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A234639 Numbers n for which sigma(sigma(sigma(n))) is odd.

%I A234639 #14 Jul 06 2019 10:37:08
%S A234639 1,2,4,16,25,29,50,64,99,100,104,116,125,137,243,279,281,309,317,343,
%T A234639 361,373,396,408,424,440,456,461,464,486,488,500,509,519,534,538,548,
%U A234639 558,568,585,590,603,618,632,638,641,646,686,693,703,709,718,725,746,747,783
%N A234639 Numbers n for which sigma(sigma(sigma(n))) is odd.
%C A234639 Also, numbers such that sigma(sigma(n)) is in A028982, i.e., a square or twice a square.
%C A234639 See A234640 for the subsequence of odd terms. See also A234638, and A234641 with further cross references.
%H A234639 R. J. Mathar, <a href="/A234639/b234639.txt">Table of n, a(n) for n = 1..404</a>
%p A234639 n := 1 :
%p A234639 for k from 1 to 10000 do
%p A234639     numtheory[sigma](k) ;
%p A234639     numtheory[sigma](%) ;
%p A234639     numtheory[sigma](%) ;
%p A234639     if type(%,'odd') then
%p A234639         printf("%d %d\n",n,k) ;
%p A234639         n := n+1 ;
%p A234639     end if;
%p A234639 end do: # _R. J. Mathar_, Oct 21 2014
%t A234639 sssoQ[n_]:=OddQ[Nest[DivisorSigma[1,#]&,n,3]]; Select[Range[800],sssoQ] (* _Harvey P. Dale_, Jul 06 2019 *)
%o A234639 (PARI) for(n=1,999,sigma(sigma(sigma(n)))%2 && print1(n","))
%K A234639 nonn
%O A234639 1,2
%A A234639 _M. F. Hasler_, Dec 28 2013