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%I #10 Sep 08 2022 08:45:28
%S 0,1,0,2,21,4,84,449,252,2662,9933,10616,75768,229825,374472,2050778,
%T 5575269,11965468,54216876,141011585,359708580,1420577566,3680660445,
%U 10395035312,37193449776,98083939969,292682641104,977230325234
%N Expansion of x/(1 - 2*x^2 - 21*x^3).
%H G. C. Greubel, <a href="/A122509/b122509.txt">Table of n, a(n) for n = 0..1000</a>
%H <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (0,2,21).
%F G.f.: x/(1 - 2*x^2 - 21*x^3).
%t CoefficientList[Series[x/(1 -2*x^2 -21*x^3), {x, 0, 30}], x]
%o (Magma) I:=[0,1,0]; [n le 3 select I[n] else 2*Self(n-2) +21*Self(n-3): n in [1..31]]; // _G. C. Greubel_, Jul 11 2021
%o (Sage)
%o def A122509_list(prec):
%o P.<x> = PowerSeriesRing(ZZ, prec)
%o return P( x/(1 -2*x^2 -21*x^3) ).list()
%o A122509_list(30) # _G. C. Greubel_, Jul 11 2021
%K nonn,easy
%O 0,4
%A _Roger L. Bagula_, Sep 15 2006
%E Edited by _G. C. Greubel_, Jul 11 2021
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