 Leech lattice

In mathematics, the Leech lattice is an even unimodular lattice Λ_{24} in 24dimensional Euclidean space E^{24} found by John Leech (1967).
Contents
History
Many of the crosssections of the Leech lattice, including the Coxeter–Todd lattice and Barnes–Wall lattice, in 12 and 16 dimensions, were found much earlier than the Leech lattice. O'Connor & Pall (1944) discovered a related odd unimodular lattice in 24 dimensions, now called the odd Leech lattice, whose even sublattice has index 2 in the Leech lattice. The Leech lattice was discovered in 1965 by John Leech (1967, 2.31, p. 262), by improving some earlier sphere packings he found in (Leech 1964).
Conway (1968) calculated the order of the automorphism group of the Leech lattice, and discovered three new sporadic groups as a byproduct: the Conway groups, Co_{1}, Co_{2}, Co_{3}.
Bei dem Versuch, eine form aus dem einer solchen Klassen wirklich anzugeben, fand ich mehr als 10 verscheidene Klassen in Γ_{24}
Witt (1941, p.324)Witt (1941, p.324), has a single rather cryptic sentence mentioning that he found more than 10 even unimodular lattices in 24 dimensions without giving further details. In a seminar in 1970 Ernst Witt claimed that one of the lattices he found in 1940 was the Leech lattice. See his collected works (Witt 1998, p. 328–329) for more comments and for some notes Witt wrote about this in 1972.
Characterization
The Leech lattice Λ_{24} is the unique lattice in E^{24} with the following list of properties:
 It is unimodular; i.e., it can be generated by the columns of a certain 24×24 matrix with determinant 1.
 It is even; i.e., the square of the length of any vector in Λ_{24} is an even integer.
 The length of any nonzero vector in Λ_{24} is at least 2.
Properties
The last condition is equivalent to the condition that unit balls centered at the points of Λ_{24} do not overlap. Each is tangent to 196,560 neighbors, and this is known to be the largest number of nonoverlapping 24dimensional unit balls that can simultaneously touch a single unit ball (compare with 6 in dimension 2, as the maximum number of pennies which can touch a central penny; see kissing number). This arrangement of 196560 unit balls centred about another unit ball is so efficient that there is no room to move any of the balls; this configuration, together with its mirrorimage, is the only 24dimensional arrangement where 196560 unit balls simultaneously touch another. This property is also true in 1, 2 and 8 dimensions, with 2, 6 and 240 unit balls, respectively, based on the integer lattice, hexagonal tiling and E8 lattice, respectively.
It has no root system and in fact is the first unimodular lattice with no roots (vectors of norm less than 4), and therefore has a centre density of 1. By multiplying this value by the volume of a unit ball in 24 dimensions, , one can derive its absolute density.
Conway (1983) showed that the Leech lattice is isometric to the set of simple roots (or the Dynkin diagram) of the reflection group of the 26dimensional even Lorentzian unimodular lattice II_{25,1}. By comparison, the Dynkin diagrams of II_{9,1} and II_{17,1} are finite.
Constructions
The Leech lattice can be constructed in a variety of ways. As with all lattices, it can be constructed via its generator matrix, a 24×24 matrix with determinant 1.
Using the binary Golay code
The Leech lattice can be explicitly constructed as the set of vectors of the form 2^{−3/2}(a_{1}, a_{2}, ..., a_{24}) where the a_{i} are integers such that
and for each fixed residue class modulo 4, the 24 bit word, whose 1's correspond to the coordinates i such that a_{i} belongs to this residue class, is a word in the binary Golay code. The Golay code, together with the related Witt Design, features in a construction for the 196560 minimal vectors in the Leech lattice.
Using the Lorentzian lattice II_{25,1}
The Leech lattice can also be constructed as where w is the Weyl vector:
in the 26dimensional even Lorentzian unimodular lattice II_{25,1}. The existence of such an integral vector of norm zero relies on the fact that 1^{2} + 2^{2} + ... + 24^{2} is a perfect square (in fact 70^{2}); the number 24 is the only integer bigger than 1 with this property. This was conjectured by Édouard Lucas, but the proof came much later, based on elliptic functions.
The vector in this construction is really the Weyl vector of the even sublattice D_{24} of the odd unimodular lattice I^{25}. More generally, if L is any positive definite unimodular lattice of dimension 25 with at least 4 vectors of norm 1, then the Weyl vector of its norm 2 roots has integral length, and there is a similar construction of the Leech lattice using L and this Weyl vector.
Based on other lattices
Conway & Sloane (1982) described another 23 constructions for the Leech lattice, each based on a Niemeier lattice. It can also be constructed by using three copies of the E8 lattice, in the same way that the binary Golay code can be constructed using three copies of the extended Hamming code, H_{8}. This construction is known as the Turyn construction of the Leech lattice.
