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 A234638 #16 Apr 05 2019 12:05:28 %S A234638 1,3,7,10,17,21,22,30,31,46,51,52,55,66,70,71,81,93,94,97,106,115,119, %T A234638 127,138,154,156,165,170,199,210,213,214,217,232,235,241,253,265,282, %U A234638 291,298,310,318,322,337,343,345,357,364,374,381,382,385 %N A234638 Numbers n for which sigma(sigma(n)) is odd. %C A234638 See A234641 for numbers n such that n^2 is in this sequence. %C A234638 The primes in this sequence are 3 and A066436. - _Robert Israel_, Apr 05 2019 %H A234638 Robert Israel, <a href="/A234638/b234638.txt">Table of n, a(n) for n = 1..10000</a> %e A234638 3 is in the sequence because sigma(sigma(3)) = sigma(4) = 7, which is odd. %e A234638 7 is in the sequence because sigma(sigma(7)) = sigma(8) = 15, which is odd. %e A234638 8 is not in the sequence because sigma(sigma(8)) = sigma(15) = 24, which is even. %p A234638 filter:= proc(n) uses numtheory; sigma(sigma(n))::odd end proc: %p A234638 select(filter, [$1..1000]); # _Robert Israel_, Apr 05 2019 %t A234638 Select[Range[400], OddQ[DivisorSigma[1, DivisorSigma[1, #]]] &] (* _Alonso del Arte_, Dec 29 2013 *) %o A234638 (PARI) is(n)=bittest(sigma(sigma(n)),0) %o A234638 (PARI) for(n=1,999,is(n)&&print1(n",")) %Y A234638 Cf. A000203, A028982, A066436, A234641. %K A234638 nonn %O A234638 1,2 %A A234638 _M. F. Hasler_, Dec 28 2013