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Re: Primality sieve challenge

Newsgroups comp.lang.javascript
Date 2016-01-19 10:55 -0800
References (3 earlier) <37264925.IaaGi9xk8D@PointedEars.de> <87ziw27kln.fsf@bsb.me.uk> <2865358.ifOrN9Bpor@PointedEars.de> <0f34c52b-ae45-49b0-8363-080d3d11d5fb@googlegroups.com> <3532030.noGN7VB15C@PointedEars.de>
Message-ID <06c1db3d-5ab3-4d76-bb98-b48adfbaa900@googlegroups.com> (permalink)
Subject Re: Primality sieve challenge
From Scott Sauyet <scott.sauyet@gmail.com>

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Thomas 'PointedEars' Lahn wrote:
> Scott Sauyet wrote:
>> Thomas 'PointedEars' Lahn wrote:
>>> Ben Bacarisse wrote:
>>>> but all those that have the form 6k + {0, 2, 3, 4} are clearly
>>>> composite.
>>> I can see why it is so for {0, 2, 4} (all multiples of 6 are divisible by
>>> 2, so are the sums when added 2 or 4), but why also for the summand 3?
>> 
>> Because 6k + 3 is divisible by 3.
> 
> Why?  I can see that it follows for k = 1 (9), k = 2 (15), k = 3 (21), and 
> for several greater k, but why for *all* k?

It's extremely similar to the reason you described for {0, 2, 4}:

All multiples of 6 are divisible by 3, so are the sums when added 3.  If
this is still surprising, I'd recommend any introductory text on number
theory.


>> And if it's greater than 5 it's clearly not equal to 3, so it's not prime.
> 
> I find that a specious argument at best.

A number that is divisible by a prime, but is not actually equal to that
prime is, by definition, composite.  I can't see why you consider that
specious.

 
>> Since all positive integers > 5 are (trivially) of one of forms `6k + 0`,
>> `6k + 1`, `6k + 2`, `6k + 3`, `6k + 4`, or `6k + 5`, and we've easily
>> demonstrated that all those of the form `6k + {0, 2, 3, 4}` are composite,
> 
> Yes.
> 
>> all primes must be of the form `6k + 1` or `6k + 5`.
> 
> Again, why?  If something is true for A and B, it does not follow that it is 
> not true for C ∉ {A, B}: p(A) ∧ p(B) ↛ ¬p(C); here p(X) := “X is composite 
> (not prime)”.  What relation I am missing here?

I'm not following your objection.  All integers greater than 1 are either 
prime or composite.  All integers are in one of the sets (to use Ben's
notation) Rr = { 6k + r | k ∈ N } for some r ∈ {0, 1, 2, 3, 4, 5}.  All
sufficently large integers in R0, R2, R3, R4 have already been shown to be
composite, so any suffiently large primes must be in R1 and R5.  I've
already shown that R5 is equivalent to { 6k - 1 | k ∈ N }.

Note that there is nothing magical about the number 6 here.  All primes 
greater than 2 are also of one of the forms `8k + 1`, `8k + 3`, `8k + 5`, 
or `8k + 7`, and in general for all `n`, thereis some `p` such that all 
primes greater than `p` are of the form { nk + r | k ∈ N } for some `r`
coprime to `n`.  (`r` is coprime to `n` if the greatest common divisor of
`r` and `n` is 1.)

Does that make it any clearer?

