OFFSET
1,1
COMMENTS
Subsequence of A058501.
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
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
EXAMPLE
2 is in this sequence because 2 is prime and sigma(2-1) = 1 is odd.
MATHEMATICA
Select[Range[20000], PrimeQ[#] && OddQ[DivisorSigma[1, #-1]] &] (* Amiram Eldar, Dec 04 2018 *)
PROG
(PARI) lista(nn) = {forprime(p=2, nn, if (sigma(p-1) % 2, print1(p, ", ")); ); } \\ Michel Marcus, Oct 30 2014
(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
(GAP) Filtered(Filtered([1..25000], i->IsPrime(i)), p->IsOddInt(Sigma(p-1))); # Muniru A Asiru, Dec 05 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Juri-Stepan Gerasimov, Oct 27 2014
EXTENSIONS
More terms from Michel Marcus, Oct 30 2014
STATUS
approved