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 A268965 Number of n X 2 0..2 arrays with some element plus some horizontally or antidiagonally adjacent neighbor totalling two not more than once. 1
 9, 60, 336, 1728, 8448, 39936, 184320, 835584, 3735552, 16515072, 72351744, 314572800, 1358954496, 5838471168, 24964497408, 106300440576, 450971566080, 1906965479424, 8040178778112, 33809982554112, 141836999983104 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS R. H. Hardin, Table of n, a(n) for n = 1..210 FORMULA Empirical: a(n) = 8*a(n-1) - 16*a(n-2). Conjectures from Colin Barker, Jan 17 2019: (Start) G.f.: 3*x*(3 - 4*x) / (1 - 4*x)^2. a(n) = 4^(n-1) * (6*n+3). (End) EXAMPLE Some solutions for n=4: ..0..0. .0..0. .0..1. .2..2. .1..2. .1..0. .2..2. .0..2. .2..1. .0..1 ..0..2. .1..1. .0..0. .1..1. .2..0. .0..0. .2..1. .2..1. .0..2. .2..2 ..2..2. .2..1. .2..2. .2..2. .0..1. .2..2. .2..1. .0..0. .1..2. .2..0 ..1..0. .2..1. .2..2. .2..1. .2..1. .1..0. .2..2. .0..0. .2..2. .1..2 CROSSREFS Column 2 of A268971. Sequence in context: A268972 A288962 A074431 * A081904 A085373 A241976 Adjacent sequences:  A268962 A268963 A268964 * A268966 A268967 A268968 KEYWORD nonn AUTHOR R. H. Hardin, Feb 16 2016 STATUS approved

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Last modified January 23 10:50 EST 2020. Contains 331171 sequences. (Running on oeis4.)