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 A007148 Number of self-complementary 2-colored bracelets (turnover necklaces) with 2n beads. (Formerly M0774) 8
 1, 2, 3, 6, 10, 20, 37, 74, 143, 284, 559, 1114, 2206, 4394, 8740, 17418, 34696, 69194, 137971, 275280, 549258, 1096286, 2188333, 4369162, 8724154, 17422652, 34797199, 69505908, 138845926, 277383872, 554189329, 1107297290, 2212558942 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 REFERENCES N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 E. M. Palmer and R. W. Robinson, Enumeration of self-dual configurations Pacific J. Math., 110 (1984), 203-221. F. Ruskey, Necklaces, Lyndon words, De Bruijn sequences, etc. [Cached copy, with permission, pdf format only] FORMULA a(n) = 2^(n-2) + (1/(4n)) * Sum_{d|n} phi(2d)*2^(n/d). - N. J. A. Sloane, Sep 25 2012 a(n) = (1/2)*(A000079(n-1) + A000013(n)). MAPLE # see A245558 L := proc(n, k)     local a, j ;     a := 0 ;     for j in numtheory[divisors](igcd(n, k)) do         a := a+numtheory[mobius](j)*binomial(n/j, k/j) ;     end do:     a/n ; end proc: A007148 := proc(n)     local a, k, l;     a := 0 ;     for k from 1 to n do         for l in numtheory[divisors](igcd(n, k)) do             a := a+L(n/l, k/l)*ceil(k/2/l) ;         end do:     end do:     a; end proc: seq(A007148(n), n=1..20) ; # R. J. Mathar, Jul 23 2017 MATHEMATICA a[n_] := (1/2)*(2^(n-1) + Total[ EulerPhi[2*#]*2^(n/#) &  /@ Divisors[n]]/(2*n)); Table[ a[n], {n, 1, 33}] (* Jean-François Alcover, Oct 25 2011 *) PROG (PARI) a(n)= (1/2) *(2^(n-1)+sumdiv(n, k, eulerphi(2*k)*2^(n/k))/(2*n)) (Python) from sympy import divisors, totient def a(n):     if n==1: return 1     return 2**(n - 2) + sum(totient(2*d)*2**(n//d) for d in divisors(n))//(4*n) print([a(n) for n in range(1, 31)]) # Indranil Ghosh, Jul 24 2017 CROSSREFS Cf. A000013, A000079, A007147. Different from, but easily confused with, A045690 and A093371. Sequence in context: A164047 A158291 A045690 * A093371 A003214 A331693 Adjacent sequences:  A007145 A007146 A007147 * A007149 A007150 A007151 KEYWORD nonn,nice,easy AUTHOR EXTENSIONS Description corrected by Christian G. Bower STATUS approved

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Last modified August 9 20:09 EDT 2020. Contains 336326 sequences. (Running on oeis4.)