Each of the notes below resulted from my efforts to understand certain problem area. It can take a long while to read related texts and papers, work out examples, grasp relations etc. When this is done, I often write several pages, trying to put things the way I understood them. The presentation is then re-visited from time to time for further corrections and extensions. Though these expositions are originally intended for my own use, I hope that they can be helpful for others also.
No priority is assumed by default: most of the results presented are not mine, even if I fail to indicate explicitly the origin. Apologies in advance for all possible inaccuracies, mistakes, and ambiguities. You can use the notes "as they are": no warranty explicit or implied on anything :-)