<![CDATA[Math Is Fun Forum / Fast way to find primes.]]> 2020-10-17T03:32:01Z FluxBB https://www.mathisfunforum.com/viewtopic.php?id=22923 <![CDATA[Re: Fast way to find primes.]]> Did you know that the number one or number three the number 9 and the number 27 if you continue to add 30 too it as many times as you want and I'm pretty sure can never be a prime 21 +30=51,3+30=33,27+30=57,9+30=39if you noticed none of those are Prime's and any those four numbers you could continue to as 30 2 it's answer forever and now don't never be a prime meaning every third number ending in 1/3 7 or 9 and that form could never be a prime it's eliminate a lot of them

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https://www.mathisfunforum.com/profile.php?id=221975 2020-10-17T03:32:01Z https://www.mathisfunforum.com/viewtopic.php?pid=415405#p415405
<![CDATA[Re: Fast way to find primes.]]> RatnaPrabhu wrote:

CHIKA OFILI Test for Divisibility by 7

For Centuries All Top Mathematicians would have had a Go at finding a Test for Divisibility by 7.

Now, a 12 year old Chika Olifi has come out with a very Simple Test.

Consider your Number as AB, where B is the last Digit and A is the rest of the Digits considered as a Single Number.
Chika Ofili Test is to Calculate A + 5B. If that is Divisible by 7, then our Original Number is Divisible by 7.

For Example, . . . Consider 588 . . . A is 58 and B is 8 . . .  A + 5B is 58 + 5 * 8 = 98 . . . this being Divisible by 7, it means 588 is Divisible by 7.

This can be Proved by a few simple steps, which our Maths Fans can try out.

That would be useful to answer some circular questions.

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https://www.mathisfunforum.com/profile.php?id=212031 2019-11-21T02:38:54Z https://www.mathisfunforum.com/viewtopic.php?pid=411394#p411394
<![CDATA[Re: Fast way to find primes.]]> CHIKA OFILI Test for Divisibility by 7

For Centuries All Top Mathematicians would have had a Go at finding a Test for Divisibility by 7.

Now, a 12 year old Chika Olifi has come out with a very Simple Test.

Consider your Number as AB, where B is the last Digit and A is the rest of the Digits considered as a Single Number.
Chika Ofili Test is to Calculate A + 5B. If that is Divisible by 7, then our Original Number is Divisible by 7.

For Example, . . . Consider 588 . . . A is 58 and B is 8 . . .  A + 5B is 58 + 5 * 8 = 98 . . . this being Divisible by 7, it means 588 is Divisible by 7.

This can be Proved by a few simple steps, which our Maths Fans can try out.

]]>
https://www.mathisfunforum.com/profile.php?id=220844 2019-11-20T14:17:05Z https://www.mathisfunforum.com/viewtopic.php?pid=411388#p411388
<![CDATA[Re: Fast way to find primes.]]> Primenumbers wrote:

I'm not sure but I'm guessing this would be a faster way to compute finding primes. I.e. using Euclid's algorithm.
See............

https://en.wikipedia.org/wiki/Euclidean_algorithm

Primality Tests are usually done on numbers in the range of 2^1024 to 2^2048 (or much, much bigger numbers, when it's simply about "finding a prime", not crypto). Enumerating (and multiplying!) all the primes below those numbers is an insurmountable task.

If enumerating all primes was easy, RSA would be useless :-p

Edit: I just saw this thread is kinda old (still first page), I hope this doesn't count as necromancy ]]>
https://www.mathisfunforum.com/profile.php?id=213156 2016-10-22T21:49:22Z https://www.mathisfunforum.com/viewtopic.php?pid=389386#p389386
<![CDATA[Re: Fast way to find primes.]]> This is called the wheel.

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https://www.mathisfunforum.com/profile.php?id=33790 2016-03-14T13:13:45Z https://www.mathisfunforum.com/viewtopic.php?pid=377717#p377717
<![CDATA[Re: Fast way to find primes.]]>

A= all the primes below

multiplied together
If (A-p) and p do not share a common factor then p = prime.

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https://www.mathisfunforum.com/profile.php?id=197584 2016-03-14T11:53:11Z https://www.mathisfunforum.com/viewtopic.php?pid=377709#p377709
<![CDATA[Re: Fast way to find primes.]]> I see the technique. I tried it in several numbers and it worked.

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https://www.mathisfunforum.com/profile.php?id=195744 2016-03-14T08:41:47Z https://www.mathisfunforum.com/viewtopic.php?pid=377702#p377702
<![CDATA[Re: Fast way to find primes.]]> I'm not sure but I'm guessing this would be a faster way to compute finding primes. I.e. using Euclid's algorithm.
See............

https://en.wikipedia.org/wiki/Euclidean_algorithm

]]>
https://www.mathisfunforum.com/profile.php?id=197584 2016-03-13T16:53:58Z https://www.mathisfunforum.com/viewtopic.php?pid=377661#p377661
<![CDATA[Fast way to find primes.]]> Take your number, p, which you want to find out if it's prime or not.
Square root it. Multiply all the primes below the square root together.
Minus p from this number and see if there are any common factors between the two. If there is p is not prime, if there isn't p is prime.

Example:

p=51
square root p=7ish
2*3*5*7=210
210-51=159 common factor=3
51 is not prime

p=53
square root p=7ish
2*3*5*7=210
210-53=157 no common factors
53 is prime.

Note: It is impossible if the two numbers add up to a no. factorable by everything, for one to be factorable by a number and the other not. They must either both be factorable or both not.

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https://www.mathisfunforum.com/profile.php?id=197584 2016-03-13T14:58:19Z https://www.mathisfunforum.com/viewtopic.php?pid=377653#p377653