Short star products for quantum symmetric pairs and applications

We prove that the star product for quantum symmetric pair coideal subalgebras is short. We apply this result to obtain new conceptual proofs, from first principles, of several fundamental facts about quantum symmetric pairs. In particular, we establish the existence of the algebra anti-automorphism $\sigma_\tau$ and of the bar involution, without making use of the quasi K-matrix. We give a new elementary proof of a conjecture by Balagovi\'c and Kolb, sometimes referred to as the fundamental lemma for quantum symmetric pairs. We obtain a conceptual formula expressing the tensor quasi K-matrix in terms of the much studied quasi R-matrix and the Letzter map. This also allows for a new independent proof of the intertwiner property of the quasi K-matrix.