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 togetherIf (A-p) and p do not share a common factor then p = prime.]]>

See............

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

]]>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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