OFFSET
1,2
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..270 (terms 1..95 from Vincenzo Librandi)
Umberto Cerruti, Numeri Armonici e Numeri Perfetti, Blog matematico (in Italian), 2013. The sequence is on page 13.
MATHEMATICA
B[n_] := (n DivisorSigma[0, n] + DivisorSigma[2, n])/(2 DivisorSigma[1, n]); Select[Range[5 10^5], IntegerQ[B[#]] && ! PrimeQ[#] &]
PROG
(Magma) IsInteger := func<i | i eq Floor(i)>; [n: n in [1..5*10^5] | IsInteger((n*NumberOfDivisors(n)+DivisorSigma(2, n))/(2*SumOfDivisors(n))) and not IsPrime(n)];
CROSSREFS
KEYWORD
nonn
AUTHOR
Bruno Berselli, Oct 11 2013
STATUS
approved