## 24506Re: [PrimeNumbers] n, 2n-1, 2n+1 all prime or prime-power (maybe n-2 also)

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• Oct 1, 2012
First, you missed an easy one:

4, 7, 9

Second, the next one seems to be:

(3^541-1)/2, 3^541-2, 3^541

As far as the conjecture about the small examples with double
powers being the only ones, that would seem to be related to
the ABC Conjecture.

On 10/1/2012 9:33 AM, WarrenS wrote:
> If we demand that n, 2n-1, and 2n+1 all simultaneously be prime or prime-power,
> then initial examples found by hand are
>
> n, 2n-1, 2n+1
> 2, 3, 5
> 3, 5, 7
> 5, 9*, 11
> 9*, 17, 19
> 13, 25*, 27*
> 41, 81*, 83
> 121*,241,243*
> (next one over 10^12)
>
> where * for prime powers.
> You can easily see that is is impossible for all three to be prime (except in the
> first two lines) since at least one member of the troika must be divisible by 3. Hence the only way to accomplish it, is to make that one be a power of 3.
> So then we necessarily have an exponentially-sparse set of primes, in that sense resembling the famous "Mersenne primes" 2^p-1 as well as "Proth primes" like 3*2^n+1.
> Another kind of resemblance is the fact that if Q=3^k then
> Q+2 will be easy to test for primality if we know (Q+1)/2 is prime [since then it
> would be a "safeprime"]. In the other direction, (Q-1)/2 might also be easy to test for primality...
>
> Perhaps the 13,25,27 and 121,241,243 lines are the only ones featuring a
> a double (or triple) *.
>
> It seems to me that this is an interesting class of primes.
> Perhaps even more interesting are the "pack of four" cases where n-2 is ALSO prime or prime-power.
>
>
>
>
>
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