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 Jun 24 2014 03:22:41
%S 2,4,8,16,18,20,24,32,36,40,48,50,52,64,68,72,80,88,96,98,100,104,112,
%T 116,128,136,144,148,152,160,162,164,176,180,192,196,200,208,212,224,
%U 232,240,242,244,256,272,288,292,296,304,320,324,328,338,344,352
%N Numbers such that numerator(sigma(n)/n) is odd and denominator(sigma(n)/n) is even.
%C a(n) contains powers of 2 (A000079 except 1), and hemiperfect numbers (A055153, A141645, A159271, A160678).
%H Vincenzo Librandi, <a href="/A216781/b216781.txt">Table of n, a(n) for n = 1..1000</a>
%e sigma(2)/2 = 3/2 (odd/even).
%t Select[Range[1000], OddQ[Numerator[DivisorSigma[1, #]/#]] && EvenQ[Denominator[DivisorSigma[1, #]/#]] &] (* _Vincenzo Librandi_, Jun 24 2014 *)
%o (PARI) oeab(n) = {for (i=1, n, ab = sigma(i)/i; if ((numerator(ab) % 2 == 1) && (denominator(ab) % 2 == 0), print1(i, ", ")););}
%Y Cf. A216780, A216782, A000079, A055153, A141645, A159271, A160678.
%K nonn
%O 1,1
%A _Michel Marcus_, Sep 16 2012