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 A211781 Numbers n such that Sum_(d_
 2, 3, 4, 5, 7, 11, 13, 17, 19, 23, 27, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 225, 227, 229, 233, 239, 241, 251, 252 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Union primes (A000040) and sequence of composite numbers: 4, 27, 225, 252, 672, 1200, ... LINKS EXAMPLE Number 225 is in sequence because 1/1 + 2/3 + 2/5 + 3/9 + 4/15 + 3/25 + 6/45 + 6/75 = 3 (integer). MAPLE with(numtheory); A211781:= proc(q) local b, d, j, n; for n from 2 to q do   b:=divisors(n); d:=add(tau(b[j])/b[j], j=1..nops(b))-tau(n)/n;   if trunc(d)=d then print(n); fi; od; end: A211781(10000); # Paolo P. Lava, Feb 01 2013 MATHEMATICA t = {}; Do[d2 = Sum[DivisorSigma[0, d]/d, {d, Most[Divisors[n]]}]; If[IntegerQ[d2], AppendTo[t, n]], {n, 2, 252}]; t (* T. D. Noe, Apr 26 2012 *) CROSSREFS Sequence in context: A036844 A284696 A033070 * A046022 A175787 A073019 Adjacent sequences:  A211778 A211779 A211780 * A211782 A211783 A211784 KEYWORD nonn AUTHOR Jaroslav Krizek, Apr 20 2012 STATUS approved

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Last modified September 26 02:39 EDT 2020. Contains 337346 sequences. (Running on oeis4.)