Barycentric decompositions for extensive monotone divergences

We study sets of divergences or dissimilarity measures in a generalized real-algebraic setting which includes the cases of classical and quantum multivariate divergences. We show that a special subset of divergences, the so-called test spectrum, characterizes the rest of the divergences through barycentres and that the extreme points of relevant convex subsets of general divergences are contained within the test spectrum. Only some special parts of the test spectrum may contain non-extreme elements. We are able to fully characterize the test spectrum in the case of classical multivariate divergences. The quantum case is much more varied, and we demonstrate that essentially all the bivariate and multivariate quantum divergences suggested previously in literature are within the test spectrum and extreme within the set of all quantum (multivariate) divergences. This suggests that the variability of quantum divergences is real since all the previously suggested divergences are independent of each other.