A kind of "Zettelkasten" for mathematics
Not strictly a Zettelkasten, but almost. I was ready "How to write mathematics" by P. R. Halmos. and on page 134, he discusses how he organizes a book or long article:
At one stage of writing a 300-page book I had 1000 sheets of paper, each with a mathematical statement on it, a theorem, a lemma, or even a minor comment, complete with proof. The sheets were numbered, any which way. My job was to indicate on each sheet the numbers of the sheets whose statement must logically come before, and then to arrange the sheets in linear order so that no sheet comes after one on which it's mentioned.
It is of course not quite a Zettelkasten, since the numbers are only used to describe predecessors, but I think it is still interesting to look at these examples of almost-Zettelkasten. I think it gives credibility to the Zettelkasten technique that it is not really something entirely new, but that the elements can be recognized elsewhere.
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