[seqfan] Re: A relation between A153971 and A098828?
dwilson at gambitcomm.com
Wed Jan 7 18:54:11 CET 2009
Maximilian Hasler wrote:
> Inspired by the second part of the comment, I think I found the inteded meaning:
> for(n=1,99, isprime( p = 2*n^2 + 6*n + 3 ) & !isprime(2^(p-1)+3) &
> So it should be
> A153971 Primes of the form p = 2*n^2 + 6*n + 3 such that 2^(p-1)+3 is
> not prime.
> But personnally I find this a quite arbitrary definition....
All primes (indeed all numbers) of the form p = 2n^2 + 6n + 3 have
2^(p-1)+3 divisible by 7 or 13. Thus the only p for which 2^(p-1)+3 is
prime is p = 3 with 2^(p-1)+3 = 7. All prime p > 3 yield 2^(p-1)+3 > 13,
and being divisible by 7 or 13, is composite. So your sequence becomes
A153971 Primes of the form p = 2n^2 + 6n + 3 except p = 3.
which is to say, A098828 with 3 omitted. Not worth keeping.
Even if we tweak A153971 to "Primes p with 2^(p-1)+3 composite" and add
in missing values, it is still a pretty contorted definition. Better
would be the more inclusive "Numbers with 2^p+3 prime", that is, A057732.
Maybe a mercy killing is in order.
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