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A013708 a(n) = 3^(2n+1). 22
3, 27, 243, 2187, 19683, 177147, 1594323, 14348907, 129140163, 1162261467, 10460353203, 94143178827, 847288609443, 7625597484987, 68630377364883, 617673396283947, 5559060566555523, 50031545098999707 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

1/3 + 1/27 + 1/243 +... = 3/8 - Gary W. Adamson, Aug 29 2008

Number n such that if a=n, b=8n, c=15n, d=36n*sqrt(3n), then a^3+b^3+c^3=d^2; ex: a=3, b=24, c=45, d=324, 3^3+24^3+45^3=324^2 [From Vincenzo Librandi, Nov 20 2010]

LINKS

Table of n, a(n) for n=0..17.

Tanya Khovanova, Recursive Sequences

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

FORMULA

a(n)=9*a(n-1), n>0 ; a(0)=3 . G.f.: 3/(1-9x). - Philippe Deléham, Nov 23 2008

a(n) = A000244(2n+1). - R. J. Mathar, Jul 10 2015

MATHEMATICA

NestList[9#&, 3, 20] (* Harvey P. Dale, Apr 21 2014 *)

PROG

(PARI) a(n)=3^(2*n+1) \\ Charles R Greathouse IV, Aug 05 2015

CROSSREFS

Sequence in context: A065100 A035088 A268094 * A102518 A074117 A168495

Adjacent sequences:  A013705 A013706 A013707 * A013709 A013710 A013711

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane.

STATUS

approved

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Last modified April 28 05:37 EDT 2017. Contains 285557 sequences.