OFFSET
1,2
LINKS
Colin Barker, Table of n, a(n) for n = 1..325
Index entries for linear recurrences with constant coefficients, signature (1,1331714,-1331714,-1,1).
FORMULA
EXAMPLE
16065 is in the sequence because 2*16065+1 = 32131, 4*16065+1 = 64261, and 32131 and 64261 are both triangular numbers.
MATHEMATICA
Rest@ CoefficientList[Series[459 x^2 (35 + 1154 x + 35 x^2)/((1 - x) (1 - 1154 x + x^2) (1 + 1154 x + x^2)), {x, 0, 12}], x] (* Michael De Vlieger, Jul 02 2016 *)
PROG
(PARI) isok(n) = ispolygonal(2*n+1, 3) && ispolygonal(4*n+1, 3)
(PARI) concat(0, Vec(459*x^2*(35+1154*x+35*x^2)/((1-x)*(1-1154*x+x^2)*(1+1154*x+x^2)) + O(x^20)))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Colin Barker, Jul 02 2016
STATUS
approved