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Is This Prime?

isthisprime.com

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Re: Is This Prime?

#111

Post by Dylan16807 »

jcims wrote: post #82
tshaddox wrote: post #47

Earlier quoted context omitted.

49 looks prime to me. Another commentator says it doesn't count because it's a square number, but square numbers don't really have a particular "look" to me in the same way that 2-digit numbers ending in 5 or with digits summing to a multiple of 3 do.

Based on my failure at this game, anything times 19 looks prime to me.

Well you only have 4 of those under 100.

The "looks prime" rule assumes you check for multiples of 3, so that removes one.

All the rest end in an even digit or 5.

Re: Is This Prime?

#112

Post by spoonjim »

I think this question is framed the wrong way for me. I think if the question was "Is this composite?" I would get much better scores. The way I think is "Does it have a factor?" and inverting that Boolean trips me up.

Re: Is This Prime?

#113

Post by barbegal »

whatshisface wrote: post #101
johnday wrote: post #13

Earlier quoted context omitted.

The prime numbers are critical in cryptography. Almost all of our current digital security infrastructure is based on the concept of multiplying large numbers together modulo suitably big prime numbers. Any major step towards understanding them (such as a closed-form equation for primes) would have major mathematical knock-on effects which may or may not undermine these methods, or provide us with a basis for even st…

> Almost all of our current digital security infrastructure is based on the concept of multiplying large numbers together modulo suitably big prime numbers. That's not true of elliptic curves.

Elliptic curve cryptography uses modulo of large prime numbers. Curve25519 uses 2^255 - 19 for example

Re: Is This Prime?

#114

Post by CamperBob2 »

Tempest1981 wrote: post #93
kccqzy wrote: post #61

Earlier quoted context omitted.

When I was a fifth grader we were required to memorize all prime numbers below 20.

We memorized pi to 7 decimal places... a few memorized it to 20+

    Now I, even I, would celebrate
    In rhymes unapt, the great 
    Immortal Syracusan, rivalled (sic) nevermore
    Who, in his wondrous lore,
    Passed on before,
    Left men his guidance
    How to circles mensurate.

Re: Is This Prime?

#119

Post by techbio »

lordnacho wrote: post #109
lalaithion wrote: post #17

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 stri…

Somehow I've memorised 10243 in order to have a "big prime" that most people can't just determine. Note its the lowest 5 digits.

I've now memorized 10247, the second-lowest over 5 digits. Everyone else get in line.
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