John Conway’s mental factoring method

[EdS2]

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I've come across this before, and find it rather interesting: a series of tactics for factoring, based on convenient facts about the numbers 150 and (for the advanced practitioner) 2000. Probably mainly of interest in mental arithmetic, but just possibly could be the basis of a programming challenge.

John Conway’s mental factoring method and friends

There's a link within to a 5 page PDF
Factoring Numbers with Conway’s 150 Method by Arthur T. Benjamin, 2018

Quote:The advantage of Conway’s 150 method is that it tests for many prime factors at the same time without the need to restart with the original number as we change trial divisors. It works especially well for testing 3- and 4-digit numbers and is well suited for hand calculations (on paper) or mental calculations (on your hand) as we explain. The method is based on the fact that the numbers 150 to 156 contain an unusual number of small primes as divisors. Using it, one can quickly determine if a 3- or 4-digit number is divisible by any prime number below 37.

(posted as a "general math and science" topic)

[Gil]

Very easy to use — as a mental exercice — also is John Conways Doomsday_rule or algorithm to calculate the weekday for anydate, before C.E., or after, with Julian or Gregorian calendar.

See https://en.m.wikipedia.org/wiki/Doomsday_rule

Regards,
Gil

[EdS2]

That's nice, thanks - and here we are, in between Pi Day and 4th April. (Good link, Lewis Carroll also getting a mention.)