Polyiamond
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 top row of hexiamonds in the above figure are known as the bar, crook, crown, sphinx, snake,
and yacht. The bottom row of 6-polyiamonds are known as
the chevron, signpost, lobster, hook, hexagon,
and butterfly.
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.
SEE ALSO: Polyabolo,
Polyhex,
Polyiamond Tiling,
Polyomino,
Triangular Snake Graph
REFERENCES:
Beeler, M. Item 112 in Beeler, M.; Gosper, R. W.; and Schroeppel, R. HAKMEM. Cambridge, MA: MIT Artificial Intelligence Laboratory, Memo AIM-239, pp. 48-50,
Feb. 1972. http://www.inwap.com/pdp10/hbaker/hakmem/polyominos.html#item112.
Clarke, A. L. "Polyiamonds." http://www.recmath.com/PolyPages/PolyPages/Polyiamonds.htm.
Gardner, M. "Mathematical Games: On Polyiamonds: Shapes That are Made Out of
Equilateral Triangles." Sci. Amer. 211, Dec. 1964.
Gardner, M. "Polyiamond." Ch. 18 in The Sixth Book of Mathematical Games from Scientific American. Chicago, IL: University
of Chicago Press, pp. 173-182, 1984.
Golomb, S. W. Polyominoes: Puzzles, Patterns, Problems, and Packings, 2nd ed. Princeton, NJ: Princeton
University Press, pp. 90-92, 1994.
Keller, M. "Counting Polyforms." http://members.aol.com/wgreview/polyenum.html.
Myers, J. "Polyomino Tiling." http://www.srcf.ucam.org/~jsm28/tiling/.
O'Beirne, T. H. "Pentominoes and Hexiamonds." New Scientist 12,
379-380, 1961.
Pegg, E. Jr. "Iamonds." http://www.mathpuzzle.com/iamond.htm.
Reeve, J. E. and Tyrrell, J. A. "Maestro Puzzles." Math. Gaz. 45,
97-99, 1961.
Sloane, N. J. A. Sequences A000577/M2374 and A006534/M3287 in "The On-Line Encyclopedia
of Integer Sequences."
Torbijn, I. P. J. "Polyiamonds." J. Recr. Math. 2,
216-227, 1969.
Vichera, M. "Polyforms." http://www.vicher.cz/puzzle/polyforms.htm.
von Seggern, D. CRC Standard Curves and Surfaces. Boca Raton, FL: CRC Press, pp. 342-343,
1993.
Referenced on Wolfram|Alpha:
Polyiamond
CITE THIS AS:
Weisstein, Eric W. "Polyiamond." From
MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Polyiamond.html
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