OFFSET
1,2
COMMENTS
Also positive integers y in the solutions to 6*x^2 - 7*y^2 - 4*x + 7*y - 2 = 0, the corresponding values of x being A254855.
LINKS
Colin Barker, Table of n, a(n) for n = 1..1000
Index entries for linear recurrences with constant coefficients, signature (1,26,-26,-1,1).
FORMULA
a(n) = a(n-1)+26*a(n-2)-26*a(n-3)-a(n-4)+a(n-5).
G.f.: x*(x^3+13*x^2-x-1) / ((x-1)*(x^4-26*x^2+1)).
EXAMPLE
15 is in the sequence because the 15th centered heptagonal number is 736, which is also the 16th octagonal number.
MATHEMATICA
LinearRecurrence[{1, 26, -26, -1, 1}, {1, 2, 15, 40, 377}, 30] (* Harvey P. Dale, Apr 30 2019 *)
PROG
(PARI) Vec(x*(x^3+13*x^2-x-1)/((x-1)*(x^4-26*x^2+1)) + O(x^100))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Colin Barker, Feb 08 2015
STATUS
approved