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A067725 a(n) = 3*n^2 + 6*n. 20
0, 9, 24, 45, 72, 105, 144, 189, 240, 297, 360, 429, 504, 585, 672, 765, 864, 969, 1080, 1197, 1320, 1449, 1584, 1725, 1872, 2025, 2184, 2349, 2520, 2697, 2880, 3069, 3264, 3465, 3672, 3885, 4104, 4329, 4560, 4797, 5040, 5289, 5544, 5805, 6072, 6345, 6624 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Numbers h such that 3*(3 + h) is a perfect square. - Alex Healy, Tj Tullo, Avery Pickford, Sep 20 2004

Equivalently, numbers k such that k/3+1 is a square. - Bruno Berselli, Apr 10 2018

LINKS

G. C. Greubel, Table of n, a(n) for n = 0..5000

Index entries for linear recurrences with constant coefficients, signature (3,-3,1).

FORMULA

a(n) = 3*A005563(n). - Zerinvary Lajos, Mar 06 2007

a(n) = a(n-1) + 6*n + 3, with n>0, a(0)=0. - Vincenzo Librandi, Aug 08 2010

From Colin Barker, Apr 11 2012: (Start)

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

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

E.g.f.: 3*x*(x + 3)*exp(x). - G. C. Greubel, Jul 20 2017

MATHEMATICA

Select[ Range[10000], IntegerQ[ Sqrt[ 3(3 + # )]] & ]

PROG

(PARI) a(n)=3*n*(n+2) \\ Charles R Greathouse IV, Dec 07 2011

CROSSREFS

Cf. A005563.

Cf. 8: A067728, 7: A067727, 6: A067726, 5: A067724.

Sequence in context: A063066 A262044 A097658 * A213903 A001106 A023551

Adjacent sequences:  A067722 A067723 A067724 * A067726 A067727 A067728

KEYWORD

nonn,easy

AUTHOR

Robert G. Wilson v, Feb 05 2002

EXTENSIONS

Edited by N. J. A. Sloane, Sep 14 2008 at the suggestion of R. J. Mathar

STATUS

approved

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Last modified February 21 18:45 EST 2019. Contains 320376 sequences. (Running on oeis4.)