OFFSET
1,2
COMMENTS
Also positive integers x in the solutions to x^2 - 7*y^2 + x + 7*y - 2 = 0, the corresponding values of y being A253879.
LINKS
Colin Barker, Table of n, a(n) for n = 1..832
Index entries for linear recurrences with constant coefficients, signature (17,-17,1).
FORMULA
a(n) = 17*a(n-1)-17*a(n-2)+a(n-3).
G.f.: -x*(x^2+5*x+1) / ((x-1)*(x^2-16*x+1)).
a(n) = (-2+(8-3*sqrt(7))^n*(3+sqrt(7))-(-3+sqrt(7))*(8+3*sqrt(7))^n)/4. - Colin Barker, Mar 04 2016
EXAMPLE
22 is in the sequence because the 22nd triangular number is 253, which is also the 9th centered heptagonal number.
MATHEMATICA
LinearRecurrence[{17, -17, 1}, {1, 22, 358}, 20] (* Harvey P. Dale, Sep 10 2022 *)
PROG
(PARI) Vec(-x*(x^2+5*x+1)/((x-1)*(x^2-16*x+1)) + O(x^100))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Colin Barker, Jan 17 2015
STATUS
approved