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A301976 Number of no-leaf subgraphs of the 3 X n grid.

%I A301976 #21 Oct 06 2018 04:14:59
%S A301976 1,5,43,463,5193,58653,663203,7500343,84825873,959351093,10849935003,
%T A301976 122709094303,1387798370393,15695530423373,177511143297043,
%U A301976 2007591024144903,22705175829637153,256787863292718693,2904183928335418123,32845338488555237743
%N A301976 Number of no-leaf subgraphs of the 3 X n grid.
%C A301976 Also, the number of ways to lay unit-length matchsticks on a 3 X n grid of points in such a way that no end is "orphaned".
%C A301976 Conjecture: a(n) mod 10 = 3 for n > 2.
%H A301976 Peter Kagey, <a href="/A301976/b301976.txt">Table of n, a(n) for n = 1..949</a>
%F A301976 Conjectures from _Colin Barker_, Mar 30 2018: (Start)
%F A301976 G.f.: x*(1 + x)*(1 - 8*x - 3*x^2) / (1 - 12*x + 6*x^2 + 20*x^3 + 5*x^4).
%F A301976 a(n) = 12*a(n-1) - 6*a(n-2) - 20*a(n-3) - 5*a(n-4) for n>4.
%F A301976 (End)
%e A301976 Three of the a(4) = 463 subgraphs of the 3 X 4 grid with no leaf vertices are
%e A301976   +---+   +---+      +   +   +---+      +   +   +---+
%e A301976   |   |   |   |              |   |              |   |
%e A301976   +---+---+   +,     +   +---+---+, and +---+   +---+.
%e A301976   |   |       |          |   |          |   |
%e A301976   +---+---+---+      +   +---+   +      +---+   +   +
%Y A301976 A093129 is analogous for 2 X (n+1) grids.
%K A301976 nonn
%O A301976 1,2
%A A301976 _Peter Kagey_, Mar 29 2018