kissing number

In one dimension,Note that in one dimension, "spheres" are just pairs of points separated by the unit distance. (The vertical dimension of one-dimensional illustration is merely evocative.) Unlike in higher dimensions, it is necessary to specify that the interior of the spheres (the unit-length open intervals) do not overlap in order for there to be a finite packing density. the kissing number is 2:

center

In two dimensions, the kissing number is 6:

center

Proof: Consider a circle with center C that is touched by circles with centers C₁, C₂, .... Consider the rays C C_i. These rays all emanate from the same center C, so the sum of angles between adjacent rays is 360°.

Assume by contradiction that there are more than six touching circles. Then at least two adjacent rays, say C C₁ and C C₂, are separated by an angle of less than 60°. The segments C C_i have the same length – 2r – for all i. Therefore, the triangle C C₁ C₂ is isosceles, and its third side – C₁ C₂ – has a side length of less than 2r. Therefore, the circles 1 and 2 intersect – a contradiction.See also Lemma 3.1 in {{Cite journal | last1 = Marathe | first1 = M. V. | last2 = Breu | first2 = H. | last3 = Hunt | first3 = H. B. | last4 = Ravi | first4 = S. S. | last5 = Rosenkrantz | first5 = D. J. | title = Simple heuristics for unit disk graphs | doi = 10.1002/net.3230250205 | journal = Networks | volume = 25 | issue = 2 | pages = 59 | year = 1995 | arxiv = math/9409226 }}

File:Kissing-3d.png. This leaves slightly more than 0.1 of the radius between two nearby spheres.]]

In three dimensions, the kissing number is 12, but the correct value was much more difficult to establish than in dimensions one and two. It is easy to arrange 12 spheres so that each touches a central sphere, with a lot of space left over, and it is not obvious that there is no way to pack in a 13th sphere. (In fact, there is so much extra space that any two of the 12 outer spheres can exchange places through a continuous movement without any of the outer spheres losing contact with the center one.) This was the subject of a famous disagreement between mathematicians Isaac Newton and David Gregory. Newton correctly thought that the limit was 12; Gregory thought that a 13th could fit. Some incomplete proofs that Newton was correct were offered in the nineteenth century, most notably one by Reinhold Hoppe, but the first correct proof (according to Brass, Moser, and Pach) did not appear until 1953.{{cite book |first=John H. |last=Conway |author-link=John Horton Conway |author2=Neil J.A. Sloane |author-link2=Neil Sloane |year=1999 |title=Sphere Packings, Lattices and Groups |edition=3rd |publisher=Springer-Verlag |location=New York |isbn=0-387-98585-9|page=[https://books.google.com/books?id=upYwZ6cQumoC&pg=PA21 21]}}{{cite book |first1=Peter |last1=Brass |first2=W. O. J. |last2=Moser |first3=János |last3=Pach |author-link3=János Pach |title=Research problems in discrete geometry |publisher=Springer |year=2005 |isbn=978-0-387-23815-9 |page=[https://books.google.com/books?hl=en&id=cT7TB20y3A8C&pg=PA93 93]}}{{cite book

| last = Zong | first = Chuanming

| editor1-last = Goodman | editor1-first = Jacob E.

| editor2-last = Pach | editor2-first = János

| editor3-last = Pollack | editor3-first = Richard

| contribution = The kissing number, blocking number and covering number of a convex body

| doi = 10.1090/conm/453/08812

| location = Providence, RI

| mr = 2405694

| pages = 529–548

| publisher = American Mathematical Society

| series = Contemporary Mathematics

| title = Surveys on Discrete and Computational Geometry: Twenty Years Later (AMS-IMS-SIAM Joint Summer Research Conference, June 18ÔÇô22, 2006, Snowbird, Utah)

| volume = 453

| year = 2008| isbn = 9780821842393

}}.

The twelve neighbors of the central sphere correspond to the maximum bulk coordination number of an atom in a crystal lattice in which all atoms have the same size (as in a chemical element). A coordination number of 12 is found in a cubic close-packed or a hexagonal close-packed structure.

