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 A220720 Number of ways to reciprocally link elements of an n X 3 array either to themselves or to exactly two horizontal, diagonal or antidiagonal neighbors. 1
 1, 7, 30, 126, 534, 2275, 9667, 41089, 174660, 742428, 3155820, 13414405, 57020437, 242375995, 1030264362, 4379330826, 18615162402, 79127219431, 336345003127, 1429697162869, 6077194423824, 25832248272072, 109804788896376 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Column 3 of A220725. LINKS R. H. Hardin, Table of n, a(n) for n = 1..210 FORMULA Empirical: a(n) = 3*a(n-1) + 4*a(n-2) + 5*a(n-3) + 3*a(n-4) - 2*a(n-5). Empirical g.f.: x*(1 + 4*x + 5*x^2 + 3*x^3 - 2*x^4) / ((1 + x)*(1 - 4*x - 5*x^3 + 2*x^4)). - Colin Barker, Aug 02 2018 EXAMPLE Some solutions for n=3 0=self 1=nw 3=ne 4=w 6=e 7=sw 9=se (reciprocal directions total 10): .69.49.00...69.47.00...00.00.00...00.79.00...69.47.00...00.69.47...00.79.00 .00.17.17...39.16.47...00.69.47...39.69.14...39.19.00...00.36.14...36.49.17 .36.34.00...00.13.00...00.36.14...00.16.14...00.16.14...00.00.00...00.36.14 CROSSREFS Cf. A220725. Sequence in context: A037709 A037611 A171472 * A024311 A210100 A297080 Adjacent sequences:  A220717 A220718 A220719 * A220721 A220722 A220723 KEYWORD nonn AUTHOR R. H. Hardin, Dec 18 2012 STATUS approved

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Last modified January 17 09:32 EST 2020. Contains 330949 sequences. (Running on oeis4.)