cp's OEIS Frontend

A221821 Number of n X 2 arrays of occupancy after each element stays put or moves to some horizontal or vertical neighbor, without consecutive moves in the same direction.

%I A221821 #9 Aug 11 2018 07:11:05
%S A221821 3,31,252,2113,17684,147920,1237439,10351593,86594795,724396370,
%T A221821 6059833949,50692672540,424062286786,3547432284249,29675536361423,
%U A221821 248246446379403,2076670068892280,17372085836189001,145323694322280832
%N A221821 Number of n X 2 arrays of occupancy after each element stays put or moves to some horizontal or vertical neighbor, without consecutive moves in the same direction.
%C A221821 Column 2 of A221824.
%H A221821 R. H. Hardin, <a href="/A221821/b221821.txt">Table of n, a(n) for n = 1..210</a>
%F A221821 Empirical: a(n) = 9*a(n-1) - 4*a(n-2) - 11*a(n-3) - 2*a(n-4) + 22*a(n-5) - 16*a(n-6) + 8*a(n-7).
%F A221821 Empirical g.f.: x*(3 + 4*x - 15*x^2 + 2*x^3 + 22*x^4 - 16*x^5 + 8*x^6) / (1 - 9*x + 4*x^2 + 11*x^3 + 2*x^4 - 22*x^5 + 16*x^6 - 8*x^7). - _Colin Barker_, Aug 11 2018
%e A221821 Some solutions for n=3:
%e A221821 ..0..2....1..1....1..0....2..0....0..1....1..2....1..0....1..0....0..1....2..0
%e A221821 ..0..1....1..2....0..3....1..1....3..1....0..2....2..0....1..3....1..2....2..0
%e A221821 ..0..3....1..0....0..2....0..2....0..1....1..0....2..1....1..0....2..0....2..0
%Y A221821 Cf. A221824.
%K A221821 nonn
%O A221821 1,1
%A A221821 _R. H. Hardin_, Jan 26 2013