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 A220713 Number of ways to reciprocally link elements of an n X 2 array either to themselves or to exactly two king-move neighbors, without consecutive collinear links. 1
 1, 8, 26, 118, 463, 1930, 7843, 32209, 131698, 539466, 2208130, 9041065, 37013388, 151537869, 620403077, 2539982216, 10398860814, 42573713750, 174299854287, 713596365222, 2921515440791, 11960897338493, 48968785309970 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Column 2 of A220718. LINKS R. H. Hardin, Table of n, a(n) for n = 1..210 FORMULA Empirical: a(n) = 2*a(n-1) + 8*a(n-2) + 3*a(n-3) - 2*a(n-4) - 3*a(n-5) + a(n-6). Empirical g.f.: x*(1 + 6*x + 2*x^2 - x^3 - 3*x^4 + x^5) / (1 - 2*x - 8*x^2 - 3*x^3 + 2*x^4 + 3*x^5 - x^6). - Colin Barker, Aug 02 2018 EXAMPLE Some solutions for n=3 0=self 1=nw 2=n 3=ne 4=w 6=e 7=sw 8=s 9=se (reciprocal directions total 10): .69.47...68.47...89.00...00.00...00.00...00.00...00.78...69.48...00.00...89.78 .36.14...23.00...29.17...69.47...68.48...00.78...39.27...00.12...68.47...23.12 .00.00...00.00...36.14...36.14...26.24...36.24...36.14...00.00...23.00...00.00 CROSSREFS Cf. A220718. Sequence in context: A140788 A082573 A112645 * A260962 A274733 A223312 Adjacent sequences: A220710 A220711 A220712 * A220714 A220715 A220716 KEYWORD nonn AUTHOR R. H. Hardin, Dec 18 2012 STATUS approved

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Last modified June 8 08:19 EDT 2023. Contains 363157 sequences. (Running on oeis4.)