I imagine something like Lovasz' Combinatorial Problems and Exercises, maybe also in a format amenable to spaced repetition. How many of us are qualified to contribute though? I certainly am not.
We could also write separate exercise books for different topics, and then each of us could specialize and sort of distributed pair-teach-learn in Bittorrent style. For example, Patrick has been helping me a lot (and I think he enjoys teaching me). I'd obviously like to return the favor, perhaps in another subject.
Note: This was originally posted in the discussion area, but motions to move it to the top level were made.
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My own desire to improve my rationality coupled with some posts criticizing LessWrong not too long ago led to an idea. For reference, the posts I mean are these: