Polyiamond
A generalization of the Polyominoes using a collection of equal-sized EquilateralTriangles (instead of Squares) arranged with coincident sides. Polyiamonds aresometimes 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, ... (Sloane's A000577). The number ofone-sided polyiamonds composed of triangles are 1, 1, 1, 4, 6, 19, 43, 121, ... (Sloane's A006534). NoHoles are possible with fewer than seven triangles.
The top row of 6-polyiamonds in the above figure are known as theBar, Crook, Crown, Sphinx,Snake, and Yacht. The bottom row of 6-polyiamonds are known as the Chevron,Signpost, Lobster, Hook, Hexagon, and Butterfly.
See also Polyabolo, Polyhex, Polyomino
References
Beeler, M.; Gosper, R. W.; and Schroeppel, R. HAKMEM. Cambridge, MA: MIT Artificial Intelligence Laboratory, Memo AIM-239, Feb. 1972.
Gardner, M. ``Mathematical Games.'' Sci. Amer., Dec. 1964.
Golomb, S. W. Polyominoes: Puzzles, Patterns, Problems, and Packings, 2nd ed. Princeton, NJ: Princeton University Press, pp. 90-92, 1994.
Sloane, N. J. A. SequencesA000577/M2374and A006534/M3287in ``An On-Line Version of the Encyclopedia of Integer Sequences.''http://www.research.att.com/~njas/sequences/eisonline.html and Sloane, N. J. A. and Plouffe, S.The Encyclopedia of Integer Sequences. San Diego: Academic Press, 1995.
von Seggern, D. CRC Standard Curves and Surfaces. Boca Raton, FL: CRC Press, pp. 342-343, 1993.
- Affinity
- Affix
- Aggregate
- AGM
- Agnesi's Witch
- Agnésienne
- Agonic Lines
- Ahlfors-Bers Theorem
- A-Integrable
- Airy Differential Equation
- Airy-Fock Functions
- Airy Functions
- Airy Projection
- Aitken Interpolation
- Aitken's Delta Squared Process
- Ajima-Malfatti Points
- Albanese Variety
- Albers Conic Projection
- Albers Equal-Area Conic Projection
- Alcuin's Sequence
- Aleksandrov-Cech Cohomology
- Aleksandrov's Uniqueness Theorem
- Aleph
- Aleph-0
- Aleph-1