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%I #17 Mar 18 2019 19:30:37
%S 6,12,36,48,90,180,294,576,1134,2160,4158,8496,16458,33096,65880,
%T 131712,262242,525852,1048686,2099520,4195296,8392824,16777350,
%U 33564672,67109250,134234256,268438860,536904480,1073741994,2147556240,4294967478,8590066944
%N a(n) = Sum_{d|n} (2^d - (-1)^d)*phi(3*n/d).
%H Michael De Vlieger, <a href="/A306899/b306899.txt">Table of n, a(n) for n = 1..3320</a>
%H Dennis S. Bernstein, Omran Kouba, <a href="https://arxiv.org/abs/1901.10703">Counting Colorful Necklaces and Bracelets in Three Colors</a>, arXiv:1901.10703 [math.CO], 2019.
%p See A306888.
%t Table[DivisorSum[n, (2^# - (-1)^#) EulerPhi[3 n/#] &], {n, 10^4}] (* _Michael De Vlieger_, Mar 18 2019 *)
%o (PARI) a(n) = sumdiv(n, d, (2^d - (-1)^d)*eulerphi(3*n/d)); \\ _Michel Marcus_, Mar 16 2019
%Y Cf. A306888, A306896, A306898.
%K nonn
%O 1,1
%A _Michael De Vlieger_ and _N. J. A. Sloane_, Mar 15 2019
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