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 A211778 Composite numbers n such that Sum_{d_
 8, 12, 32, 80, 81, 128, 189, 196, 324, 336, 448, 512, 576, 768, 1600, 1936, 2025, 2048, 2187, 2500, 5292, 5324, 5616, 5780, 8192, 8748, 9477, 11264, 12096, 13520, 14400, 14800, 15552, 15625, 20736, 32768, 35721, 36864, 49152, 53248, 59049, 69696, 73575, 73872 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Complement of primes (A000040) with respect to A211777. LINKS EXAMPLE For n = 32 holds: 1/1 + 1/2 + 2/4 + 4/8 + 8/16 = 3 (integer). MATHEMATICA t = {}; Do[If[! PrimeQ[n], d2 = Sum[EulerPhi[d]/d, {d, Most[Divisors[n]]}]; If[IntegerQ[d2], AppendTo[t, n]]], {n, 2, 10000}]; t (* T. D. Noe, Apr 26 2012 *) PROG (PARI) is(n)=denominator(sumdiv(n, d, if(d

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Last modified January 21 04:53 EST 2020. Contains 331104 sequences. (Running on oeis4.)