In four dimensions, the kissing number is 24. This was proven in 2003 by Oleg Musin.{{cite journal |author=O. R. Musin |title=The problem of the twenty-five spheres |year=2003 |journal=Russ. Math. Surv. |volume=58 |pages=794–795 |doi=10.1070/RM2003v058n04ABEH000651 |issue=4|bibcode=2003RuMaS..58..794M |s2cid=250839515 }}{{Cite journal|last1=Pfender|first1=Florian|last2=Ziegler|first2=Günter M.|author-link2=Günter M. Ziegler|title=Kissing numbers, sphere packings, and some unexpected proofs|journal=Notices of the American Mathematical Society|date=September 2004|pages=873–883|url=https://www.ams.org/notices/200408/fea-pfender.pdf}}. Previously, the answer was thought to be either 24 or 25: it is straightforward to produce a packing of 24 spheres around a central sphere (one can place the spheres at the vertices of a suitably scaled 24-cell centered at the origin), but, as in the three-dimensional case, there is a lot of space left over — even more, in fact, than for n = 3 — so the situation was even less clear.

The existence of the highly symmetrical E₈ lattice and Leech lattice has allowed known results for n = 8 (where the kissing number is 240), and n = 24 (where it is 196,560).{{cite journal| last=Levenshtein | first=Vladimir I. | author-link=Vladimir Levenshtein | year=1979 | title=О границах для упаковок в n-мерном евклидовом пространстве |trans-title=On bounds for packings in n-dimensional Euclidean space | journal=Doklady Akademii Nauk SSSR | volume=245 | issue=6 | language=ru | pages=1299–1303}}{{cite journal

| last1=Odlyzko | first1=A. M. | authorlink1=Andrew Odlyzko

| last2=Sloane | first2=N. J. A. | authorlink2=N.J.A. Sloane

| title=New bounds on the number of unit spheres that can touch a unit sphere in n dimensions

| journal=Journal of Combinatorial Theory | series=Series A

| volume=26

| issue=2

| date=1979

| pages=210–214

| doi=10.1016/0097-3165(79)90074-8 | doi-access=free}}

The kissing number in n dimensions is unknown for other dimensions.

If arrangements are restricted to lattice arrangements, in which the centres of the spheres all lie on points in a lattice, then this restricted kissing number is known for n = 1 to 9 and n = 24 dimensions.{{MathWorld | urlname=KissingNumber |title=Kissing Number}} For 5, 6, and 7 dimensions the arrangement with the highest known kissing number found so far is the optimal lattice arrangement, but the existence of a non-lattice arrangement with a higher kissing number has not been excluded.

The following table lists some known bounds on the kissing number in various dimensions.{{cite journal|last1=Machado|first1=Fabrício C.|last2=Oliveira|first2=Fernando M.|title=Improving the Semidefinite Programming Bound for the Kissing Number by Exploiting Polynomial Symmetry|year=2018|pages=362–369|volume=27|journal=Experimental Mathematics|issue=3|arxiv=1609.05167|doi=10.1080/10586458.2017.1286273|s2cid=52903026}} The dimensions in which the kissing number is known are listed in boldface.

File:Kissing growth.svg in n. The base of exponential growth is not known. The grey area in the above plot represents the possible values between known upper and lower bounds. Circles represent values that are known exactly.]]

The kissing number problem can be generalized to the problem of finding the maximum number of non-overlapping congruent copies of any convex body that touch a given copy of the body. There are different versions of the problem depending on whether the copies are only required to be congruent to the original body, translates of the original body, or translated by a lattice. For the regular tetrahedron, for example, it is known that both the lattice kissing number and the translative kissing number are equal to 18, whereas the congruent kissing number is at least 56.{{Cite journal|last1=Lagarias|first1=Jeffrey C.|last2=Zong|first2=Chuanming|title=Mysteries in packing regular tetrahedra|journal=Notices of the American Mathematical Society|date=December 2012|pages=1540–1549|url=https://www.ams.org/notices/201211/rtx121101540p.pdf}}

There are several approximation algorithms on intersection graphs where the approximation ratio depends on the kissing number.{{Cite journal|last1=Kammer|first1=Frank|last2=Tholey|first2=Torsten|title=Approximation Algorithms for Intersection Graphs|journal=Algorithmica |volume=68|issue=2|date=July 2012|pages=312–336|doi=10.1007/s00453-012-9671-1|s2cid=3065780|url=https://opus.bibliothek.uni-augsburg.de/opus4/frontdoor/index/index/docId/1303 }} For example, there is

a polynomial-time 10-approximation algorithm to find a maximum non-intersecting subset of a set of rotated unit squares.

