Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #9 Feb 13 2022 13:38:38
%S 5,13,17,29,37,41,53,61,73,89,97,101,109,113,137,149,157,173,181,193,
%T 197,229,233,241,245,257,269,277,281,293,313,317,325,337,349,353,373,
%U 389,397,401,409,421,425,433,449,457,461,509,521,541,557,569,577,593,601,605,613,617,637,641,653,661,673,677,701,709
%N Odd numbers k that are not multiples of 3 and for which sigma(k) is congruent to 2 modulo 4.
%C a(25) = 245 = 5* 7^2 is the first term that is not prime.
%H <a href="/index/Si#SIGMAN">Index entries for sequences related to sigma(n)</a>
%t Select[Range[1, 700, 2], !Divisible[#, 3] && Mod[DivisorSigma[1, #], 4] == 2 &] (* _Amiram Eldar_, Feb 13 2022 *)
%o (PARI) isA351535(n) = ((n%2) && (0!=(n%3)) && (2 == (sigma(n)%4)));
%Y Intersection of A001651 and A191218. Complement of A351533 in A191218.
%Y Cf. A002144 (subsequence).
%K nonn
%O 1,1
%A _Antti Karttunen_, Feb 13 2022