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%I #12 Jun 22 2018 15:11:14
%S 1,0,3,-1,0,3,-1,0,6,0,0,9,0,0,15,1,0,21,0,0,33,1,0,45,0,0,66,1,0,87,
%T -1,0,123,-1,0,162,-1,0,222,0,0,288,1,0,384,-1,0,495,1,0,648,0,0,825,
%U 2,0,1062,-2,0,1341,-2,0,1707,-1,0,2136,1,0,2688,2,0,3339,-1,0,4164,2,0,5136,1,0
%N McKay-Thompson series of class 27a for Monster.
%H G. C. Greubel, <a href="/A058600/b058600.txt">Table of n, a(n) for n = -1..2500</a>
%H D. Ford, J. McKay and S. P. Norton, <a href="http://dx.doi.org/10.1080/00927879408825127">More on replicable functions</a>, Comm. Algebra 22, No. 13, 5175-5193 (1994).
%H <a href="/index/Mat#McKay_Thompson">Index entries for McKay-Thompson series for Monster simple group</a>
%F Expansion of A + 3*q^2/A, where A = q*(eta(q^3)/eta(q^27)), in powers of q. - _G. C. Greubel_, Jun 22 2018
%e T27a = 1/q + 3*q - q^2 + 3*q^4 - q^5 + 6*q^7 + 9*q^10 + 15*q^13 + q^14 + ...
%t eta[q_]:= q^(1/24)*QPochhammer[q]; A:= q*(eta[q^3]/eta[q^27]); a:= CoefficientList[Series[A + 3*q^2/A, {q, 0, 60}], q]; Table[a[[n]], {n, 0, 50}] (* _G. C. Greubel_, Jun 22 2018 *)
%o (PARI) q='q+O('q^50); A = eta(q^3)/eta(q^27); Vec(A + 3*q^2/A) \\ _G. C. Greubel_, Jun 22 2018
%Y Cf. A000521, A007240, A014708, A007241, A007267, A045478, etc.
%K sign
%O -1,3
%A _N. J. A. Sloane_, Nov 27 2000
%E Terms a(24) onward added by _G. C. Greubel_, Jun 22 2018
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