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 A100587 Number of nonempty subsets of divisors of n. 17
 1, 3, 3, 7, 3, 15, 3, 15, 7, 15, 3, 63, 3, 15, 15, 31, 3, 63, 3, 63, 15, 15, 3, 255, 7, 15, 15, 63, 3, 255, 3, 63, 15, 15, 15, 511, 3, 15, 15, 255, 3, 255, 3, 63, 63, 15, 3, 1023, 7, 63, 15, 63, 3, 255, 15, 255, 15, 15, 3, 4095, 3, 15, 63, 127, 15, 255, 3, 63, 15, 255, 3, 4095, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS A119347(n) <= a(n). - Reinhard Zumkeller, Jun 27 2015 LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 Wasin So, Integral circulant graphs, Discr. Math. 306 (1) (2006) 153-158 FORMULA a(n) = -1 + 2^tau(n), where tau(n) = DivisorSigma(0, n) = A000005(n). EXAMPLE For all prime numbers p, a(p)=3, since those subsets are {{1,p},{1},{p}}. MAPLE A100587:=n->-1+2^numtheory[tau](n): seq(A100587(n), n=1..100); # Wesley Ivan Hurt, Dec 12 2015 MATHEMATICA Table[2^DivisorSigma[0, n] - 1, {n, 73}] (* Michael De Vlieger, Dec 11 2015 *) PROG (PARI) a(n) = 2^(numdiv(n)) - 1; \\ Michel Marcus, Dec 15 2013 (Haskell) a100587 = (subtract 1) . (2 ^) . a000005' -- Reinhard Zumkeller, Jun 27 2015 CROSSREFS Cf. A000005, A066781, A100371. Cf. A119347. Sequence in context: A156838 A274845 A324048 * A324548 A249383 A187419 Adjacent sequences: A100584 A100585 A100586 * A100588 A100589 A100590 KEYWORD nonn,easy AUTHOR Labos Elemer, Dec 01 2004 STATUS approved

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Last modified September 8 20:40 EDT 2024. Contains 375759 sequences. (Running on oeis4.)