OFFSET
1,2
COMMENTS
Also positive integers x in the solutions to 4*x^2 - 5*y^2 - 3*x + 5*y - 1 = 0, the corresponding values of y being A133273.
LINKS
Colin Barker, Table of n, a(n) for n = 1..750
Index entries for linear recurrences with constant coefficients, signature (19,-19,1).
FORMULA
a(n) = (6 + (5+2*sqrt(5))*(9+4*sqrt(5))^(-n) + (5-2*sqrt(5))*(9+4*sqrt(5))^n)/16.
a(n) = 19*a(n-1) - 19*a(n-2) + a(n-3) for n>3.
G.f.: x*(1 - 8*x + x^2) / ((1 - x)*(1 - 18*x + x^2)).
EXAMPLE
11 is in the sequence because the 11th 10-gonal number is 451, which is also the 10th centered 10-gonal number.
PROG
(PARI) Vec(x*(1 - 8*x + x^2) / ((1 - x)*(1 - 18*x + x^2)) + O(x^30))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Colin Barker, Dec 25 2016
STATUS
approved