  -- Scott

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Thread

Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-17 01:40 -0800
  Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-17 02:20 -0800
  Re: Primality sieve challenge Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-01-17 16:24 +0000
    Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-17 09:03 -0800
      Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-17 09:17 -0800
        Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-17 09:22 -0800
          Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-17 11:32 -0800
          Re: Primality sieve challenge Stefan Weiss <krewecherl@gmail.com> - 2016-01-18 00:13 +0100
            Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-18 04:10 -0800
          Re: Primality sieve challenge Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-01-18 01:20 +0000
            Re: Primality sieve challenge "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2016-01-18 12:16 +0100
              Re: Primality sieve challenge Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-01-18 11:25 +0000
                Re: Primality sieve challenge "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2016-01-18 12:43 +0100
                Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-18 04:18 -0800
                Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-18 04:25 -0800
              Re: Primality sieve challenge Gene Wirchenko <genew@telus.net> - 2016-01-18 09:52 -0800
                Re: Primality sieve challenge "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2016-01-18 19:23 +0100
                Re: Primality sieve challenge "Michael Haufe (TNO)" <tno@thenewobjective.com> - 2016-01-18 10:48 -0800
                Re: Primality sieve challenge Gene Wirchenko <genew@telus.net> - 2016-01-18 13:09 -0800
                Re: Primality sieve challenge Scott Sauyet <scott.sauyet@gmail.com> - 2016-01-18 13:14 -0800
                Re: Primality sieve challenge "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2016-01-18 22:45 +0100
                Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-18 14:23 -0800
                Re: Primality sieve challenge Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-01-18 22:56 +0000
                Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-18 15:02 -0800
                Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-18 16:04 -0800
                Re: Primality sieve challenge Stefan Weiss <krewecherl@gmail.com> - 2016-01-19 02:42 +0100
                Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-18 23:34 -0800
                Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-19 01:34 -0800
                Re: Primality sieve challenge "Michael Haufe (TNO)" <tno@thenewobjective.com> - 2016-01-19 06:23 -0800
                Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-19 07:10 -0800
                Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-19 07:15 -0800
                Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-19 08:34 -0800
                Re: Primality sieve challenge Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-01-19 16:54 +0000
                Re: Primality sieve challenge "Michael Haufe (TNO)" <tno@thenewobjective.com> - 2016-01-19 08:57 -0800
                Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-19 10:18 -0800
                Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-19 10:59 -0800
                Re: Primality sieve challenge Stefan Weiss <krewecherl@gmail.com> - 2016-01-20 01:33 +0100
                Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-20 02:10 -0800
                Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-20 02:21 -0800
                Re: Primality sieve challenge Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-01-20 10:45 +0000
                Re: Primality sieve challenge "Michael Haufe (TNO)" <tno@thenewobjective.com> - 2016-01-20 06:43 -0800
                Re: Primality sieve challenge Scott Sauyet <scott.sauyet@gmail.com> - 2016-01-20 15:21 -0800
                Re: Primality sieve challenge Stefan Weiss <krewecherl@gmail.com> - 2016-01-21 03:18 +0100
                Re: Primality sieve challenge Jon Ribbens <jon+usenet@unequivocal.co.uk> - 2016-01-21 12:12 +0000
                Re: Primality sieve challenge Dr J R Stockton <reply1600@merlyn.demon.co.uk.invalid> - 2016-01-21 23:20 +0000
                Re: Primality sieve challenge Stefan Weiss <krewecherl@gmail.com> - 2016-01-22 19:44 +0100
                Re: Primality sieve challenge Dr J R Stockton <reply1600@merlyn.demon.co.uk.invalid> - 2016-01-23 21:50 +0000
                Re: Primality sieve challenge Stefan Weiss <krewecherl@gmail.com> - 2016-01-24 01:26 +0100
                Re: Primality sieve challenge "Michael Haufe (TNO)" <tno@thenewobjective.com> - 2016-01-23 17:54 -0800
                Re: Primality sieve challenge "Chris M. Thomasson" <nospam@no-spam.ws> - 2016-01-24 13:34 -0800
                Re: Primality sieve challenge Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-01-25 00:21 +0000
                Re: Primality sieve challenge Stefan Weiss <krewecherl@gmail.com> - 2016-01-25 03:08 +0100
                Re: Primality sieve challenge "Michael Haufe (TNO)" <tno@thenewobjective.com> - 2016-01-24 19:35 -0800
                Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-25 03:53 -0800
                Re: Primality sieve challenge Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-01-18 19:32 +0000
        Re: Primality sieve challenge Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-01-17 19:51 +0000
          Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-17 13:43 -0800
  Re: Primality sieve challenge "Chris M. Thomasson" <nospam@nospam.nospam> - 2016-01-17 12:46 -0800
    Re: Primality sieve challenge Gene Wirchenko <genew@telus.net> - 2016-01-18 09:50 -0800
      Re: Primality sieve challenge Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-01-18 20:09 +0100
        Re: Primality sieve challenge Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-01-18 19:28 +0000
          Re: Primality sieve challenge Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-01-18 21:37 +0100
            Re: Primality sieve challenge Scott Sauyet <scott.sauyet@gmail.com> - 2016-01-18 13:12 -0800
              Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-18 13:46 -0800
                Re: Primality sieve challenge jonas.thornvall@gmail.com - 2016-01-18 14:00 -0800
              Re: Primality sieve challenge Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-01-19 13:47 +0100
                Re: Primality sieve challenge Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-01-19 14:04 +0000
                Re: Primality sieve challenge Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-01-19 19:59 +0100
                Re: Primality sieve challenge Scott Sauyet <scott.sauyet@gmail.com> - 2016-01-19 10:55 -0800
                Re: Primality sieve challenge Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-01-19 20:04 +0100
                Re: Primality sieve challenge Scott Sauyet <scott.sauyet@gmail.com> - 2016-01-19 13:42 -0800
                Re: Primality sieve challenge Erwin Moller <erwinmollerusenet@xs4all.nl> - 2016-01-21 17:21 +0100
                Re: Primality sieve challenge Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-02-03 19:52 +0100
        Re: Primality sieve challenge Dr J R Stockton <reply1600@merlyn.demon.co.uk.invalid> - 2016-01-19 19:43 +0000

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