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A005656
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Number of bracelets (turn over necklaces) with n red, 1 pink and n - 3 blue beads; also reversible strings with n red and n-3 blue beads.
(Formerly M2920)
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2
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0, 0, 1, 3, 12, 44, 170, 651, 2520, 9752, 37854, 147070, 572264, 2229096, 8692788, 33933459, 132594480, 518584880, 2029976630, 7952706234, 31179618184, 122331419080, 480283635468, 1886828198398
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OFFSET
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1,4
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REFERENCES
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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) = (1/2)*(binomial(2*n - 3, n - 3) + binomial(n - 2, floor((n - 3)/2))). - Michael Somos
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MAPLE
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MATHEMATICA
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Table[(1/2) (Binomial[2 n - 3, n - 3] + Binomial[n - 2, Floor[(n - 3) / 2]]), {n, 40}] (* Vincenzo Librandi, Oct 08 2017 *)
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PROG
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(PARI) C(n, k)= if(k<0||k>n, 0, n!/k!/(n-k)!);
a(n)= (1/2) *(C(2*n-3, n-3)+C(n-2, (n-3)\2));
(Magma) [(1/2)*(Binomial(2*n-3, n-3) + Binomial(n-2, Floor((n-3)/2))): n in [1..30]]; // Vincenzo Librandi, Oct 08 2017
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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