# [seqfan] Re: A180268

Ray Chandler rayjchandler at sbcglobal.net
Mon Aug 23 23:11:02 CEST 2010

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Second sequence is 2n+1-sigma(n) = n-(sigma(n)-n-1) which is A157449.

Not that it is more interesting as a result.  And I get the same terms as
KB.

Ray Chandler

> Is there any reason to think this is an interesting sequence?
> sigma(n) - n - 1 is Chowla's function (for n > 1), and the
> primality of the Chowla function has some interest (it's
> A085842).  But what is
> sigma(n) + 2n + 1 and why do we care if it's prime?  Is there
> any relationship between the two?  They're primes at a
> distance of 3n + 2, but that's not particularly interesting of itself.
>
> Of course this comes back to the general, hard question of
> "what makes a sequence interesting?".  The trouble is that if
> the sequence *is* interesting, it doesn't make that apparent.
>
> Charles Greathouse
> Analyst/Programmer
> Case Western Reserve University
>
> On Mon, Aug 23, 2010 at 4:07 PM, Klaus Brockhaus
> <klaus-brockhaus at t-online.de> wrote:
> > %S A180268 988,1612,3172,5332,7852
> > %N A180268 Numbers n with property that A000203(n)-n-1 is a prime
> > number and 2*n+1-A000203(n) is a prime number.
> >
> > Utterly wrong or some additional condition is missing. I get
> >
> > 4, 10, 30, 42, 60, 70, 78, 102, 138, 186, 198, 216, 222, 228, 240,
> > 246, 258, 270, 282, 360, 372, 390, 414, 438, 492, 498, 546,
> 582, 600,
> > 606, 642, 708, 720, 756, 762, 774, 786, 810, 852, 870, 930,
> 942, 954,
> > 988, 1002, 1014, 1020, 1026, 1038, 1068, 1086, 1182, 1266,
> 1290, 1314,
> > 1362, 1368, 1386, 1398, 1470, 1542, 1584, 1612, 1626, 1638, 1656,
> > 1680, 1686, 1698, 1710, 1722, 1740, 1794, 1836
> >
> > which is not in the OEIS.
> >
> > KB

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