A note on trilinear forms for reducible representations and Beilinson's conjectures

Abstract

We extend Prasad's results on the existence of trilinear forms on representations of GL2 of a local field, by permitting one or more of the representations to be reducible principal series, with infinite-dimensional irreducible quotient. We apply this in a global setting to compute (unconditionally) the dimensions of the subspaces of motivic cohomology of the product of two modular curves constructed by Beilinson.