OFFSET
1,2
COMMENTS
Also positive integers x in the solutions to 6*x^2 - 7*y^2 - 6*x + 7*y = 0, the corresponding values of y being A253458.
LINKS
Colin Barker, Table of n, a(n) for n = 1..708
Index entries for linear recurrences with constant coefficients, signature (27,-27,1).
FORMULA
a(n) = 27*a(n-1)-27*a(n-2)+a(n-3).
G.f.: x*(13*x-1) / ((x-1)*(x^2-26*x+1)).
a(n) = sqrt((-2-(13-2*sqrt(42))^n-(13+2*sqrt(42))^n)*(2-(13-2*sqrt(42))^(1+n)-(13+2*sqrt(42))^(1+n)))/(4*sqrt(6)). - Gerry Martens, Jun 04 2015
EXAMPLE
14 is in the sequence because the 14th centered hexagonal number is 547, which is also the 13th centered heptagonal number.
PROG
(PARI) Vec(x*(13*x-1)/((x-1)*(x^2-26*x+1)) + O(x^100))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Colin Barker, Jan 01 2015
STATUS
approved