OFFSET
1,1
COMMENTS
Except the first term, main diagonal of A155546. - Vincenzo Librandi, Mar 04 2013
LINKS
Bruno Berselli, Table of n, a(n) for n = 1..1000
Index entries for linear recurrences with constant coefficients, signature (3,-3,1).
FORMULA
G.f.: -x*(5-14*x+5*x^2)/(1-x)^3.
a(n) = a(-n+1) = 2*n^2-2*n-5.
a(n) = A046092(n-1)-5.
Sum_{n>=1} 1/a(n) = Pi*tan(sqrt(11)*Pi/2)/(2*sqrt(11)). - Amiram Eldar, Dec 23 2022
MATHEMATICA
Table[2 n^2 - 2 n - 5, {n, 50}]
PROG
(Magma) [m: m in [-5..5000] | IsSquare(2*m+11)];
(Maxima) makelist(coeff(taylor(-(5-14*x+5*x^2)/(1-x)^3, x, 0, n), x, n), n, 0, 50);
(Magma) I:=[-5, -1, 7]; [n le 3 select I[n] else 3*Self(n-1)-3*Self(n-2)+Self(n-3): n in [1..50]]; // Vincenzo Librandi, Mar 04 2013
(PARI) a(n)=2*n^2-2*n-5 \\ Charles R Greathouse IV, Jun 17 2017
CROSSREFS
KEYWORD
sign,easy
AUTHOR
Bruno Berselli, Mar 01 2013
STATUS
approved