OFFSET
1,2
COMMENTS
Also positive integers x in the solutions to 3*x^2 - 7*y^2 - x + 7*y - 2 = 0, the corresponding values of y being A254653.
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^2-4*x+1)*(x^2+7*x+1) / ((x-1)*(x^4-110*x^2+1)).
EXAMPLE
4 is in the sequence because the 4th pentagonal number is 22, which is also the 3rd centered heptagonal number.
MATHEMATICA
LinearRecurrence[{1, 110, -110, -1, 1}, {1, 4, 88, 421, 9661}, 30] (* Harvey P. Dale, Dec 09 2018 *)
PROG
(PARI) Vec(-x*(x^2-4*x+1)*(x^2+7*x+1)/((x-1)*(x^4-110*x^2+1)) + O(x^100))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Colin Barker, Feb 04 2015
STATUS
approved