[seqfan] Re: Error in A180269 Primes of the form 3^3+3*n+1

Charles Greathouse charles.greathouse at case.edu
Mon Aug 23 22:42:31 CEST 2010


Good catch.  The sequence should be
Primes of the form 3^n + 3n + 1.
and the offset should be 1.

Also, the keyword easy should be removed, since the sequence grows
exponentially and checking large numbers for primality is hard.

It's probably also worth mentioning the indexes n:
0,1,3,41,45,83,95,181,193,195,311,325,901,1825,...
although I wouldn't submit this as its own sequence.

Charles Greathouse
Analyst/Programmer
Case Western Reserve University

On Mon, Aug 23, 2010 at 4:32 PM, Jonathan Post <jvospost3 at gmail.com> wrote:
> A180269 Primes of the form 3^3+3*n+1.
> But the PARI seems to be for the formula 3^k+3*k+1 instead.
> The formula as given in the seq's title is equivalent to "Prtimes of
> the form 27 + 3*n + 1" hence "primes of the form 3*n + 28"
> which would begin 31, 37, 43, 61, 67, 73, 79, 97, 103, 109, 127, 139, 151, ...
> which is A002476  Primes of form 6n + 1 for n>3
> or A002476(n+3)
> unless I'm also making an elementary mistake.
>
>
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