A weighted sums of squares decomposition of positive Borel measurable
functions on a bounded Borel subset of the Euclidean space is obtained via
duality from the spectral theorem for tuples of commuting self-adjoint
operators. The analogous result for polynomials or certain rational functions
was amply exploited during the last decade in a variety of applications.
We describe algebraic certificates of positivity for functions belonging to a
finitely generated algebra of Borel measurable functions, with particular
emphasis to algebras generated by semi-algebraic functions. In which case the
standard global optimization problem with constraints given by elements of the
same algebra is reduced via a natural change of variables to the better
understood case of polynomial optimization. A collection of simple examples and
numerical experiments complement the theoretical parts of the article.
A refined notion of curvature for a linear system of Hermitian vector spaces,
in the sense of Grothendieck, leads to the unitary classification of a large
class of analytic Hilbert modules. Specifically, we study Hilbert sub-modules,
for which the localizations are of finite (but not constant) dimension, of an
analytic function space with a reproducing kernel.