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A107660 Sum 3^max(k,n-k),k=0..n. 2
1, 6, 21, 72, 225, 702, 2133, 6480, 19521, 58806, 176661, 530712, 1592865, 4780782, 14344533, 43040160, 129127041, 387400806, 1162222101, 3486725352, 10460235105, 31380882462, 94142824533, 282429005040, 847287546561 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Third column of A107661.

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 0..1000

Index entries for linear recurrences with constant coefficients, signature (3,3,-9).

FORMULA

G.f.: (1+3*x)/((1-3*x)*(1-3*x^2)).

a(2n) = 3^(2n+1) - 2*3^n; a(2n+1) = 9^(n+1) - 3^(n+1).

a(n) = A167993(n+2) + 3*A167993(n+1). - R. J. Mathar, Aug 16 2013

MATHEMATICA

CoefficientList[Series[(1 + 3 x) / ((1 - 3 x) (1 - 3 x^2)), {x, 0, 40}], x] (* Vincenzo Librandi, Aug 17 2013 *)

CROSSREFS

Cf. A107659, A107661.

Sequence in context: A022814 A000390 A000391 * A200665 A200466 A200249

Adjacent sequences:  A107657 A107658 A107659 * A107661 A107662 A107663

KEYWORD

nonn,easy

AUTHOR

Mitch Harris

STATUS

approved

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Last modified November 29 21:32 EST 2021. Contains 349416 sequences. (Running on oeis4.)