A234639 - OEIS

%I #14 Jul 06 2019 10:37:08

%S 1,2,4,16,25,29,50,64,99,100,104,116,125,137,243,279,281,309,317,343,

%T 361,373,396,408,424,440,456,461,464,486,488,500,509,519,534,538,548,

%U 558,568,585,590,603,618,632,638,641,646,686,693,703,709,718,725,746,747,783

%N Numbers n for which sigma(sigma(sigma(n))) is odd.

%C Also, numbers such that sigma(sigma(n)) is in A028982, i.e., a square or twice a square.

%C See A234640 for the subsequence of odd terms. See also A234638, and A234641 with further cross references.

%H R. J. Mathar, <a href="/A234639/b234639.txt">Table of n, a(n) for n = 1..404</a>

%p n := 1 :

%p for k from 1 to 10000 do

%p     numtheory[sigma](k) ;

%p     numtheory[sigma](%) ;

%p     numtheory[sigma](%) ;

%p     if type(%,'odd') then

%p         printf("%d %d\n",n,k) ;

%p         n := n+1 ;

%p     end if;

%p end do: # _R. J. Mathar_, Oct 21 2014

%t sssoQ[n_]:=OddQ[Nest[DivisorSigma[1,#]&,n,3]]; Select[Range[800],sssoQ] (* _Harvey P. Dale_, Jul 06 2019 *)

%o (PARI) for(n=1,999,sigma(sigma(sigma(n)))%2 && print1(n","))

%K nonn

%O 1,2

%A _M. F. Hasler_, Dec 28 2013