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A096304
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Numbers k such that 3k does not divide (6k-4)!/((3k-2)!*(3k-1)!).
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6
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1, 2, 3, 4, 5, 9, 10, 11, 12, 13, 14, 27, 28, 29, 30, 31, 32, 36, 37, 38, 39, 40, 41, 81, 82, 83, 84, 85, 86, 90, 91, 92, 93, 94, 95, 108, 109, 110, 111, 112, 113, 117, 118, 119, 120, 121, 122, 243, 244, 245, 246, 247, 248, 252, 253, 254, 255, 256, 257, 270, 271
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OFFSET
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1,2
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COMMENTS
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Equivalently, members of A019469 divisible by 3, divided by 3.
Ralf Stephan's formula is that terms k written in ternary have an arbitrary least significant digit and above that only 0's and 1's (per A340051). - Kevin Ryde, May 22 2021
{3a(n)-1:n>=1} is the set of positive integers k such that the k-th central binomial coefficient is not divisible by (k+1)*(2k-1). Such integers k are characterized by the following property: k is congruent to 2 (mod 3), and at least one of k-1, k+1 has no 2's in its base-3 expansion. - Valerio De Angelis, Aug 08 2022
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LINKS
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FORMULA
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a(n) = 9 * A005836(floor(n/6)) + (n mod 6) (conjectured) (confirmed, see links).
G.f.: x*(1+2*x)/(1-x^3) + 3/(1-x) * Sum_{i>=0} 3^i * x^(3*2^i) / (1 + x^(3*2^i)). - Kevin Ryde, May 22 2021
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MATHEMATICA
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Select[Range[300], Mod[(6#-4)!/((3#-2)!(3#-1)!), 3#]!=0&] (* Harvey P. Dale, Jun 11 2019 *)
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PROG
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(PARI) for(n=1, 300, if(((6*n-4)!/(3*n-2)!/(3*n-1)!)%(3*n), print1(n", ")))
(PARI) a(n) = my(r); [n, r]=divrem(n, 3); fromdigits(concat(binary(n), r), 3); \\ Kevin Ryde, May 22 2021
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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