A175659 Eight bishops and one elephant on a 3 X 3 chessboard: a(n)= (3^(n+1)-Jacobsthal(n+1))-(3^n-Jacobsthal(n)), with Jacobsthal=A001045.

1, 6, 16, 52, 156, 476, 1436, 4332, 13036, 39196, 117756, 353612, 1061516, 3185916, 9560476, 28686892, 86071596, 258236636, 774753596, 2324348172, 6973219276, 20920007356, 62760721116, 188283561452, 564853480556

Offset

0,2

Comments

The a(n) represent the number of n-move routes of a fairy chess piece starting in the central square (m = 5) on a 3 X 3 chessboard. This fairy chess piece behaves like a bishop on the eight side and corner squares but on the central square the bishop turns into a raging elephant, see A175654.

The sequence above corresponds to 4 A[5] vectors with decimal values 343, 349, 373 and 469. These vectors lead for the side squares to A000079 and for the corner squares to A093833 (a(n)=3^n-Jacobsthal(n)).

Links

Vincenzo Librandi, Table of n, a(n) for n = 0..1000

Index entries for linear recurrences with constant coefficients, signature (4,-1,-6).

Formula

G.f.: (1+2*x-7*x^2)/(1-4*x+x^2+6*x^3).

a(n) = 4*a(n-1)-a(n-2)-6*a(n-3) with a(0)=1, a(1)=6 and a(2)=16.

a(n) = (-2*(-1)^n)/3-2^n/3+2*3^n. [Colin Barker, Oct 07 2012]

Maple

nmax:=24; m:=5; A[5]:= [1, 0, 1, 0, 1, 0, 1, 1, 1]: A:=Matrix([[0, 0, 0, 0, 1, 0, 0, 0, 1], [0, 0, 0, 1, 0, 1, 0, 0, 0], [0, 0, 0, 0, 1, 0, 1, 0, 0], [0, 1, 0, 0, 0, 0, 0, 1, 0], A[5], [0, 1, 0, 0, 0, 0, 0, 1, 0], [0, 0, 1, 0, 1, 0, 0, 0, 0], [0, 0, 0, 1, 0, 1, 0, 0, 0], [1, 0, 0, 0, 1, 0, 0, 0, 0]]): for n from 0 to nmax do B(n):=A^n: a(n):= add(B(n)[m, k], k=1..9): od: seq(a(n), n=0..nmax);

Mathematica

CoefficientList[Series[(1 + 2 x - 7 x^2) / (1 - 4 x + x^2 + 6 x^3), {x, 0, 40}], x] (* Vincenzo Librandi, Jul 21 2013 *)

Prog

(Magma) I:=[1, 6, 16]; [n le 3 select I[n] else 4*Self(n-1)-Self(n-2)-6*Self(n-3): n in [1..35]]; // Vincenzo Librandi, Jul 21 2013

Crossrefs

Cf. A000079, A001045, A093833, A175654, A175655 (central square).

Sequence in context: A301978 A275585 A026086 * A221270 A316984 A192000

Adjacent sequences: A175656 A175657 A175658 * A175660 A175661 A175662

Keyword

easy,nonn

Author

Johannes W. Meijer, Aug 06 2010

Status

approved