The kissing number problem can be stated as the existence of a solution to a set of inequalities. Let $x\_n$ be a set of N D-dimensional position vectors of the centres of the spheres. The condition that this set of spheres can lie round the centre sphere without overlapping is:Numbers m and n run from 1 to N. $x\; =\; (x\_n)\_N$ is the sequence of the N positional vectors. As the condition behind the second universal quantifier ($\backslash forall$) does not change if m and n are exchanged, it is sufficient to let this quantor extend just over

$$\backslash exist\; x\backslash \; \backslash left\backslash \{\; \backslash forall\_n\; \backslash \{x\_n^\backslash textsf\{T\}\; x\_n\; =\; 1\backslash \}\; \backslash land\; \backslash forall\_\{m,n:\; m\; \backslash neq\; n\}\; \backslash \{\; (x\_n\; -\; x\_m)^\backslash textsf\{T\}(x\_n\; -\; x\_m)\; \backslash geq\; 1\backslash \}\; \backslash right\backslash \}$$

Thus the problem for each dimension can be expressed in the existential theory of the reals. However, general methods of solving problems in this form take at least exponential time which is why this problem has only been solved up to four dimensions. By adding additional variables, $y\_\{nm\}$ this can be converted to a single quartic equation in N(N − 1)/2 + DN variables:Concerning the matrix $y\; =\; (y\_\{mn\})\_\{N\backslash times\{N\}\}$ only the entries having m < n are needed. Or, equivalent, the matrix can be assumed to be antisymmetric. Anyway the matrix has justN(N − 1)/2 free scalar variables. In addition, there are N D-dimensional vectors x_n, which correspondent to a matrix $x\; =\; (x\_\{nd\})\_\{N\; \backslash times\; D\}$ of N column vectors.

Therefore, to solve the case in D = 5 dimensions and N = 40 + 1 vectors would be equivalent to determining the existence of real solutions to a quartic polynomial in 1025 variables. For the D = 24 dimensions and N = 196560 + 1, the quartic would have 19,322,732,544 variables. An alternative statement in terms of distance geometry is given by the distances squared $R\_\{mn\}$ between the m^th and n^th sphere:

This must be supplemented with the condition that the Cayley–Menger determinant is zero for any set of points which forms a (D + 1) simplex in D dimensions, since that volume must be zero. Setting $R\_\{mn\}\; =\; 1\; +\; \{y\_\{mn\}\}^2$ gives a set of simultaneous polynomial equations in just y which must be solved for real values only. The two methods, being entirely equivalent, have various different uses. For example, in the second case one can randomly alter the values of the y by small amounts to try to minimise the polynomial in terms of the y.

• Equilateral dimension
• Spherical code
• Soddy's hexlet
• Cylinder sphere packing

{{reflist}}

• {{cite arXiv |eprint=2411.04916 |last1=Cohn |first1=Henry |last2=Li |first2=Anqi |title=Improved kissing numbers in seventeen through twenty-one dimensions |date=2024 |class=math.MG }}
• T. Aste and D. Weaire The Pursuit of Perfect Packing (Institute Of Physics Publishing London 2000) {{isbn|0-7503-0648-3}}
• [https://www.math.rwth-aachen.de/~Gabriele.Nebe/LATTICES/kiss.html Table of the Highest Kissing Numbers Presently Known] maintained by Gabriele Nebe and Neil Sloane (lower bounds)
• {{cite journal

| last1 = Bachoc | first1 = Christine | author1-link = Christine Bachoc

| last2 = Vallentin | first2 = Frank

| arxiv = math.MG/0608426

| doi = 10.1090/S0894-0347-07-00589-9

| issue = 3

| journal = Journal of the American Mathematical Society

| mr = 2393433

| pages = 909–924

| title = New upper bounds for kissing numbers from semidefinite programming

| volume = 21

| year = 2008| bibcode = 2008JAMS...21..909B | s2cid = 204096 }}

• {{cite web |last1=Grime |first1=James |title=Kissing Numbers |url=https://www.youtube.com/watch?v=LZ7X_YOfJqY |archive-url=https://ghostarchive.org/varchive/youtube/20211212/LZ7X_YOfJqY| archive-date=2021-12-12 |url-status=live|website=youtube |date=10 October 2018 |publisher=Brady Haran |access-date=11 October 2018 |format=video}}{{cbignore}}

{{Isaac Newton}}

{{Packing problem}}

Category:Discrete geometry

Category:Packing problems