OFFSET
1,2
COMMENTS
Also positive integers y in the solutions to 3*x^2 - 7*y^2 - x + 7*y - 2 = 0, the corresponding values of x being A254652.
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*(2*x^3+55*x^2-2*x-1) / ((x-1)*(x^4-110*x^2+1)).
EXAMPLE
3 is in the sequence because the 3rd centered heptagonal number is 22, which is also the 4th pentagonal number.
PROG
(PARI) Vec(x*(2*x^3+55*x^2-2*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