prime number challenge

08082016, 04:56 PM
Post: #13




RE: prime number challenge
(08082016 04:15 PM)TASP Wrote: has any individual demonstrated a talent for IDing primes by sight ?? Chances of correctly identifying a relatively small prime (let's say 4 digits) are not too bad: * The number has to end in 1,3,7 or 9 (disregarding the obvious 2 and 5), this alone allows you to discard 60% of the numbers. * Quick divisibility by 3 by sum of digits: this one discards other 13% roughly. So you can easily discard 73% of the numbers, then out of the 2700 numbers that passed the 2 tests above, 1227 are primes (total primes = 1229, less the numbers 2 and 5). So you have roughly a 45% chance of detecting a prime number <10000 at naked eye. Almost a coin flip. Now if you want more accuracy, *just* memorize all these: Divisibility rules up to 50 

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Messages In This Thread 
prime number challenge  Don Shepherd  08072016, 12:30 AM
RE: prime number challenge  Gerson W. Barbosa  08072016, 04:36 AM
RE: prime number challenge  Don Shepherd  08072016, 04:44 AM
RE: prime number challenge  Gerson W. Barbosa  08072016, 05:24 AM
RE: prime number challenge  Paul Dale  08072016, 05:53 AM
RE: prime number challenge  Gerson W. Barbosa  08072016, 11:56 AM
RE: prime number challenge  Paul Dale  08072016, 12:22 PM
RE: prime number challenge  Gerson W. Barbosa  08072016, 04:29 PM
RE: prime number challenge  Massimo Gnerucci  08072016, 05:40 PM
RE: prime number challenge  Paul Dale  08072016, 09:57 PM
RE: prime number challenge  Gerson W. Barbosa  08072016, 11:11 PM
RE: prime number challenge  TASP  08082016, 04:15 PM
RE: prime number challenge  Claudio L.  08082016 04:56 PM
RE: prime number challenge  TASP  08082016, 06:29 PM
RE: prime number challenge  EdS2  08142016, 05:14 PM

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