Thanks a lot for this endorsement of his functional analysis book. I never got around to understanding functional analysis properly; mainly because I don't work on PDE's/control theory that would have used the ideas more heavily. Added to my todo list.

Not really on topic, but I would like to use this opportunity to express gratitude to the HN community - although this forum is mostly tech-centric, it is wonderful to see things like this post get interest and discussion.