1. Packing Circles with Two Different Sized Circles
Given positive integers 1<m≤n, we wish to pack m circles of one size and n circles of another size into a unit circle so that the total area of the circles is maximized. What are the maximal area packings for various m and n? The corresponding problem for squares was the problem of the month in April 2000. What are the best solutions for packing circles in a square or squares in a circle?
2. Covering a Square with Circles of Any Size
The problem of covering a unit square with n circles of equal size has been well studied. You can the best known solutions here. A variant of this problem is to cover a unit square with n circles of any size and minimal combined area. What are the minimal covers for various n? What are the minimal circle covers of an equilateral triangle of length 1?
3. Packing Discrete Circles on a Square Torus
Say we have circles of diameter d, all centered at half lattice points, packed inside a square torus with side n. If d and n are fixed, how many nonoverlapping circles C(n,d) will fit? For each n, C(n,d) is a nonincreasing piecewise constant function of d, which can be illustrated by exhibiting the packings at the discontinuities. For example, for n=5, the discontinuities occur for d=1, √2, √5, 2√2, and 5. For each n, can you find the discontinuities of C(n,d), and the value of C(n,d) at those points? It is clear that C(n,1)=n^{2} and C(n,n)=1 are always discontinuities. What other general formulas are true?
4. Packing Circles of Radii 1, 2, 3, ^{. . .}, n in a Circle.
What is the smallest Circle that contains nonoverlapping circles of radii 1, 2, 3, ^{. . .}, n?
m=2, n=2 area 13π/18 = 2.268+  m=2, n=3 area 2.345+ (David W. Cantrell)  m=2, n=4 area 3π/4 = 2.356+ 
m=2, n=5 area 7π/9 = 2.443+ (David W. Cantrell)  m=2, n=6 area 2.438+ (David W. Cantrell)  m=3, n=3 area 2.502+ 
m=3, n=4 area 2.445+ (David W. Cantrell)  m=3, n=5 area 2.459+  m=3, n=6 area 2.537+ (Philippe Fondanaiche) 
m=3, n=8, 9 area 2.537+, 2.601+ (Philippe Fondanaiche)  m=4, n=4, 5 area = 2.525+, 2.618+  m=4, n=6 area = 2.523+ (David W. Cantrell) 
m=4, n=8, 9 area = 2.541+, 2.590+ (Philippe Fondanaiche)  m=5, n=5, 6, 7 area 2.459+, 2.520+, 2.662+ (David W. Cantrell) 
m=6, n=6 area = 2.531+ (David W. Cantrell)  m=2, 3, 4, 5, 6, n=7 area 2.531+, 2.575+, 2.662+, 2.618+, 2.706+ (David W. Cantrell, Maurizio Morandi) 
2. Here are the best known coverings of a square by circles:
n=1 area π/2 = 1.570+  n=3 area 1.442+  n=4 area 1.397+ 
n=5 area 5π/12 = 1.309+  n=6 area 1.2965+ (David W. Cantrell)  n=7 area 1.2813+ (Philippe Fondanaiche) 
n=8 area 1.2687+ (David W. Cantrell)  n=9 area 1.2558+ (David W. Cantrell)  n=13 area 1.185+ 
And here are the best known coverings of a equilateral triangle by circles:
n=1 area π/3 = 1.047+  n=2 area 7π/24 = .916+ (Hans Melissen)  n=3 area π/4 = .785+ (Hans Melissen) 
n=4 area 5π/24 = .654+ (Hans Melissen)  n=5 area .644+ (David W. Cantrell) 
n=6 area .630+ (David W. Cantrell)  n=7 area .624+ (David W. Cantrell) 
3. Here are the small discontinuities of discrete packings of circles on a torus:
n=1, d=1 1 circle 










Joseph DeVincentis noted that the possible discontinuities are square roots of the sum of two squares of integers, and showed C(n,√2)=n n/2 and C(n, n/2 √2)=2 are always discontinuities for n≥4. He also showed that C(2n,2)=n^{2}, but wasn't sure this was a discontinuity.
4. The Al Zimmerman Programming Contest featured this problem. The best results for small n are shown below. Fixed circles are shown in blue, and rattlers are shown in purple.
1.  2.  3.  
r = 1 Trivial.  r = 3 Trivial.  r = 5 Trivial. 
4.  5.  6.  
r = 7 Trivial.  r = 9.001397+ Found by Klaus Nagel and Hugo Pfoertner in October 2005.  r = 11.057040+ Found by Fred Mellender in October 2005. 
7.  8.  9.  
r = 13.462110+ Found by Gerrit de Blaauw in October 2005.  r = 16.221746+ Found by Gerrit de Blaauw in October 2005.  r = 19.233193+ Found by Gerrit de Blaauw in October 2005. 
10.  11.  12.  
r = 22.000193+ Found by Steve Trevorrow in November 2005.  r = 24.960634+ Found by Gerrit de Blaauw in October 2005.  r = 28.371389+ Found by Steve Trevorrow in November 2005. 
13.  14.  15.  
r = 31.545867+ Found by Steve Trevorrow in November 2005.  r = 35.095647+ Found by Tomas Rokicki in November 2005.  r = 38.837995+ Found by Tomas Rokicki in November 2005. 
16.  17.  18.  
r = 42.458116+ Found by Tomas Rokicki in November 2005.  r = 46.291242+ Found by Tomas Rokicki in November 2005.  r = 50.119762+ Found by Boris von Loesch in November 2005. 
19.  20.  21.  
r = 54.240293+ Found by Tomas Rokicki in November 2005.  r = 58.400567+ Found by Addis Locatelli and Schoen in November 2005.  r = 62.558877+ Found by Boris von Loesch in November 2005. 
22.  23.  
r = 66.760286+ Found by Tomas Rokicki in November 2005.  r = 71.199461+ Found by Boris von Loesch in December 2005. 
If you can extend any of these results, please email me. Click here to go back to Math Magic. Last updated 7/14/07.