8 ms·
This reminds me of my favorite in-person magic trick to do. First, memorize all the two-digit primes. 25 numbers isn't that hard to memorize. Then, tell someo
by lalaithion 5y ago
This reminds me of my favorite in-person magic trick to do.
First, memorize all the two-digit primes. 25 numbers isn't that hard to memorize.
Then, tell someone "Oh, I can instantly tell whether a number is prime or not. Give me a number, I'll tell you whether it's prime."
If they tell you a number between 1 and 100, use your memorized list. Otherwise, it's a game of cold reading; if they just generated a random string of many digits, there's a low chance that the number actually is prime. "21923847" is a keysmash, and it's almost certainly not prime, because the frequency of primes goes down as numbers get bigger. And most people will ask you a number, hear "no", and then go check the number's primality. Eventually, they'll look up a number in advance; that number is almost certainly prime.
- float4 5y agoAnd then one day, somebody asks you calmly: "what about 2^31-1"? Better memorise the small Mersenne exponents as well! https://oeis.org/A000043 https://oeis.org/A000043
- the_arun 5y agoAfter 8 attempts, I ran into "Game Over". Was just thinking about the satisfaction for the author of this site. There is just enormous fun in saying - Game Over - to another human :)
- deleted 5y ago[deleted]
- aidenn0 5y agoAlso, most composites are divisible by 2, 3 or 5, all of which are very easy to check (for 3 just sum up the digits and if the result is divisible by 3, the original number is divisible by 3). 49, 77, and 91 are the only composites under 100 that is not divisible by one of those 3 numbers, making memorizing 25 numbers not even necessary. 49 and 77 are obviously composite to me making 91 the only number I bothered memorizing.
- pkulak 5y ago53?
- dhruvrrp 5y ago53 is prime
- falseprofit 5y ago57?
- sincerely 5y ago5+7=12 thus divisible by 3 and not prime
- techbio 5y agohttps://en.wikipedia.org/wiki/Shure_SM57 https://en.wikipedia.org/wiki/Shure_SM57
- techbio 5y agohttps://mathoverflow.net/questions/326912/story-of-grothendiecks-prime-number-57 https://mathoverflow.net/questions/326912/story-of-grothendi...
- OriginalNebula 5y ago21923847?
- nxpnsv 5y agoOnly 14 away from a prime though
- pkulak 5y agoOh, shoot, brain fart. I was thinking OP was listing primes and forgot one. :D
- mjthompson 5y agoI hope the twin prime conjecture holds, since it would be pretty incredible to think that, despite what you said, there are infinitely many twin primes.
- lordnacho 5y agoSomehow I've memorised 10243 in order to have a "big prime" that most people can't just determine. Note its the lowest 5 digits.
- maximus-decimus 5y agoThat's a lot of effort just for tricking people into thinking you can tell if a number is prime.