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%I #16 Jun 29 2023 20:04:17
%S 1,3,12,27,108,243,972,2187,8748,19683,78732,177147,708588,1594323,
%T 6377292,14348907,57395628,129140163,516560652,1162261467,4649045868,
%U 10460353203,41841412812,94143178827,376572715308
%N A133566 * A000244.
%C A133646 is a companion case.
%H Harvey P. Dale, <a href="/A133647/b133647.txt">Table of n, a(n) for n = 0..1000</a>
%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (0, 9).
%F A133566 * A000244, where A133566 = an infinite lower triangular matrix and A000244 = [3^0, 3^1, 3^2, ...]. For odd n, a(n) = 3^n. For even n, a(n) = 4 * 3^(n-1).
%F G.f.: (1+3*x+3*x^2)/(1-9*x^2). - _Philippe Deléham_, Apr 06 2012
%F a(n+2) = 9*a(n), a(0) = 1, a(1) = 3, a(2) = 12. - _Philippe Deléham_, Apr 06 2012
%e a(3) = 27 = 3^3.
%e a(4) = 108 = 4 * 3^3.
%t Join[{1},Flatten[NestList[9#&,{3,12},20]]] (* or *) LinearRecurrence[{0,9},{1,3,12},40] (* _Harvey P. Dale_, Aug 01 2019 *)
%Y Cf. A133566, A133646, A000244.
%K nonn
%O 0,2
%A _Gary W. Adamson_, Sep 19 2007
%E More terms from _Philippe Deléham_, Apr 06 2012
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