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A026121 3^n*(3^n-1)/2. 5

%I

%S 0,3,36,351,3240,29403,265356,2390391,21520080,193700403,1743362676,

%T 15690441231,141214502520,1270932117003,11438393835996,

%U 102945558872871,926510072902560,8338590785263203,75047317454789316,675425858255365311

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

%H Vincenzo Librandi, <a href="/A026121/b026121.txt">Table of n, a(n) for n = 0..200</a>

%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (12,-27).

%F a(n) = C(3^n,2), n>=0. - _Zerinvary Lajos_, Jan 07 2008

%F a(0) = 0, a(1)=3, a(n)=12*a(n-1)-27*a(n-2). - _Harvey P. Dale_, Oct 07 2012

%F G.f.: 3*x / ( (9*x-1)*(3*x-1) ). a(n) = 3*A016142(n-1). - _R. J. Mathar_, Mar 24 2013

%p seq(binomial(3^n,2),n=0..19); # _Zerinvary Lajos_, Jan 07 2008

%t Binomial[3^Range[0,20],2] (* or *) LinearRecurrence[{12,-27},{0,3},30] (* _Harvey P. Dale_, Oct 07 2012 *)

%o (MAGMA) [3^n*(3^n-1)/2: n in [0..30]]; // _Vincenzo Librandi_, May 01 2011

%o (PARI) a(n)=binomial(3^n,2) \\ _Charles R Greathouse IV_, Jun 11 2015

%K nonn,easy

%O 0,2

%A _N. J. A. Sloane_.

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Last modified November 12 09:25 EST 2019. Contains 329054 sequences. (Running on oeis4.)