prime number challenge
08-08-2016, 04:56 PM
Post: #13
 Claudio L. Senior Member Posts: 1,834 Joined: Dec 2013
RE: prime number challenge
(08-08-2016 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 - 08-07-2016, 12:30 AM RE: prime number challenge - Gerson W. Barbosa - 08-07-2016, 04:36 AM RE: prime number challenge - Don Shepherd - 08-07-2016, 04:44 AM RE: prime number challenge - Gerson W. Barbosa - 08-07-2016, 05:24 AM RE: prime number challenge - Paul Dale - 08-07-2016, 05:53 AM RE: prime number challenge - Gerson W. Barbosa - 08-07-2016, 11:56 AM RE: prime number challenge - Paul Dale - 08-07-2016, 12:22 PM RE: prime number challenge - Gerson W. Barbosa - 08-07-2016, 04:29 PM RE: prime number challenge - Massimo Gnerucci - 08-07-2016, 05:40 PM RE: prime number challenge - Paul Dale - 08-07-2016, 09:57 PM RE: prime number challenge - Gerson W. Barbosa - 08-07-2016, 11:11 PM RE: prime number challenge - TASP - 08-08-2016, 04:15 PM RE: prime number challenge - Claudio L. - 08-08-2016 04:56 PM RE: prime number challenge - TASP - 08-08-2016, 06:29 PM RE: prime number challenge - EdS2 - 08-14-2016, 05:14 PM

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