Positivity properties of canonical bases

We prove that the canonical basis of a modified quantum group U̇ exhibits strong positivity properties for the canonical basis elements arising from spherical parabolic subalgebras. Our main result establishes that the structure constants for both the multiplication with arbitrary canonical basis elements in U̇ and the action on the canonical basis elements of arbitrary tensor products of simple lowest and highest weight modules by these elements belong to Nv, v^-1. This implies, in particular, for quantum groups of finite type, the structure constants for multiplication and for action on tensor product with respect to canonical basis are governed by positive coefficients. A key ingredient is the thickening construction, an algebraic technique that embeds a suitable approximation of the tensor of a lowest weight module and a highest weight module of U̇ into the negative part U^- of a larger quantum group. This allows us to inherit the desired positivity for the tensor product from the well-established positivity of the canonical basis of U^-.