# Circle packing in a circle

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Circle packing in a circle is a two-dimensional packing problem with the objective of packing unit circles into the smallest possible larger circle.

Minimum solutions (in case several minimal solutions have been shown to exist, only one variant appears in the table):[1]

Number of unit circles Enclosing >circle radius Density Optimality Diagram
1 1 1.0000 Trivially optimal.
2 2 0.5000 Trivially optimal.
3 $1+\frac{2}{3} \sqrt{3}$ ≈ 2.154... 0.6466... Trivially optimal.
4 $1+\sqrt{2}$ ≈ 2.414... 0.6864... Trivially optimal.
5 $1+\sqrt{2(1+\frac{1}{\sqrt{5}})}$ ≈ 2.701... 0.6854... Trivially optimal. Also proved optimal by Graham in 1968.[2]
6 3 0.6667... Trivially optimal. Also proved optimal by Graham in 1968.[2]
7 3 0.7778... Trivially optimal.
8 $1+\frac{1}{\sin(\frac{\pi}{7})}$ ≈ 3.304... 0.7328... Proved optimal by Pirl in 1969.[3]
9 $1+\sqrt{2(2+\sqrt{2})}$ ≈ 3.613... 0.6895... Proved optimal by Pirl in 1969.[3]
10 3.813... 0.6878... Proved optimal by Pirl in 1969.[3]
11 $1+\frac{1}{\sin(\frac{\pi}{9})}$ ≈ 3.923... 0.7148... Proved optimal by Melissen in 1994.[4]
12 4.029... 0.7392... Proved optimal by Fodor in 2000.[5]
13 $2 + \sqrt{5}$ ≈4.236... 0.7245... Proved optimal by Fodor in 2003.[6]
14 4.328... 0.7474... Conjectured optimal.[7]
15 $1 + \sqrt{6 + \frac{2}{\sqrt{5}} + 4 \sqrt{1 +\frac{2}{\sqrt{5}}}}$ ≈ 4.521... 0.7339... Conjectured optimal.[7]
16 4.615... 0.7512... Conjectured optimal.[7]
17 4.792... 0.7403... Conjectured optimal.[7]
18 $1+\sqrt{2}+\sqrt{6}$ ≈ 4.863... 0.7611... Conjectured optimal.[7]
19 $1+\sqrt{2}+\sqrt{6}$ ≈ 4.863... 0.8034... Proved optimal by Fodor in 1999.[8]
20 5.122... 0.7623... Conjectured optimal.[7]

Similar data is available at the web site Packomania for up to 2600 circles.

## References

1. ^ Erich Friedman, Circles in Circles on Erich's Packing Center
2. ^ a b R.L. Graham, Sets of points with given minimum separation (Solution to Problem El921), Amer. Math. Monthly 75 (1968) 192-193.
3. ^ a b c U. Pirl, Der Mindestabstand von n in der Einheitskreisscheibe gelegenen Punkten, Mathematische Nachrichten 40 (1969) 111-124.
4. ^ H. Melissen, Densest packing of eleven congruent circles in a circle, Geometriae Dedicata 50 (1994) 15-25.
5. ^ F. Fodor, The Densest Packing of 12 Congruent Circles in a Circle, Beiträge zur Algebra und Geometrie, Contributions to Algebra and Geometry 41 (2000) ?, 401–409.
6. ^ F. Fodor, The Densest Packing of 13 Congruent Circles in a Circle, Beiträge zur Algebra und Geometrie, Contributions to Algebra and Geometry 44 (2003) 2, 431–440.
7. Graham RL, Lubachevsky BD, Nurmela KJ,Ostergard PRJ. Dense packings of congruent circles in a circle. Discrete Math 1998;181:139–154.
8. ^ F. Fodor, The Densest Packing of 19 Congruent Circles in a Circle, Geom. Dedicata 74 (1999), 139–145.