OFFSET
1,2
COMMENTS
Also positive integers y in the solutions to 3*x^2 - 7*y^2 - 3*x + 7*y = 0, the corresponding values of x being A253476.
LINKS
Colin Barker, Table of n, a(n) for n = 1..980
Index entries for linear recurrences with constant coefficients, signature (1,110,-110,-1,1).
FORMULA
a(n) = a(n-1)+110*a(n-2)-110*a(n-3)-a(n-4)+a(n-5).
G.f.: -x*(x^4+9*x^3-74*x^2+9*x+1) / ((x-1)*(x^4-110*x^2+1)).
EXAMPLE
10 is in the sequence because the 10th centered heptagonal number is 316, which is also the 15th centered triangular number.
MATHEMATICA
LinearRecurrence[{1, 110, -110, -1, 1}, {1, 10, 46, 1045, 5005}, 30] (* Harvey P. Dale, Aug 13 2018 *)
PROG
(PARI) Vec(-x*(x^4+9*x^3-74*x^2+9*x+1)/((x-1)*(x^4-110*x^2+1)) + O(x^100))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Colin Barker, Jan 02 2015
STATUS
approved