Higher-dimensional algebra .2. 2-Hilbert spaces

A 2-Hilbert space is a category with structures and properties analogous to those of a Hilbert space. More precisely, we define a 2-Hilbert space to be an Abelian category enriched over Hilb with a *-structure, conjugate-linear on the hem-sets, satisfying [f(g), h] = [g, f*h] = [f, hg*]. We also define monoidal, braided monoidal, and symmetric monoidal versions of 2-Hilbert spaces, which we call 2-H*-algbebras, braided 2-H*-algebras, and symmetric 2-H*-algbebras, and we describe the relation between these and tangles in two, three, and four dimensions, respectively. We prove a generalized Doplicher-Roberts theorem starting that every symmetric 2-H*-algebra is equivalent to the category Rep(G) of continuous unitary finite-dimensional representations of some compact supergroupoid G. The equivalence is given by a categorified version of the Gelfand transform; we also construct a categorified version of the Fourier transform when G is a compact Abelian group. Finally, we characterize Rep(G) by its universal properties when G is a compact classical group. For example, Rep(U(n)) is the free connected symmetric 2-H*-algebra on one even object of dimension n. (C) 1997 Academic Press.