M))=\bigoplus_{i\geq p}\Omega^i(M)$ of $\Omega^{\ast}(M)$, which converges to the twisted de Rham cohomology $H^*(M,H)$. We also show that the differentials in the spectral sequence can be given in terms of cup products and specific elements of Massey products as well, which generalizes a result of Atiyah and Segal. Some results about the indeterminacy of differentials are also given in this paper.

Abstract: Let $p$ be an odd prime and $A$ be the mod $p$ Steenrod algebra. For computing the stable homotopy groups of spheres with the classical Adams spectral sequence, we must compute the $E_2$-term of the Adams spectral sequence, $\mathrm{Ext}_A^{\ast,\ast} (\mathbb{Z}_p,\mathbb{Z}_p)$. In this paper we prove that in the cohomology of $A$, the product $k_0 h_n \tilde{\delta}_{s+4} \in \mathrm{Ext}_{A}^{s+7, t(s,n)+s}(\mathbb{Z}_p, \mathbb{Z}_p)$, is nontrivial for $n \geq 5$, and trivial for $n=3, 4$, where $\tilde{\delta}_{s+4}$ is actually $\tilde{\alpha}_{s+4}^{(4)}$ described by Wang and Zheng, $p \geq 11$, $0 \leq s < p – 4$ and $t(s,n)=2(p-1)[(s + 2) + (s + 4)p + (s + 3)p^2 + (s + 4)p^3 + p^n]$.

Abstract: Let p be an odd prime greater than seven and A the mod p Steenrod algebra. In this paper we prove that in the cohomology of A the product $h_1 h_n \tilde \delta _{s + 4}\in \mathrm{Ext}_A^{s+6,t(s,n)+s}(\mathbb{Z}_p,\mathbb{Z}_p)$ is nontrivial for $n \geq 5$, and trivial for $n=3, 4$, where $ \tilde \delta _{s + 4}$ is actually $\tilde \alpha _{s+4}^{(4)}$ described by X. Wang and Q. Zheng, $0 \leq s < p – 4$, $t(s,n) = 2(p-1)[(s + 1) + (s + 3)p + (s + 3)p^2 + (s + 4)p^3 + p^n ].$ We show our results by explicit combinatorial analysis of the (modified) May spectral sequence. The method of proof is very elementary.