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 #20 Jul 17 2019 03:18:07
%S 1,2,1,1,1,2,1,4,3,2,1,1,1,2,1,5,1,6,1,1,1,2,1,4,1,2,1,1,1,2,1,2,1,2,
%T 1,3,1,2,1,4,1,2,1,1,3,2,1,1,1,2,1,1,1,2,1,4,1,2,1,1,1,2,3,7,1,2,1,1,
%U 1,2,1,12,1,2,1,1,1,2,1,5,5,2,1,1,1,2
%N Denominators of averages of squares of the divisors of n.
%C Average of squares of the divisors of n = A001157(n)/A000005(n).
%C Iff a(n) = 1, n is in A020486.
%H Amiram Eldar, <a href="/A158298/b158298.txt">Table of n, a(n) for n = 1..10000</a>
%t Array[Denominator[DivisorSigma[2, #]/DivisorSigma[0, #]] &, 100] (* _Amiram Eldar_, Jul 15 2019 *)
%o (PARI) a(n) = denominator(sigma(n, 2)/numdiv(n)) \\ _Michel Marcus_, Jun 13 2013
%Y Cf. A001157, A000005, A020486, A158299 (for numerators).
%K nonn,frac
%O 1,2
%A _Jaroslav Krizek_, Mar 15 2009
%E Data corrected and extended by _Amiram Eldar_, Jul 15 2019