  We discuss semicanonical bases from the point of view of Cohomological Hall
algebras via the "dimensional reduction" from 3-dimensional Calabi-Yau
categories to 2-dimensional ones. Also, we discuss the notion of motivic
Donaldson-Thomas invariants (as defined by M. Kontsevich and Y. Soibelman) in
the framework of 2-dimensional Calabi-Yau categories. In particular we propose
a conjecture which allows one to define Kac polynomials for a 2-dimensional
Calabi-Yau category (this is a theorem of S. Mozgovoy in the case of
preprojective algebras).
