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 A268633 Number of n X 2 0..2 arrays with some element plus some horizontally or vertically adjacent neighbor totalling two exactly once. 2
 3, 24, 120, 504, 1944, 7128, 25272, 87480, 297432, 997272, 3306744, 10865016, 35429400, 114791256, 369882936, 1186176312, 3788111448, 12053081880, 38225488248, 120875192568, 381221761176, 1199453833944, 3765727153080 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Column 2 of A268639. LINKS R. H. Hardin, Table of n, a(n) for n = 1..210 FORMULA Empirical: a(n) = 6*a(n-1) - 9*a(n-2) for n>3. Conjectures from Colin Barker, Mar 21 2018: (Start) G.f.: 3*x*(1 + x)^2 / (1 - 3*x)^2. a(n) = 8*3^(n-2)*(2*n-1) for n>1. (End) EXAMPLE Some solutions for n=8: ..0..1. .1..2. .2..1. .1..2. .2..2. .0..0. .0..0. .2..2. .1..2. .1..2 ..1..0. .2..1. .1..0. .2..2. .1..2. .0..0. .0..0. .2..1. .2..1. .2..1 ..2..1. .1..0. .2..1. .2..1. .2..1. .0..1. .0..0. .2..1. .2..2. .2..1 ..2..1. .0..0. .1..0. .0..0. .2..2. .1..2. .1..0. .1..2. .1..2. .2..2 ..2..2. .0..0. .2..2. .1..0. .2..2. .0..1. .0..1. .2..2. .0..0. .1..2 ..2..1. .0..0. .2..2. .0..1. .0..1. .0..0. .1..0. .2..2. .1..0. .2..1 ..1..0. .1..2. .2..2. .0..0. .0..0. .2..1. .2..2. .2..1. .0..0. .2..2 ..0..0. .0..1. .2..2. .0..0. .0..0. .2..2. .2..2. .1..2. .0..1. .2..2 CROSSREFS Cf. A268639. Sequence in context: A127520 A175115 A301812 * A324065 A151883 A009134 Adjacent sequences:  A268630 A268631 A268632 * A268634 A268635 A268636 KEYWORD nonn AUTHOR R. H. Hardin, Feb 09 2016 STATUS approved

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Last modified June 5 11:52 EDT 2020. Contains 334840 sequences. (Running on oeis4.)