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A007727
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Number of 2n-bead black-white strings with n black beads and fundamental period 2n.
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14
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1, 2, 4, 18, 64, 250, 900, 3430, 12800, 48600, 184500, 705430, 2703168, 10400598, 40113164, 155117250, 601067520, 2333606218, 9075085776, 35345263798, 137846344000, 538257870990, 2104098258284, 8233430727598, 32247600966144
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OFFSET
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0,2
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COMMENTS
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LINKS
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FORMULA
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MAPLE
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if n = 0 then
1;
else
add(numtheory[mobius](n/d)*binomial(2*d, d), d =numtheory[divisors](n)) ;
end if ;
end proc:
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MATHEMATICA
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a[n_] := If[n == 0, 1, Sum[MoebiusMu[n/d] Binomial[2d, d], {d, Divisors[n]}]];
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PROG
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(PARI) { a(n) = if(n>0, sumdiv(n, d, moebius(n/d)*binomial(2*d, d)), 0); }
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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Doug Bowman, bowman(AT)math.uiuc.edu.
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EXTENSIONS
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STATUS
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approved
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