A158453 Number of connected spanning subgraphs in 3 X n grid.

1, 23, 431, 7857, 142625, 2587279, 46929343, 851213073, 15439417633, 280042119887, 5079439221503, 92131506968913, 1671092849279201, 30310492056413839, 549775513106272063, 9971897330025100689, 180871526632046468257, 3280670474584477332047, 59505213248435614382975

Offset: 1

Author

Alois P. Heinz, Mar 19 2009

Keywords

• nonn
• easy

Links

• Alois P. Heinz, Table of n, a(n) for n = 1..250
• Index entries for linear recurrences with constant coefficients, signature (22,-73,54,-8).

Crossrefs

Column k=3 of A359993.

Programs

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);

Formula

G.f.: (2*x^3-x^2-x) / (54*x^3-73*x^2-1+22*x-8*x^4).