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%I #8 Aug 02 2018 15:20:00
%S 1,2,7,21,68,221,717,2330,7571,24601,79940,259761,844081,2742802,
%T 8912607,28961101,94107748,305798741,993678757,3228912810,10492201691,
%U 34093920401,110786605380,359995910921,1169789934761,3801177874402
%N Number of ways to reciprocally link elements of a 2 X n array either to themselves or to exactly two horizontal, diagonal or antidiagonal neighbors.
%C Row 2 of A220725.
%H R. H. Hardin, <a href="/A220726/b220726.txt">Table of n, a(n) for n = 1..210</a>
%F Empirical: a(n) = 3*a(n-1) + a(n-2) - 2*a(n-4).
%F Empirical g.f.: x*(1 - x - 2*x^3) / (1 - 3*x - x^2 + 2*x^4). - _Colin Barker_, Aug 02 2018
%e All solutions for n=3 0=self 1=nw 3=ne 4=w 6=e 7=sw 9=se (reciprocal directions total 10):
%e .00.00.00...00.67.47...69.49.00...00.69.47...69.47.00...69.46.47...00.79.00
%e .00.00.00...36.34.00...00.16.14...00.36.14...36.14.00...00.13.00...36.46.14
%Y Cf. A220725.
%K nonn
%O 1,2
%A _R. H. Hardin_, Dec 18 2012
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