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%I #7 Jul 24 2018 10:03:31
%S 3,15,57,225,895,3551,14089,55905,221827,880191,3492529,13858081,
%T 54987775,218187167,865749521,3435226017,13630706691,54085572239,
%U 214607297385,851544879425,3378863116543,13407063134527,53198172193817
%N Hilltop maps: number of n X 2 binary arrays indicating the locations of corresponding elements not exceeded by any horizontal or antidiagonal neighbor in a random 0..3 n X 2 array.
%C Column 2 of A218810.
%H R. H. Hardin, <a href="/A218804/b218804.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) + a(n-4) + a(n-5) + a(n-6) + a(n-7).
%F Empirical g.f.: x*(1 + x)^2*(3 + x^4) / (1 - 3*x - 3*x^2 - 3*x^3 - x^4 - x^5 - x^6 - x^7). - _Colin Barker_, Jul 24 2018
%e Some solutions for n=3:
%e ..1..0....0..1....1..0....0..0....1..1....0..1....1..0....0..1....0..1....0..1
%e ..0..0....1..0....1..1....1..1....0..0....1..1....1..1....1..1....1..1....0..1
%e ..1..0....0..0....0..1....0..1....1..0....0..0....1..0....1..1....0..1....1..1
%Y Cf. A218810.
%K nonn
%O 1,1
%A _R. H. Hardin_, Nov 06 2012
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