%I #8 Jul 30 2019 18:19:41
%S 1,5,12,76,136,139,178,269,276,308,427,429,446,455,501,581,611,612,
%T 738,932,1576,1637,2952,2969,3184,3204,4647,4975,5400,5458,6199,7152,
%U 8816,9120,9180,9196,9272,9294,9504,9584,9720,9950,9960
%N Numbers n such that sigma(n) is a harmonic number.
%C Recall that n is harmonic if the harmonic mean of its divisors is an integer, i.e. if n * tau(n) / sigma(n) is an integer. (Tattersall, p. 147)
%D Tattersall, J. "Elementary Number Theory in Nine Chapters", Cambridge Univ. Press, 2001.
%H Amiram Eldar, <a href="/A074245/b074245.txt">Table of n, a(n) for n = 1..10000</a>
%e sigma(12) = 28 and 28 * tau(28) / sigma(28) = 28 * 6 / 56 = 3, an integer, so 12 is a term of the sequence.
%t isHarmonic[n_] := IntegerQ[n*DivisorSigma[0, n] / DivisorSigma[1, n]]; Select[Range[10^4], isHarmonic[DivisorSigma[1, # ]] &]
%Y Cf. A001599.
%K nonn
%O 1,2
%A _Joseph L. Pe_, Sep 19 2002
