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%I #24 Nov 14 2021 23:53:19
%S 2,3,5,17,19,37,73,101,163,197,257,401,577,677,883,1153,1297,1459,
%T 1601,1801,2179,2593,2917,3137,3529,4051,4357,5477,7057,8101,8713,
%U 8837,10369,11251,12101,13457,14401,15139,15377,15877,16901,17299,17957,18433,19603
%N Primes p such that sigma(p-1) is odd.
%C Subsequence of A058501.
%C Union of A002496 and A090698. - _Ivan Neretin_, Dec 04 2018
%C Except for the terms 2 and 3, union of the primes of the form 4*k^2 + 1 and the primes of the form 18*k^2 + 1. - _Jianing Song_, Nov 14 2021
%H Charles R Greathouse IV, <a href="/A249410/b249410.txt">Table of n, a(n) for n = 1..10000</a>
%e 2 is in this sequence because 2 is prime and sigma(2-1) = 1 is odd.
%t Select[Range[20000], PrimeQ[#] && OddQ[DivisorSigma[1, #-1]] &] (* _Amiram Eldar_, Dec 04 2018 *)
%o (PARI) lista(nn) = {forprime(p=2, nn, if (sigma(p-1) % 2, print1(p, ", ")););} \\ _Michel Marcus_, Oct 30 2014
%o (PARI) list(lim)=my(v=List([2]),t); forstep(n=2,sqrt(lim),2, if(isprime(t=n^2+1), listput(v,t))); for(n=1,sqrtint(lim\2), if(isprime(t=2*n^2+1), listput(v,t))); Set(v) \\ _Charles R Greathouse IV_, Nov 04 2014
%o (GAP) Filtered(Filtered([1..25000],i->IsPrime(i)),p->IsOddInt(Sigma(p-1))); # _Muniru A Asiru_, Dec 05 2018
%Y Cf. A000203, A058501, A090698.
%K nonn
%O 1,1
%A _Juri-Stepan Gerasimov_, Oct 27 2014
%E More terms from _Michel Marcus_, Oct 30 2014