OFFSET
1,1
LINKS
Index entries for linear recurrences with constant coefficients, signature (0,6,0,-1).
FORMULA
G.f.: 2*x*(1+x)*(1+x+x^2) / ( (x^2+2*x-1)*(x^2-2*x-1) ). - R. J. Mathar, Nov 27 2011
a(n) = 2*A077241(n-1). - R. J. Mathar, Nov 27 2011
a(n) = 6*a(n-2) - a(n-4). - Colin Barker, Oct 20 2014
EXAMPLE
a(1)=2 because 2*4 + 17 = 25 = 5^2.
MATHEMATICA
Select[Range[6000000], IntegerQ[Sqrt[2#^2+17]]&] (* Harvey P. Dale, Aug 18 2012 *)
LinearRecurrence[{0, 6, 0, -1}, 2{1, 2, 8, 13}, 30] (* Robert G. Wilson v, Dec 02 2014 *)
PROG
(PARI) Vec(2*x*(1+x)*(1+x+x^2) / ((x^2+2*x-1)*(x^2-2*x-1)) + O(x^50)) \\ Colin Barker, Oct 20 2014
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Richard Choulet, Sep 21 2008
EXTENSIONS
Corrected by R. J. Mathar, Nov 27 2011
Editing and more terms from Colin Barker, Oct 20 2014
STATUS
approved