This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A191218 #26 Feb 16 2022 23:10:44
%S A191218 5,13,17,29,37,41,45,53,61,73,89,97,101,109,113,117,137,149,153,157,
%T A191218 173,181,193,197,229,233,241,245,257,261,269,277,281,293,313,317,325,
%U A191218 333,337,349,353,369,373,389,397,401,405,409,421,425,433,449,457,461,477
%N A191218 Odd numbers n such that sigma(n) is congruent to 2 modulo 4.
%C A191218 Exactly the numbers of the form p^{4k+1}*m^2 with p a prime congruent to 1 modulo 4 and m a positive integer coprime with p. The odd perfect numbers are all of this form.
%C A191218 See A228058 for the terms where m > 1. - _Antti Karttunen_, Apr 22 2019
%H A191218 Antti Karttunen, <a href="/A191218/b191218.txt">Table of n, a(n) for n = 1..20000</a>
%H A191218 <a href="/index/Si#SIGMAN">Index entries for sequences related to sigma(n)</a>
%e A191218 For n=3 one has a(3)=17 since sigma(17) = 18 = 4*4 +2 is congruent to 2 modulo 4
%p A191218 with(numtheory): genodd := proc(b) local n,s,d; for n from 1 to b by 2 do s := sigma(n);
%p A191218 if modp(s,4)=2 then print(n); fi; od; end;
%t A191218 Select[Range[1,501,2],Mod[DivisorSigma[1,#],4]==2&] (* _Harvey P. Dale_, Nov 12 2017 *)
%o A191218 (PARI) forstep(n=1,10^3,2,if(2==(sigma(n)%4),print1(n,", "))) \\ _Joerg Arndt_, May 27 2011
%o A191218 (PARI) list(lim)=my(v=List()); forstep(e=1,logint(lim\=1,5),4, forprimestep(p=5,sqrtnint(lim,e),4, my(pe=p^e); forstep(m=1,sqrtint(lim\pe),2, if(m%p, listput(v,pe*m^2))))); Set(v) \\ _Charles R Greathouse IV_, Feb 16 2022
%Y A191218 Subsequence of A191217.
%Y A191218 Cf. A228058, A324898 (subsequences).
%Y A191218 Cf. A000203, A191219, A324647, A324718, A324719.
%K A191218 nonn,easy
%O A191218 1,1
%A A191218 _Luis H. Gallardo_, May 26 2011