OFFSET
0,1
COMMENTS
This Pell equation is used to find the 12-gonal square numbers (see A342709).
The corresponding solutions y are in A033890.
Essentially the same as A246453. - R. J. Mathar, Mar 24 2021
LINKS
FORMULA
a(n) = 7*a(n-1) - a(n-2).
a(n) = 2*T(2*n+1, 3/2), where T(n,x) denotes the n-th Chebyshev polynomial of the first kind. - Peter Bala, Jul 02 2022
EXAMPLE
a(1)^2 - 5 * A033890(1)^2 = 18^2 - 5 * 8^2 = 4.
MATHEMATICA
LinearRecurrence[{7, -1}, {3, 18}, 20] (* Amiram Eldar, Mar 19 2021 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Bernard Schott, Mar 19 2021
STATUS
approved