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A000013
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Definition (1): Number of n-bead binary necklaces with beads of 2 colors where the colors may be swapped but turning over is not allowed.
(Formerly M0313 N0115)
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28
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1, 1, 2, 2, 4, 4, 8, 10, 20, 30, 56, 94, 180, 316, 596, 1096, 2068, 3856, 7316, 13798, 26272, 49940, 95420, 182362, 349716, 671092, 1290872, 2485534, 4794088, 9256396, 17896832, 34636834, 67110932, 130150588, 252648992, 490853416
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OFFSET
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0,3
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COMMENTS
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Definition (2): Equivalently, number of different output sequences from an n-stage pure cycling shift register when 2 sequences are considered the same if one is the complement of the other.
Definition (3): Also number of different output sequences from an n-stage pure cycling shift register constrained so contents have even weight.
Definition (4): Also number of output sequences from (n-1)-stage shift register which feeds back the mod 2 sum of the contents of the register.
The equivalence of definitions (1) and (2) follows at once from the definitions.
If u is an output sequence of type (2) then its derivative is of type (3) - so (2) and (3) count the same things.
If we have a shift register of type (4), append a new cell which contains the mod 2 sum of the contents to get a shift register of type (3). So (3) and (4) count the same things.
If n is even, a(n) = A000116(n/2). If 2^(n+1)-1 is prime, then a(n) = A128976(n+1), the number of cycles in the digraph of the Lucas-Lehmer operator LL(x)=x^2-2 acting on Z/(2^(n+1)-1). - M. F. Hasler, May 19 2007
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REFERENCES
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N. J. Fine, Classes of periodic sequences, Illinois J. Math., 2 (1958), 285-302.
E. N. Gilbert and J. Riordan, Symmetry types of periodic sequences, Illinois J. Math., 5 (1961), 657-665.
S. W. Golomb, Shift-Register Sequences, Holden-Day, San Francisco, 1967, p. 172.
Karyn McLellan, Periodic coefficients and random Fibonacci sequences, Electronic Journal of Combinatorics, 20(4), 2013, #P32.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane, On single-deletion-correcting codes, in Codes and Designs (Columbus, OH, 2000), 273-291, Ohio State Univ. Math. Res. Inst. Publ., 10, de Gruyter, Berlin, 2002.
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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T. D. Noe, Table of n, a(n) for n = 0..200
Joerg Arndt, Matters Computational (The Fxtbook), p.151, p.408
H. Bottomley, Initial terms of A000011 and A000013
F. Ruskey, Necklaces, Lyndon words, De Bruijn sequences, etc.
N. J. A. Sloane, On single-deletion-correcting codes
N. J. A. Sloane, Maple code for this and related sequences
Index entries for sequences related to necklaces
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FORMULA
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Sum_{ d divides n } (phi(2d)*2^(n/d))/(2n) for n>0. - Michael Somos, Oct 20 1999
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EXAMPLE
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G.f. = 1 + x + 2*x^2 + 2*x^3 + 4*x^4 + 4*x^5 + 8*x^6 + 10*x^7 + 20*x^8 + ...
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MAPLE
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with(numtheory): A000013 := proc(n) local s, d; if n = 0 then RETURN(1) else s := 0; for d in divisors(n) do s := s+(phi(2*d)*2^(n/d))/(2*n); od; RETURN(s); fi; end;
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MATHEMATICA
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a[n_] := Fold[ #1 + EulerPhi[2#2]2^(n/#2)/(2n) &, 0, Divisors[n]]
a[ n_] := If[ n < 1, Boole[n == 0], DivisorSum[ n, EulerPhi[2 #] 2^(n/#) &] / (2 n)]; (* Michael Somos, Dec 19 2014 *)
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PROG
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(PARI) {a(n) = if( n<1, n==0, sumdiv(n, k, eulerphi(2*k) * 2^(n/k)) / (2*n))}; /* Michael Somos, Oct 20 1999 */
(Haskell)
a000013 0 = 1
a000013 n = sum (zipWith (*)
(map (a000010 . (* 2)) ds) (map (2 ^) $ reverse ds)) `div` (2 * n)
where ds = a027750_row n
-- Reinhard Zumkeller, Jul 08 2013
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CROSSREFS
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Cf. A000031, A000016, A000116.
Cf. A128976.
Cf. A000010, A027750.
Sequence in context: A000011 A187213 A022476 * A064484 A063776 A247181
Adjacent sequences: A000010 A000011 A000012 * A000014 A000015 A000016
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KEYWORD
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nonn,nice,easy
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AUTHOR
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N. J. A. Sloane
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STATUS
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approved
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