%I
%S 64,256,1016,3692,11752,33042,83752,195020,423460,867347,1690744,
%T 3158528,5686080,9908365,16774260,27673310,44603624,70391386,
%U 108974472,165764956,248107880,365856580,532088128,763986096,1083921900,1520770469
%N Number of length n+2 0..3 arrays with at most two downsteps in every n consecutive neighbor pairs.
%C Row 2 of A255660.
%H R. H. Hardin, <a href="/A255662/b255662.txt">Table of n, a(n) for n = 1..210</a>
%F Empirical: a(n) = (1/39916800)*n^11 + (1/453600)*n^10 + (1/11520)*n^9 + (11/6048)*n^8 + (28751/1209600)*n^7 + (4303/21600)*n^6 + (789461/725760)*n^5 + (40955/18144)*n^4 + (164359/43200)*n^3 + (110513/6300)*n^2 + (44789/1540)*n + 10.
%F Empirical g.f.: x*(64  512*x + 2168*x^2  5684*x^3 + 9864*x^4  11798*x^5 + 9944*x^6  5948*x^7 + 2500*x^8  711*x^9 + 124*x^10  10*x^11) / (1  x)^12.  _Colin Barker_, Jan 21 2018
%e Some solutions for n=4:
%e ..2....1....3....0....1....3....3....2....3....0....2....2....1....1....2....3
%e ..3....2....1....0....3....0....3....3....2....0....1....1....3....1....1....0
%e ..3....3....1....1....2....2....0....2....2....3....3....2....1....2....1....3
%e ..1....2....3....3....1....3....3....2....3....1....3....1....3....2....2....0
%e ..1....0....1....0....1....2....2....3....0....1....3....3....1....3....1....1
%e ..0....3....2....0....1....3....3....0....1....2....3....1....3....3....2....2
%Y Cf. A255660.
%K nonn
%O 1,1
%A _R. H. Hardin_, Mar 01 2015
