We show that the category A(G) of actions of a locally compact group G on C∗-algebras (with equivariant nondegenerate ∗-homomorphisms into multiplier algebras) is equivalent, via a full-crossed-product functor, to a comma category of maximal coactions of G under the comultiplication (C∗(G), δG); and also that A(G) is equivalent, via a reduced-crossed-product functor, to a comma category of norm...

We study how the comultiplication on a Hopf algebra can be modified in such a way that the new comultiplication together with the original multiplication and a suitable antipode gives a new Hopf algebra. To this end, we have to study Harrison type cocycles, and it turns out that there is a relation with the Yang-Baxter equation. The construction is applied to deform the coalgebra structure on t...

We define the reduced Khovanov homology of an open book (S, φ), and we identify a distinguished “contact element” in this group which may be used to establish the tightness of contact structures compatible with (S, φ). Our construction generalizes the relationship between the reduced Khovanov homology of a link and the Heegaard Floer homology of its branched double cover. As an application, we ...

The objective of the present paper is to give a survey of recent progress on applications of the approaches of Ringel-Hall type algebras to quantum groups and cluster algebras via various forms of Green’s formula. In this paper, three forms of Green’s formula are highlighted, (1) the original form of Green’s formula [Gre][Rin2], (2) the degeneration form of Green’s formula [DXX] and (3) the pro...

Let G be any group and let K(G) denote the multiplier Hopf algebra of complex functions with finite support in G. The product in K(G) is pointwise. The comultiplication on K(G) is defined with values in the multiplier algebra M(K(G)⊗K(G)) by the formula (∆(f))(p, q) = f(pq) for all f ∈ K(G) and p, q ∈ G. In this paper we consider multiplier Hopf algebras B (over C) such that there is an embeddi...