tell me about the games and tiny, specific, complex goals you invented as a child to keep from dying of boredom
I'll list some of mine
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prime factorize numbers in my head. i used to just pick random numbers i saw around esp. at math competitions. picked up some useful mental math tricks this way
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the main mental math trick is to check divisibility by repeatedly subtracting large multiples of the number. like check divisibility by 7 by repeatedly subtracting e.g. 70, 1400, 4900, etc. also the reverse, like subtracting into (or allowing negatives)
there's a trick based on digits like the 9 trick but harder but it's pretty annoying and doesn't generalize well. subtracting multiples works fine up to 2-digit primes
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