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A218227
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.
1
3, 15, 61, 241, 961, 3839, 15327, 61185, 244257, 975105, 3892739, 15540287, 62038717, 247666113, 988713281, 3947063807, 15757159327, 62904498689, 251122418273, 1002511271425, 4002147065347, 15977058402447, 63782362573693
OFFSET
1,1
COMMENTS
Column 2 of A218233.
LINKS
FORMULA
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).
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
EXAMPLE
Some solutions for n=3:
..1..1....1..1....0..1....0..1....0..1....1..0....1..0....1..1....0..0....0..1
..1..0....0..0....0..0....0..1....1..1....1..1....0..0....0..0....1..0....1..0
..0..0....0..0....0..1....0..1....1..0....1..1....1..1....1..1....0..1....0..1
CROSSREFS
Cf. A218233.
Sequence in context: A001655 A058749 A292483 * A072336 A356421 A122671
KEYWORD
nonn
AUTHOR
R. H. Hardin, Oct 24 2012
STATUS
approved