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 A220721 Number of ways to reciprocally link elements of an nX4 array either to themselves or to exactly two horizontal, diagonal or antidiagonal neighbors 1
 1, 21, 230, 2586, 29310, 331589, 3751888, 42452438, 480346122, 5435081884, 61497557384, 695840407309, 7873383816355, 89086767652576, 1008010323915227, 11405563810447637, 129053128472692922, 1460226802057078840 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Column 4 of A220725 LINKS R. H. Hardin, Table of n, a(n) for n = 1..210 FORMULA Empirical: a(n) = 9*a(n-1) +20*a(n-2) +64*a(n-3) +65*a(n-4) +51*a(n-5) -90*a(n-6) -76*a(n-7) -34*a(n-8) +21*a(n-9) +13*a(n-10) -a(n-11) -a(n-12) EXAMPLE Some solutions for n=3 0=self 1=nw 3=ne 4=w 6=e 7=sw 9=se (reciprocal directions total 10) .69.47.00.00...00.79.69.47...00.79.79.00...00.00.00.00...00.00.69.47 .36.14.79.00...36.49.13.17...36.34.19.17...00.69.49.00...69.49.37.17 .00.36.46.14...00.00.13.00...00.00.36.14...00.00.16.14...00.13.13.00 CROSSREFS Sequence in context: A161518 A162360 A161899 * A320588 A020267 A188354 Adjacent sequences:  A220718 A220719 A220720 * A220722 A220723 A220724 KEYWORD nonn AUTHOR R. H. Hardin Dec 18 2012 STATUS approved

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Last modified January 23 16:36 EST 2020. Contains 331172 sequences. (Running on oeis4.)