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A158453
Number of connected spanning subgraphs in 3 X n grid.
3
1, 23, 431, 7857, 142625, 2587279, 46929343, 851213073, 15439417633, 280042119887, 5079439221503, 92131506968913, 1671092849279201, 30310492056413839, 549775513106272063, 9971897330025100689, 180871526632046468257, 3280670474584477332047, 59505213248435614382975
OFFSET
1,2
FORMULA
G.f.: (2*x^3-x^2-x) / (54*x^3-73*x^2-1+22*x-8*x^4).
MAPLE
a:= n-> (Matrix([[23, 1, 0, 1/4]]). Matrix(4, (i, j)-> `if`(i=j-1, 1,
`if`(j=1, [22, -73, 54, -8][i], 0)))^n)[1, 3]:
seq(a(n), n=1..20);
CROSSREFS
Column k=3 of A359993.
Sequence in context: A286787 A226847 A203147 * A291128 A174262 A174425
KEYWORD
nonn,easy
AUTHOR
Alois P. Heinz, Mar 19 2009
STATUS
approved