The most difficult material were simply the proofs, especially of the fundamental theorem of arithmetic. The theorem makes sense intuitively, but using induction is always difficult.
The most interesting part were the tests for divisibility of certain integers. Those 4 rules seem really useful, especially the second one. I remember using it as a kid to remember the multiples of 9, 18, 27,36, 45, 54, 63, 72, 81, 90, as summing the digits give you 9. It is really neat that that extends to 1233, and other super large numbers.
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