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%I #10 Dec 05 2017 04:01:36
%S 1,2,4,12,41,176,792,3840,19291,100182,533160,2897544,16020564,
%T 89898944,510914744,2936004072,17036988567,99718480238,588166176660,
%U 3493203829992,20876368407633,125470501764720,757994313694512,4600845871441080,28047225141946116,171662437354159416
%N a(n) = Sum_{d|n} Sum{t|d} moebius(d/t)*binomial(3*t,t)/(3*d^2).
%H M. Isachenkov, I. Kirsch, V. Schomerus, <a href="http://arxiv.org/abs/1403.6857">Chiral Primaries in Strange Metals</a>, arXiv preprint arXiv:1403.6857 [hep-th], 2014. See (3.5).
%p with(numtheory);
%p f:=proc(N) local Na, n, ans;
%p ans:=0;
%p for Na in divisors(N) do
%p for n in divisors(Na) do
%p ans := ans + mobius(Na/n)*binomial(3*n,n)/(3*Na^2); od: od:
%p ans;
%p end;
%p [seq(f(n),n=1..30)];
%t a[n_] := Sum[MoebiusMu[d/t]*Binomial[3*t, t]/(3*d^2), {d, Divisors[n]}, {t, Divisors[d]}];
%t Array[a, 26] (* _Jean-François Alcover_, Dec 05 2017 *)
%K nonn
%O 1,2
%A _N. J. A. Sloane_, Jun 28 2014