Circle packing in an equilateral triangle
Circle packing in an equilateral triangle is a packing problem in discrete mathematics where the objective is to pack n unit circles into the smallest possible equilateral triangle. Optimal solutions are known for n < 13 and for any triangular number of circles, and conjectures are available for n < 28.[1][2][3]
A conjecture of Paul Erdős and Norman Oler states that, if n is a triangular number, then the optimal packings of n − 1 and of n circles have the same side length: that is, according to the conjecture, an optimal packing for n − 1 circles can be found by removing any single circle from the optimal hexagonal packing of n circles.[4] This conjecture is now known to be true for n ≤ 15.[5]
Minimum solutions for the side length of the triangle:[1]
| Number of circles |
Triangle number |
Length | Area | Figure |
|---|---|---|---|---|
| 1 | Yes | = 3.464... | 5.196... | |
| 2 | = 5.464... | 12.928... | ||
| 3 | Yes | = 5.464... | 12.928... | |
| 4 | = 6.928... | 20.784... | 
| |
| 5 | = 7.464... | 24.124... | 
| |
| 6 | Yes | = 7.464... | 24.124... | |
| 7 | = 8.928... | 34.516... | ||
| 8 | = 9.293... | 37.401... | ||
| 9 | = 9.464... | 38.784... | ||
| 10 | Yes | = 9.464... | 38.784... | |
| 11 | = 10.730... | 49.854... | ||
| 12 | = 10.928... | 51.712... | ||
| 13 | = 11.406... | 56.338... | ||
| 14 | = 11.464... | 56.908... | ||
| 15 | Yes | = 11.464... | 56.908... |
A closely related problem is to cover the equilateral triangle with a fixed number of equal circles, having as small a radius as possible.[6]
See also
- Circle packing in an isosceles right triangle
- Malfatti circles, three circles of possibly unequal sizes packed into a triangle
References
- ^ a b Melissen, Hans (1993), "Densest packings of congruent circles in an equilateral triangle", The American Mathematical Monthly, 100 (10): 916–925, doi:10.2307/2324212, JSTOR 2324212, MR 1252928.
- ^ Melissen, J. B. M.; Schuur, P. C. (1995), "Packing 16, 17 or 18 circles in an equilateral triangle", Discrete Mathematics, 145 (1–3): 333–342, doi:10.1016/0012-365X(95)90139-C, MR 1356610.
- ^ Graham, R. L.; Lubachevsky, B. D. (1995), "Dense packings of equal disks in an equilateral triangle: from 22 to 34 and beyond", Electronic Journal of Combinatorics, 2: Article 1, approx. 39 pp. (electronic), MR 1309122.
- ^ Oler, Norman (1961), "A finite packing problem", Canadian Mathematical Bulletin, 4 (2): 153–155, doi:10.4153/CMB-1961-018-7, MR 0133065.
- ^ Payan, Charles (1997), "Empilement de cercles égaux dans un triangle équilatéral. À propos d'une conjecture d'Erdős-Oler", Discrete Mathematics (in French), 165/166: 555–565, doi:10.1016/S0012-365X(96)00201-4, MR 1439300.
- ^ Nurmela, Kari J. (2000), "Conjecturally optimal coverings of an equilateral triangle with up to 36 equal circles", Experimental Mathematics, 9 (2): 241–250, doi:10.1080/10586458.2000.10504649, MR 1780209, S2CID 45127090.