Modern theory of packing spheres began with the two papers (Levenshtein 1979) and
(Odlyzko and Sloane 1979). More recent developments are chronicled in (Conway and
Sloane 1999), (Cohn and Elkies 2003), and (Cohn 2002). The last two are a sequence, which,
like Beethoven’s first two piano concerti, became published in reverse order. These are good
foundational studies for the present œuvre, and are recommended with references within.
We begin with some rudimentary definitions. The n-sphere is the sphere of dimension
n considered embedded in R
n+1
. The 1-sphere is the circle; the 2-sphere is the common
sphere; all others carry the dimension as part of the name. Unless otherwise qualified, all
spheres have radius one.
Think of the circle S1 with six circles around it. One could form six equilateral triangles
— 2-simplexes — joining the center of S1 with the centers of the surrounding circles, joining
those centers to form a hexagon. The insight looking forward is that the interior angles of
these triangles — π/3 each — exactly complete a circle. In this case note that n = 1, and that
arcsec(n + 1) divides 2π six times. The concept of tightness appears, meaning that any circle
touching S1, say T1, also touches two others.
Modern theory of packing spheres began with the two papers (Levenshtein 1979) and
(Odlyzko and Sloane 1979). More recent developments are chronicled in (Conway and
Sloane 1999), (Cohn and Elkies 2003), and (Cohn 2002). The last two are a sequence, which,
like Beethoven’s first two piano concerti, became published in reverse order. These are good
foundational studies for the present œuvre, and are recommended with references within.
We begin with some rudimentary definitions. The n-sphere is the sphere of dimension
n considered embedded in R
n+1
. The 1-sphere is the circle; the 2-sphere is the common
sphere; all others carry the dimension as part of the name. Unless otherwise qualified, all
spheres have radius one.
Think of the circle S1 with six circles around it. One could form six equilateral triangles
— 2-simplexes — joining the center of S1 with the centers of the surrounding circles, joining
those centers to form a hexagon. The insight looking forward is that the interior angles of
these triangles — π/3 each — exactly complete a circle. In this case note that n = 1, and that
arcsec(n + 1) divides 2π six times. The concept of tightness appears, meaning that any circle
touching S1, say T1, also touches two others.
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