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%I #7 Jul 24 2018 09:38:48
%S 3,15,61,241,961,3839,15327,61185,244257,975105,3892739,15540287,
%T 62038717,247666113,988713281,3947063807,15757159327,62904498689,
%U 251122418273,1002511271425,4002147065347,15977058402447,63782362573693
%N Hilltop maps: number of n X 2 binary arrays indicating the locations of corresponding elements not exceeded by any horizontal, vertical or antidiagonal neighbor in a random 0..2 n X 2 array.
%C Column 2 of A218233.
%H R. H. Hardin, <a href="/A218227/b218227.txt">Table of n, a(n) for n = 1..210</a>
%F Empirical: a(n) = 3*a(n-1) + 3*a(n-2) + 3*a(n-3) + 3*a(n-4) + a(n-5) + a(n-6) + a(n-7) + a(n-8) + a(n-9).
%F Empirical g.f.: x*(3 + 6*x + 7*x^2 + 4*x^3 + x^4 + 2*x^5 + 3*x^6 + 2*x^7 + x^8) / (1 - 3*x - 3*x^2 - 3*x^3 - 3*x^4 - x^5 - x^6 - x^7 - x^8 - x^9). - _Colin Barker_, Jul 24 2018
%e Some solutions for n=3:
%e ..1..1....1..1....0..1....0..1....0..1....1..0....1..0....1..1....0..0....0..1
%e ..1..0....0..0....0..0....0..1....1..1....1..1....0..0....0..0....1..0....1..0
%e ..0..0....0..0....0..1....0..1....1..0....1..1....1..1....1..1....0..1....0..1
%Y Cf. A218233.
%K nonn
%O 1,1
%A _R. H. Hardin_, Oct 24 2012
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