A generalization of the polyominoes using a collection of equal-sized equilateral triangles (instead
of squares) arranged with coincident sides. Polyiamonds
are sometimes simply known as iamonds.
The number of two-sided (i.e., can be picked up and flipped, so mirror image pieces are considered identical) polyiamonds made up of triangles are 1, 1, 1, 3, 4, 12, 24, 66, 160, 448, ... (OEIS
A000577). The number of one-sided polyiamonds
composed of
triangles are 1, 1, 1, 4, 6, 19, 43, 121, ... (OEIS A006534).
One of the 160 9-polyiamonds has a hole (Gardner 1984, p. 174).
The numbers of -polyiamonds
with holes for ,
10, 11, ... are 1, 4, 25, 108, 450, ... (OEIS A070764;
Myers), the first few of which are illustrated above.
triangles are 1, 1, 1, 3, 4, 12, 24, 66, 160, 448, ... (OEIS
A000577). The number of one-sided polyiamonds
composed of 
triangles are 1, 1, 1, 4, 6, 19, 43, 121, ... (OEIS A006534).
One of the 160 9-polyiamonds has a hole (Gardner 1984, p. 174).
-polyiamonds
with holes for 
,
10, 11, ... are 1, 4, 25, 108, 450, ... (OEIS A070764;
Myers), the first few of which are illustrated above.
