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 2014-12-11, 18:04 #1 jwaltos     Apr 2012 Brady 2×33×7 Posts Primality question. I came across a reference to -1 considered prime years ago. http://mathforum.org/kb/message.jspa?messageID=1093172 Is there any theoretical utility in considering this number as such?
2014-12-11, 18:12   #2
R.D. Silverman

Nov 2003

164448 Posts

Quote:
 Originally Posted by jwaltos I came across a reference to -1 considered prime years ago. http://mathforum.org/kb/message.jspa?messageID=1093172 Is there any theoretical utility in considering this number as such?
The utility is less than zero.

2014-12-11, 19:25   #3
jwaltos

Apr 2012

2×33×7 Posts

Quote:
 Originally Posted by R.D. Silverman The utility is less than zero.
Good pun and thank you for the reply.

My reason for posting is that all primes can be developed from (-1,7,11,13,17,19,23 +30*n) where 2,3,5 are not included. Riesel's 1984? book notes this in the initial first or second chapters. The sum of digits of every prime is one of 1,4,2,8,5,7 which is the cyclic reciprocal of 7 ( I have not found a proof of this and pointer would be appreciated). To me, this is a curiosity but an interesting one.

Last fiddled with by jwaltos on 2014-12-11 at 19:28

2014-12-11, 19:38   #4
TheMawn

May 2013
East. Always East.

32778 Posts

Quote:
 Originally Posted by jwaltos The sum of digits of every prime is one of 1,4,2,8,5,7 which is the cyclic reciprocal of 7 ( I have not found a proof of this and pointer would be appreciated). To me, this is a curiosity but an interesting one.
Any number whose digits add up to 3 is divisible by 3. This is something I remember learning back in early grade school as a "trick" to knowing what something's divisors might be. This applies to any number whose digits add up to 6 also. This is also true for 9, but these numbers have the added bonus of being divisible by 9 as well.

That leaves a sum of 0 (mod 10, I imagine). 19 is a counter example of that, so I'm not sure why it was missed.

 2014-12-11, 21:42 #5 jwaltos     Apr 2012 Brady 1011110102 Posts Thanks Mawn. Last fiddled with by jwaltos on 2014-12-11 at 22:30
 2014-12-11, 22:36 #6 legendarymudkip     Jun 2014 12010 Posts http://en.wikipedia.org/wiki/Fundame..._of_arithmetic 12=2^2*3 12=2^2*3*-1*-1
2014-12-12, 00:50   #7
TheMawn

May 2013
East. Always East.

11×157 Posts

Quote:
 Originally Posted by legendarymudkip http://en.wikipedia.org/wiki/Fundame..._of_arithmetic
It amazes me how much the Ancient Greek managed to accomplish without algebra.

"4 measures 12" as in if you have 12 blocks, you could measure them if you had a stick that was 4 blocks long.

2014-12-12, 03:14   #8
axn

Jun 2003

7·709 Posts

Quote:
 Originally Posted by TheMawn That leaves a sum of 0 (mod 10, I imagine). 19 is a counter example of that, so I'm not sure why it was missed.
19 = 1+9 = 10 = 1+0 = 1

Sum of digits = number mod 9, except, instead of 0, we will use 9 itself.

2014-12-12, 04:39   #9
TheMawn

May 2013
East. Always East.

32778 Posts

Quote:
 Originally Posted by axn 19 = 1+9 = 10 = 1+0 = 1 Sum of digits = number mod 9, except, instead of 0, we will use 9 itself.
Of so you keep re-adding the digits until you're left with a 1-digit number.

The thing about the 3 still holds then, @OP. If a number's digits add up to something bigger than 9 and still a product of 3, it eventually decomposes back to 3 6 or 9.

396 - > 18 -> 9.

1299 -> 21 -> 3.

Etc

2014-12-12, 17:22   #10
Jens K Andersen

Feb 2006
Denmark

2·5·23 Posts

Quote:
 Originally Posted by jwaltos My reason for posting is that all primes can be developed from (-1,7,11,13,17,19,23 +30*n) where 2,3,5 are not included.
You are missing 1, and it's more common to use 29 than -1. The usual form is (1,7,11,13,17,19,23,29 + 30*n). In case you don't know, 30 = 2*3*5 (product of the first three primes), and 1,7,11,13,17,19,23,29 are the numbers below 30 and coprime to 30, so (1,7,11,13,17,19,23,29 + 30*n) is simply the numbers with no prime factor <=5. Its' trivial to make similar expressions for numbers with no prime factor <= p for other small primes p. p=2 gives that all primes above 2 are odd (2n+1). p=3 gives that all primes above 3 are of form 6n+1 or 6n+5, but in this case it's more common to use -1 in 6n+/-1. p=7 is also relatively easy by hand. After that I recommend using a computer (it's a trivial programming exercise, please don't post the long lists of coprime numbers).

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