Illustration for n = 4:
The free polyominoes with four cells are also called free tetrominoes.
The five free tetrominoes are as shown below:
_
|_| _ _ _
|_| |_| |_|_ |_|_ _ _
|_| |_|_ |_|_| |_|_| |_|_|
|_| |_|_| |_| |_| |_|_|
.
The bounding boxes are respectively as shown below:
_
| | _ _ _ _ _ _
| | | | | | | | _ _
| | | | | | | | | |
|_| |_ _| |_ _| |_ _| |_ _|
.
4 X 1 3 X 2 3 X 2 3 X 2 2 X 2
.
The total area of the bounding boxes is 4 + 6 + 6 + 6 + 4 = 26, so a(4) = 26.
.
