OFFSET
1,3
COMMENTS
a(n) is also the number of ways to tile an unbreakable 3 X 2n bracelet with dominoes and with upside-down T-shaped tetrominoes which looks like this:
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FORMULA
Conjectures from Colin Barker, Sep 06 2020: (Start)
G.f.: x^3*(6 - 8*x + 7*x^2 - 2*x^3) / ((1 - x)*(1 - 3*x + 3*x^2 - 3*x^3 + 2*x^4 - x^5)).
a(n) = 4*a(n-1) - 6*a(n-2) + 6*a(n-3) - 5*a(n-4) + 3*a(n-5) - a(n-6) for n>6.
(End)
EXAMPLE
For n=5 the a(5) = 35 tilings are as follows: we can use 3 colors of 10-minoes, each of which can be rotated to 10 different positions or "phases", giving us 30, and we can use two (single-color) 5-minoes in five different "phases", giving us another 5 tilings, with a grand total of 30 + 5 = 35.
MATHEMATICA
B[1] = 0; B[2] = 0; B[3] = 0; B[4] = 0; B[5] = 5;
B[n_?IntegerQ] :=
B[n] = Floor[(n - 3)/2]*n +
Sum[Floor[(i + 1)/2]*B[n - 4 - i], {i, 1, n - 5}];
Table[B[2 n], {n, 1, 30}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Tianshu Ouyang and Greg Dresden, Sep 05 2020
STATUS
approved