Let (A, ∆) be a weak multiplier Hopf algebra as introduced in [32] (see also [31]). It is a pair of a non-degenerate algebra A, with or without identity, and a coproduct ∆ on A, satisfying certain properties. The main difference with multiplier Hopf algebras is that now, the canonical maps T 1 and T 2 on A ⊗ A, defined byandare no longer assumed to be bijective. Also recall that a weak multiplier Hopf algebra is called regular if its antipode is a bijective map from A to itself.In this paper, we introduce and study the notion of integrals on such regular weak multiplier Hopf algebras. A left integral is a non-zero linear functional on A that is left invariant (in an appropriate sense). Similarly for a right integral.For a regular weak multiplier Hopf algebra (A, ∆) with (sufficiently many) integrals, we construct the dual ( A, ∆). It is again a regular weak multiplier Hopf algebra with (sufficiently many) integrals. This duality extends the known duality of finite-dimensional weak Hopf algebras to this more general case. It also extends the duality of multiplier Hopf algebras with integrals, the so-called algebraic quantum groups. For this reason, we will sometimes call a regular weak multiplier Hopf algebra with enough integrals an algebraic quantum groupoid.We discuss the relation of our work with the work on duality for algebraic quantum groupoids by Timmermann [20].We also illustrate this duality with a particular example in a separate paper (see [30]). In this paper, we only mention the main definitions and results for this example. However, we do consider the two natural weak multiplier Hopf algebras associated with a groupoid in detail and show that they are dual to each other in the sense of the above duality.a (1) S(a (2) ).We are using the Sweedler notation here (see further in this introduction under the item Conventions and notations). It has been carefully argued in [33] that these maps are well-defined with values in M (A).The images ε s (A) and ε t (A) are subalgebras of M (A). They are called the source and target algebras. They are commuting non-degenerate subalgebras of M (A). In the regular case, they embed in M (A) in such a way that their multiplier algebras M (ε s (A)) and M (ε t (A)) still embed in M (A). These multiplier algebras are denoted by A s and A t resp. They are still commuting subalgebras of M (A).The source and target algebras ε s (A) and ε t (A) can be identified resp. with the left and the right leg of E. In fact we have E ∈ M (ε s (A) ⊗ ε t (A)).Remark that in earlier papers on the subject, we called A s and A t the source and target algebras. However, in the second version of our second paper on the subject [33], we changed the terminology.For more details about the short review over regular multiplier Hopf algebra given above, we refer to earlier work, in particular [32] and [33].
The examples associated with a groupoidConsider a groupoid G. One can associate two regular weak multiplier Hopf algebras.