arXiv2004
Let g be a Lie algebra over a field F of characteristic zero, let C be a certain tensor category of representations of g, and C-du a certain category of duals. In arXiv:math.AG/0409053 we associated to C and C-du by a Tannaka reconstruction a monoid M with a coordinate ring F[M] of matrix coefficients, as well as a Lie algebra Lie(M). We interpreted this situation algebraic geometrically. In particular, we showed: If the Lie algebra g is generated by one-parameter elements, then it identifies in a natural way with a subalgebra of Lie(M), and there exists a subgroup of the unit group of M, which is dense in M. We now introduce the coordinate ring of regular functions on this dense subgroup, as well as the algebra of linear regular functions on the universal enveloping algebra of g, and investigate their relation. We investigate and describe various coordinate rings of matrix coefficients associated to categories of integrable representations of g. We specialize to integrable representations of Kac-Moody algebras and free Lie algebras. Some results on coordinate rings of Kac-Moody groups obtained by V. G. Kac and D. Peterson, some coordinate rings of the associated groups of linear algebraic integrable Lie algebras defined by V. G. Kac, and some results on coordinate rings of free Kac-Moody groups obtained by Y. Billig and A. Pianzola fit into this context. We determine the Tannaka monoid associated to the full subcategory of integrable representations in the category O of a Kac-Moody algebra and to its category of full duals. Its Zariski-open dense unit group is the formal Kac-Moody group. We give various descriptions of its coordinate ring of matrix coefficients. We show that its Lie algebra is the formal Kac-Moody algebra.