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A046209
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Number of ternary Lyndon words whose digits sum to 0 mod 3; also number of trace 0 irreducible polynomials over GF(3).
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12
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1, 1, 2, 6, 16, 38, 104, 270, 726, 1960, 5368, 14736, 40880, 113828, 318848, 896670, 2532160, 7174050, 20390552, 58112088, 166037248, 475467916, 1364393896, 3922624800, 11297181456, 32588003000, 94143178098, 272342710380, 788854912240, 2287679084096, 6641649422408, 19302293185470
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OFFSET
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1,3
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COMMENTS
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Also number of ternary Lyndon words of trace 0 over GF(3).
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LINKS
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FORMULA
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a(n) = 1/(3*n) * sum(d divides n, gcd(d, 3)*mu(d)*3^(n/d) ).
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EXAMPLE
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a(4) = 6 = |{ 0012, 0021, 0111, 0102, 0222, 1122 }|.
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MATHEMATICA
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a[n_] := 1/(3n) DivisorSum[n, GCD[#, 3]*MoebiusMu[#]*3^(n/#)&]; Array[a, 32] (* Jean-François Alcover, Dec 06 2015, adapted from PARI *)
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PROG
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(PARI) a(n) = 1/(3*n) * sumdiv(n, d, gcd(d, 3)*moebius(d)*3^(n/d) ); /* Joerg Arndt, Aug 17 2012 */
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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