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**ZHero****Real Member**- Registered: 2008-06-08
- Posts: 1,889

Here's an interesting 'divisibility test' for 7 & 13 i came across; but, sadly, is applicable to numbers with more than 3 digits only!

Divide the number into groups of 3 (starting from Right) and find the difference between the sum of the numbers in odd and even places.. If the difference is '0' or a multiple of 7 or 13 (as the case may be), its divisible by it!

e.g. 4537764

==> 4 537 764

==> (764+4) - (537)

==> 231= 7*33 ! Hence divisible!

*Last edited by ZHero (2008-07-19 10:02:18)*

If two or more thoughts intersect, there has to be a point!

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,710

Thanks ZHero ... why does it work?

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,710

We do ... just have limited memory capacity

(Please don't yell)

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman

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**Monox D. I-Fly****Member**- From: Indonesia
- Registered: 2015-12-02
- Posts: 2,000

kylekatarn wrote:

you are really angry today jane...what's wrong?

yelling in several posts

Who is jane?

Actually I never watch Star Wars and not interested in it anyway, but I choose a Yoda card as my avatar in honor of our great friend bobbym who has passed away.

May his adventurous soul rest in peace at heaven.

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**ganesh****Administrator**- Registered: 2005-06-28
- Posts: 35,485

Monox D. I-Fly wrote:

kylekatarn wrote:you are really angry today jane...what's wrong?

yelling in several postsWho is jane?

JaneFairfax is a member with close to 7000 posts! I personally feel the member in question is smart!

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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**Monox D. I-Fly****Member**- From: Indonesia
- Registered: 2015-12-02
- Posts: 2,000

ganesh wrote:

Monox D. I-Fly wrote:kylekatarn wrote:you are really angry today jane...what's wrong?

yelling in several postsWho is jane?

JaneFairfax is a member with close to 7000 posts! I personally feel the member in question is smart!

Oh, a deleted post, then?

Actually I never watch Star Wars and not interested in it anyway, but I choose a Yoda card as my avatar in honor of our great friend bobbym who has passed away.

May his adventurous soul rest in peace at heaven.

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**ganesh****Administrator**- Registered: 2005-06-28
- Posts: 35,485

Monox D. I-Fly wrote:

ganesh wrote:Monox D. I-Fly wrote:Who is jane?

JaneFairfax is a member with close to 7000 posts! I personally feel the member in question is smart!

Oh, a deleted post, then?

The individual continues to be a member.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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**Primenumbers****Member**- Registered: 2013-01-22
- Posts: 149

If the number (y) is odd and divisible by 7........

(y -1)/2=a

If a is odd, b = (a-1) / 2, if a is even b = (a +y-1) / 2

If b is odd, c = (b-1) / 2, if b is even c = (a +y-1) / 2

If y is divisible by 7, c will be divisible by 7..........

Works for any Mersenne Number....

Works because whatever I do to y to get to zero I have to be able to do to a multiple of y, to get to a remainder zero for y! And 7-1/2=3 3-1/2=1 1-1/2=0.

Example:35= 5*7,

35-1/2=17, 17-1/2=8 (8+35-1)/2=21,

21= 3*7

Example: 27=3*9

27-1/2=13 13-1/2=6

6=3*2

3-1/2=1

1-1/2=0

**"Time not important. Only life important."*** - The Fifth Element 1997*

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