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A000740
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Number of 2n-bead balanced binary necklaces of fundamental period 2n, equivalent to reversed complement; also Dirichlet convolution of b_n=2^(n-1) with mu(n); also number of components of Mandelbrot set corresponding to Julia sets with an attractive n-cycle.
(Formerly M2582 N1021)
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199
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1, 1, 3, 6, 15, 27, 63, 120, 252, 495, 1023, 2010, 4095, 8127, 16365, 32640, 65535, 130788, 262143, 523770, 1048509, 2096127, 4194303, 8386440, 16777200, 33550335, 67108608, 134209530, 268435455, 536854005, 1073741823, 2147450880
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OFFSET
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1,3
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COMMENTS
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Also number of compositions of n into relatively prime parts (that is, the gcd of all the parts is 1). Also number of subsets of {1,2,..,n} containing n and consisting of relatively prime numbers. - Vladeta Jovovic, Aug 13 2003
Also number of perfect parity patterns that have exactly n columns (see A118141). - Don Knuth, May 11 2006
a(n) is odd if and only if n is squarefree (Tim Keller). - Emeric Deutsch, Apr 27 2007
a(n) is a multiple of 3 for all n>=3 (see Problem 11161 link). - Emeric Deutsch, Aug 13 2008
a(n) is the number of monic irreducible polynomials with nonzero constant coefficient in GF(2)[x] of degree n. - Michel Marcus, Oct 30 2016
a(n) is the number of aperiodic compositions of n, the number of compositions of n with relatively prime parts, and the number of compositions of n with relatively prime run-lengths. - Gus Wiseman, Dec 21 2017
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REFERENCES
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H. O. Peitgen and P. H. Richter, The Beauty of Fractals, Springer-Verlag; contribution by A. Douady, p. 165.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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a(n) = Sum_{d|n} mu(n/d)*2^(d-1), Mobius transform of A011782. Furthermore, Sum_{d|n} a(d) = 2^(n-1).
Recurrence relation: a(n) = 2^(n-1) - Sum_{d|n,d>1} a(n/d). (Lafayette College Problem Group; see the Maple program and Iglesias eq (6)). - Emeric Deutsch, Apr 27 2007
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EXAMPLE
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For n=4, there are 6 compositions of n into coprime parts: <3,1>, <2,1,1>, <1,3>, <1,2,1>, <1,1,2>, and <1,1,1,1>.
The a(6) = 27 aperiodic compositions are:
(11112), (11121), (11211), (12111), (21111),
(1113), (1122), (1131), (1221), (1311), (2112), (2211), (3111),
(114), (123), (132), (141), (213), (231), (312), (321), (411),
(15), (24), (42), (51),
(6).
The a(6) = 27 compositions into relatively prime parts are:
(111111),
(11112), (11121), (11211), (12111), (21111),
(1113), (1122), (1131), (1212), (1221), (1311), (2112), (2121), (2211), (3111),
(114), (123), (132), (141), (213), (231), (312), (321), (411),
(15), (51).
The a(6) = 27 compositions with relatively prime run-lengths are:
(11112), (11121), (11211), (12111), (21111),
(1113), (1131), (1212), (1221), (1311), (2112), (2121), (3111),
(114), (123), (132), (141), (213), (231), (312), (321), (411),
(15), (24), (42), (51),
(6).
(End)
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MAPLE
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with(numtheory): a[1]:=1: a[2]:=1: for n from 3 to 32 do div:=divisors(n): a[n]:=2^(n-1)-sum(a[n/div[j]], j=2..tau(n)) od: seq(a[n], n=1..32); # Emeric Deutsch, Apr 27 2007
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MATHEMATICA
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a[n_] := Sum[ MoebiusMu[n/d]*2^(d - 1), {d, Divisors[n]}]; Table[a[n], {n, 1, 32}] (* Jean-François Alcover, Feb 03 2012, after PARI *)
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PROG
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(PARI) a(n) = sumdiv(n, d, moebius(n/d)*2^(d-1))
(Python)
from sympy import mobius, divisors
def a(n): return sum([mobius(n / d) * 2**(d - 1) for d in divisors(n)])
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CROSSREFS
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Cf. A000837, A003239, A008683, A008965, A022553, A034738, A035928, A038199, A051168, A054525, A056267, A059966, A143424, A167606, A178472, A216954, A228369, A294859, A296302.
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KEYWORD
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nonn,nice,easy,changed
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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