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 A074247 #12 Sep 30 2019 04:23:13 %S A074247 6,28,140,496,2970,8128,27846,105664,117800,237510,539400,1421280, %T A074247 1539720,2290260,4358600,8872200,33550336,50401728,90409410,233103780, %U A074247 287425800,318177800,423184320,559903400,825120800,1632825792,2705020500,3209343200,3328809120,4720896180 %N A074247 Numbers n such that harmonic mean of the divisors of n is a prime. %C A074247 Subsequence of A001599. Harmonic mean of divisors = n * tau(n) / sigma(n). %H A074247 Amiram Eldar, <a href="/A074247/b074247.txt">Table of n, a(n) for n = 1..103</a> (calculated from the b-file at A001599) %e A074247 140*tau(140)/sigma(140) = 5, a prime, so 140 is a term of the sequence. %t A074247 Select[Range[10^5], PrimeQ[ # * DivisorSigma[0, # ] / DivisorSigma[1, # ]] &] %t A074247 Select[Range[1600000],PrimeQ[HarmonicMean[Divisors[#]]]&] (* _Harvey P. Dale_, Jul 24 2019 *) %Y A074247 Cf. A001599. %K A074247 nonn %O A074247 1,1 %A A074247 _Joseph L. Pe_, Sep 20 2002 %E A074247 a(8)-a(26) from _Donovan Johnson_, Jun 10 2010 %E A074247 More terms from _Amiram Eldar_, Sep 30 2019