A253823 Octagonal numbers (A000567) which are also centered triangular numbers (A005448).

1, 97921, 1039585, 130402572385, 1384429704481, 173658931280825761, 1843664419471976641, 231264030011583194717761, 2455233718711319470593985, 307977546462671843639087352385, 3269669116478082433043125969441, 410138010309759307549971991199125921

Offset

1,2

Links

Colin Barker, Table of n, a(n) for n = 1..327

Index entries for linear recurrences with constant coefficients, signature (1,1331714,-1331714,-1,1).

Formula

a(n) = a(n-1)+1331714*a(n-2)-1331714*a(n-3)-a(n-4)+a(n-5).

G.f.: -x*(x^4+97920*x^3-390050*x^2+97920*x+1) / ((x-1)*(x^2-1154*x+1)*(x^2+1154*x+1)).

Example

97921 is in the sequence because it is the 181st octagonal number and the 256th centered triangular number.

Prog

(PARI) Vec(-x*(x^4+97920*x^3-390050*x^2+97920*x+1)/((x-1)*(x^2-1154*x+1)*(x^2+1154*x+1)) + O(x^100))

Crossrefs

Cf. A000567, A005448, A253821, A253822.

Sequence in context: A022214 A237509 A109185 * A371950 A316488 A235196

Adjacent sequences: A253820 A253821 A253822 * A253824 A253825 A253826

Keyword

nonn,easy

Author

Colin Barker, Jan 14 2015

Status

approved