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a(n) = (3^(n-1) - 1) * (3^n - 1)/2.
1

%I #17 Apr 04 2023 07:44:09

%S 0,8,104,1040,9680,88088,795704,7170080,64556960,581091368,5230058504,

%T 47071235120,423643241840,3812795553848,34315179116504,

%U 308836669444160,2779530197184320,25015772291219528,225141952170657704,2026277574184965200,18236498181611824400

%N a(n) = (3^(n-1) - 1) * (3^n - 1)/2.

%H <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (13,-39,27).

%F From _R. J. Mathar_, Nov 07 2015: (Start)

%F G.f.: -8*x^2/((x - 1)*(3*x - 1)*(9*x - 1)).

%F a(n) = 8*A006100(n). (End)

%F E.g.f.: exp(x)*(3 - 4*exp(2*x) + exp(8*x))/6. - _Stefano Spezia_, Apr 03 2023

%p A109774:=n->(3^(n-1) - 1) * (3^n - 1)/2: seq(A109774(n), n=1..30); # _Wesley Ivan Hurt_, Jan 24 2017

%Y Cf. A006100.

%K nonn,easy

%O 1,2

%A _R. K. Guy_, Aug 14 2005

%E a(21) from _Stefano Spezia_, Apr 03 2023