OFFSET
1,2
COMMENTS
Numbers k such that 80*k^2 - 80*k + 25 is a square.
Also the indices of centered square numbers which are also centered pentagonal numbers. - Colin Barker, Jan 01 2015
LINKS
Colin Barker, Table of n, a(n) for n = 1..798
Index entries for linear recurrences with constant coefficients, signature (19,-19,1).
FORMULA
a(n+2) = 18*a(n+1) - a(n) - 8.
a(n+1) = 9*a(n) - 4 + sqrt(80*a(n)^2 - 80*a(n) + 25).
G.f.: x*(-1+9*x)/(-1+x)/(1 - 18*x + x^2). - R. J. Mathar, Nov 14 2007
a(n) = 19*a(n-1) - 19*a(n-2) + a(n-3). - Colin Barker, Jan 01 2015
MATHEMATICA
LinearRecurrence[{19, -19, 1}, {1, 10, 171}, 20] (* Harvey P. Dale, Oct 09 2020 *)
PROG
(PARI) Vec(x*(-1+9*x)/((-1+x)*(1-18*x+x^2)) + O(x^100)) \\ Colin Barker, Jan 01 2015
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Richard Choulet, Oct 16 2007
EXTENSIONS
More terms from Paolo P. Lava, Nov 25 2008
STATUS
approved