OFFSET
0,2
COMMENTS
Sequence found by reading the line from 0, in the direction 0, 9,..., in the square spiral whose vertices are the generalized 11-gonal numbers A195160. - Omar E. Pol, Sep 18 2011
LINKS
Ivan Panchenko, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (3,-3,1).
FORMULA
a(n) = a(n-1) + 36*n - 27 for n>0, a(0)=0. - Vincenzo Librandi, Dec 15 2010
a(n) = Sum_{i = 2..10} P(i,n), where P(i,m) = m*((i-2)*m-(i-4))/2. - Bruno Berselli, Jul 04 2018
From G. C. Greubel, Sep 01 2019: (Start)
G.f.: 9*x*(1+3*x)/(1-x)^3.
E.g.f.: 9*x*(1+2*x)*exp(x). (End)
From Amiram Eldar, Feb 27 2022: (Start)
Sum_{n>=1} 1/a(n) = 2*log(2)/9.
Sum_{n>=1} (-1)^(n+1)/a(n) = (Pi - 2*log(2))/18. (End)
MAPLE
seq(9*n*(2*n-1), n=0..40); # G. C. Greubel, Sep 01 2019
MATHEMATICA
Table[9*n*(2*n-1), {n, 0, 40}] (* G. C. Greubel, Sep 01 2019 *)
9*PolygonalNumber[6, Range[0, 50]] (* Harvey P. Dale, Jul 24 2022 *)
PROG
(PARI) a(n)=9*n*(2*n-1) \\ Charles R Greathouse IV, Jun 17 2017
(Magma) [9*n*(2*n-1): n in [0..40]]; // G. C. Greubel, Sep 01 2019
(Sage) [9*n*(2*n-1) for n in (0..40)] # G. C. Greubel, Sep 01 2019
(GAP) List([0..40], n-> 9*n*(2*n-1)); # G. C. Greubel, Sep 01 2019
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Omar E. Pol, Dec 22 2008
STATUS
approved