arXiv subjects
Publications and source records attributed to Mihai D. Staic.
We study some examples of braided categories and quasitriangular Hopf algebras and decide which of them is pseudosymmetric, respectively pseudotriangular. We show also that there exists a universal pseudosymmetric braided category.
We give a new explicit construction for the simplicial group $K(A,n)$. We explain the topological interpretation and discuss some possible applications.
We introduce and study algebraic structures underlying 2-dimensional Homotopy Quantum Field Theories (HQFTs) with arbitrary target spaces. These algebraic structures are formalized in the notion of a twisted Frobenius algebra. Our work generalizes results of Brightwell, Turner, and the second author on 2-dimensional HQFTs with simply-connected or aspherical targets.
We give an explicit characterization for group extensions that correspond to elements of the symmetric cohomology $HS^2(G,A)$. We also give conditions for the map $HS^n(G,A)\to H^n(G,A)$ to be injective.
For a triple $(G,A,κ)$ (where $G$ is a group, $A$ is a $G$-module and $κ:G^3\to A$ is a 3-cocycle) and a $G$-module $B$ we introduce a new cohomology theory $_2H^n(G,A,κ;B)$ which we call the secondary cohomology. We give a construction that associates to a pointed topological space $(X,x_0)$ an invariant $_2κ^4\in_2H^4(π_1(X),π_2(X),κ^3;π_3(X))$. This construction can be seen a "3-type" generalization of the classical $k$-invariant.
Motivated by the recently introduced concept of a pseudosymmetric braided monoidal category, we define the pseudosymmetric group PS_n, as the quotient of the braid group B_n by the relations σ_iσ_{i+1}^{-1}σ_i=σ_{i+1}σ_i^{-1}σ_{i+1}, with 1\leq i\leq n-2. It turns out that PS_n is isomorphic to the quotient of B_n by the commutator subgroup [P_n, P_n] of the pure braid group P_n (which amounts to saying that [P_n, P_n] coincides with the normal subgroup of B_n generated by the elements [σ_i^2, σ_{i+1}^2], with 1\leq i\leq n-2), and that PS_n is a linear group.
We study some classes of lazy cocycles, called pure (respectively neat), together with their categorical counterparts, entwined (respectively strongly entwined) monoidal categories.
A laycle is the categorical analogue of a lazy cocycle. Twines (as introduced by Bruguieres) and strong twines (as introduced by the authors) are laycles satisfying some extra conditions. If $c$ is a braiding, the double braiding $c^2$ is always a twine; we prove that it is a strong twine if and only if $c$ satisfies a sort of modified braid relation (we call such $c$ pseudosymmetric, as any symmetric braiding satisfies this relation). It is known that symmetric Yetter-Drinfeld categories are trivial; we prove that the Yetter-Drinfeld category $_H{\cal YD}^H$ over a Hopf algebra $H$ is pseudosymmetric if and only if $H$ is commutative and cocommutative. We introduce as well the Hopf algebraic counterpart of pseudosymmetric braidings under the name pseudotriangular structures and prove that all quasitriangular structures on the $2^{n+1}$-dimensional pointed Hopf algebras E(n) are pseudotriangular. We observe that a laycle on a monoidal category induces a so-called pseudotwistor on every algebra in the category, and we obtain some general results (and give some examples) concerning pseudotwistors, inspired by properties of laycles and twines.
We introduce $Δ$-groups and show how they fit in the context of lattice field theory. To a manifold $M$ we associate a $Δ$-group $Γ(M)$. We define the symmetric cohomology $HS^n(G,A)$ of a group $G$ with coefficients in a $G$-module $A$. The $Δ$-group $Γ(M)$ is determined by the action of $π_1(M)$ on $π_2(M)$ and an element of $HS^3(π_1(M),π_2(M))$.
If H is a Hopf algebra with bijective antipode and α, β\in Aut_{Hopf}(H), we introduce a category_H{\cal YD}^H(α, β), generalizing both Yetter-Drinfeld and anti-Yetter-Drinfeld modules. We construct a braided T-category {\cal YD}(H) having all these categories as components, which if H is finite dimensional coincides with the representations of a certain quasitriangular T-coalgebra DT(H) that we construct. We also prove that if (α, β) admits a so-called pair in involution, then_H{\cal YD}^H(α, β) is isomorphic to the category of usual Yetter-Drinfeld modules_H{\cal YD}^H.