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 A220597 Number of ways to reciprocally link elements of an n X 3 array either to themselves or to exactly two horizontal, vertical and antidiagonal neighbors, without 3-loops. 1
 1, 7, 65, 471, 3469, 25747, 190425, 1408823, 10424813, 77135267, 570740149, 4223043591, 31247282909, 231205914319, 1710746473929, 12658211881263, 93661059763965, 693020010391803, 5127816575062977, 37941909947454471 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Column 3 of A220602. LINKS R. H. Hardin, Table of n, a(n) for n = 1..210 FORMULA Empirical: a(n) = 6*a(n-1) + 7*a(n-2) + 25*a(n-3) - 3*a(n-4) + 9*a(n-5) + 22*a(n-6) - 5*a(n-7) - 2*a(n-8). Empirical g.f.: x*(1 + x + 16*x^2 + 7*x^3 + 16*x^4 + 23*x^5 - 5*x^6 - 2*x^7) / ((1 - x + x^2)*(1 - 5*x - 13*x^2 - 33*x^3 - 17*x^4 + 7*x^5 + 2*x^6)). - Colin Barker, Aug 01 2018 EXAMPLE Some solutions for n=3 0=self 2=n 3=ne 4=w 6=e 7=sw 8=s (reciprocal directions total 10): .68.46.48...00.67.47...00.68.48...00.68.48...00.67.47...68.46.47...00.00.78 .28.68.24...38.36.48...68.24.27...00.27.28...38.38.00...26.34.00...68.34.28 .26.24.00...26.46.24...26.34.00...36.46.24...26.24.00...00.00.00...26.46.24 CROSSREFS Cf. A220602. Sequence in context: A274823 A174508 A253802 * A264875 A083302 A099342 Adjacent sequences:  A220594 A220595 A220596 * A220598 A220599 A220600 KEYWORD nonn AUTHOR R. H. Hardin, Dec 16 2012 STATUS approved

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Last modified December 1 11:38 EST 2021. Contains 349429 sequences. (Running on oeis4.)