OFFSET
0,1
COMMENTS
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 0..10000
Vincenzo Librandi, X^2-AY^2=1, Math Forum, 2007. [Wayback Machine link]
Index entries for linear recurrences with constant coefficients, signature (3,-3,1).
FORMULA
G.f.: -21*(1 + 19*x + 22*x^2)/(x-1)^3.
a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3).
From Amiram Eldar, Mar 16 2023: (Start)
Sum_{n>=0} 1/a(n) = (coth(Pi/sqrt(21))*Pi/sqrt(21) + 1)/42.
Sum_{n>=0} (-1)^n/a(n) = (cosech(Pi/sqrt(21))*Pi/sqrt(21) + 1)/42. (End)
MATHEMATICA
LinearRecurrence[{3, -3, 1}, {21, 462, 1785}, 50] (* Vincenzo Librandi, Feb 16 2012 *)
PROG
(Magma) I:=[21, 462, 1785]; [n le 3 select I[n] else 3*Self(n-1)-3*Self(n-2)+1*Self(n-3): n in [1..40]]; // Vincenzo Librandi, Feb 16 2012
(PARI) for(n=0, 40, print1(441*n^2 + 21", ")); \\ Vincenzo Librandi, Feb 16 2012
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Vincenzo Librandi, Mar 22 2009
EXTENSIONS
Comment rewritten, formula replaced by R. J. Mathar, Oct 28 2009
STATUS
approved