ChapovskyY., and KalyuzhnyiA. A. “Factorization of Conditional Expectations on Kac Algebras and Quantum Double Coset Hypergroups”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 55, no. 12, Dec. 2003, pp. 1669-77, https://umj.imath.kiev.ua/index.php/umj/article/view/4029.