They do it with ultrafilters
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Set-Theoretic Topology
Lebesgue Measure and Integration
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Anki
, a program for making flashcards which supports LaTeX, very useful to brush up on old material
Inspiration
(the paid mind-mapping alternative to
FreeMind
, which is also very good)
Favourite Sites from around the Web
edX
Piled Higher and Deeper
xkcd
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