OFFSET
1,1
COMMENTS
Numbers k such that 11*(11 + k) is a perfect square.
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..1000
Index entries for linear recurrences with constant coefficients, signature (3,-3,1).
FORMULA
From Vincenzo Librandi, Jul 07 2012: (Start)
G.f.: 11*x*(3-x)/(1-x)^3.
a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3). (End)
From Elmo R. Oliveira, Jan 28 2025: (Start)
E.g.f.: 11*exp(x)*x*(3 + x).
a(n) = 11*A005563(n). (End)
MATHEMATICA
Select[ Range[20000], IntegerQ[ Sqrt[ 11(11 + # )]] & ]
CoefficientList[Series[11 (3 - x)/(1 - x)^3, {x, 0, 50}], x] (* Vincenzo Librandi, Jul 07 2012 *)
PROG
(PARI) a(n)=11*n*(n+2) \\ Charles R Greathouse IV, Dec 07 2011
(Magma) [11*n*(n+2): n in [1..50]]; // Vincenzo Librandi, Jul 07 2012
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Robert G. Wilson v, Feb 05 2002
STATUS
approved