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.
RatnaPrabhu, Ahmedabad, INDIA
That would be useful to answer some circular questions.
]]>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.
RatnaPrabhu, Ahmedabad, INDIA
]]>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
]]>A= all the primes below
multiplied togetherhttps://en.wikipedia.org/wiki/Euclidean_algorithm
]]>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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