As a laminated lattice
Starting with a single point, Λ_{0}, one can stack copies of the lattice Λ_{n} to form an (n + 1)dimensional lattice, Λ_{n+1}, without reducing the minimal distance between points. Λ_{1} corresponds to the integer lattice, Λ_{2} is to the hexagonal lattice, and Λ_{3} is the facecentered cubic packing. Conway & Sloane (1982b) showed that the Leech lattice is the unique laminated lattice in 24 dimensions.
As a complex lattice
The Leech lattice is also a 12dimensional lattice over the Eisenstein integers. This is known as the complex Leech lattice, and is isomorphic to the 24dimensional real Leech lattice. In the complex construction of the Leech lattice, the binary Golay code is replaced with the ternary Golay code, and the Mathieu group M_{24} is replaced with the Mathieu group M_{12}. The E_{6} lattice, E_{8} lattice and Coxeter–Todd lattice also have constructions as complex lattices, over either the Eisenstein or Gaussian integers.
Using the icosian ring
The Leech lattice can also be constructed using the ring of icosians. The icosian ring is abstractly isomorphic to the E8 lattice, three copies of which can be used to construct the Leech lattice using the Turyn construction.
Symmetries
The Leech lattice is highly symmetrical. Its automorphism group is the double cover of the Conway group Co_{1}; its order is 8 315 553 613 086 720 000. Many other sporadic simple groups, such as the remaining Conway groups and Mathieu groups, can be constructed as the stabilizers of various configurations of vectors in the Leech lattice.
Despite having such a high rotational symmetry group, the Leech lattice does not possess any lines of reflection symmetry. In other words, the Leech lattice is chiral.
The automorphism group was first described by John Conway. The 398034000 vectors of norm 8 fall into 8292375 'crosses' of 48 vectors. Each cross contains 24 mutually orthogonal vectors and their inverses, and thus describe the vertices of a 24dimensional octahedron. Each of these crosses can be taken to be the coordinate system of the lattice, and has the same symmetry of the Golay code, namely 2^{12} × M_{24}. Hence the full automorphism group of the Leech lattice has order 8292375 × 4096 × 244823040, or 8 315 553 613 086 720 000.
Geometry
Conway, Parker & Sloane (1982) showed that the covering radius of the Leech lattice is ; in other words, if we put a closed ball of this radius around each lattice point, then these just cover Euclidean space. The points at distance at least from all lattice points are called the deep holes of the Leech lattice. There are 23 orbits of them under the automorphism group of the Leech lattice, and these orbits correspond to the 23 Niemeier lattices other than the Leech lattice: the set of vertices of deep hole is isometric to the affine Dynkin diagram of the corresponding Niemeier lattice.
The Leech lattice has a density of , correct to six decimal places. Cohn and Kumar showed that it gives the densest lattice packing of balls in 24dimensional space. Their results suggest, but do not prove, that this configuration also gives the densest among all packings of balls in 24dimensional space. No arrangement of 24dimensional spheres can be denser than the Leech lattice by a factor of more than 1.65×10^{−30}, and it is highly probable that the Leech lattice is indeed optimal.
The 196560 minimal vectors are of three different varieties, known as shapes:
 1104 vectors of shape (4^{2},0^{22}), for all permutations and sign choices;
 97152 vectors of shape (2^{8},0^{16}), where the '2's correspond to octads in the Golay code, and there an even number of minus signs;
 98304 vectors of shape (3,1^{23}), where the signs come from the Golay code, and the '3' can appear in any position.
The ternary Golay code, binary Golay code and Leech lattice give very efficient 24dimensional spherical codes of 729, 4096 and 196560 points, respectively. Spherical codes are higherdimensional analogues of Tammes problem, which arose as an attempt to explain the distribution of pores on pollen grains. These are distributed as to maximise the minimal angle between them. In two dimensions, the problem is trivial, but in three dimensions and higher it is not. An example of a spherical code in three dimensions is the set of the 12 vertices of the regular icosahedron.
Theta series
One can associate to any (positivedefinite) lattice Λ a theta function given by
The theta function of a lattice is then a holomorphic function on the upper halfplane. Furthermore, the theta function of an even unimodular lattice of rank n is actually a modular form of weight n/2. The theta function of an integral lattice is often written as a power series in q = e^{2iπτ} so that the coefficient of q^{n} gives the number of lattice vectors of norm 2n. In the Leech lattice, there are 196560 vectors of norm 4, 16773120 vectors of norm 6, 398034000 vectors of norm 8 and so on. The theta series of the Leech lattice is thus:
where τ(n) represents the Ramanujan tau function, and σ_{11}(n) is the divisor function. It follows that the number of vectors of norm 2m is
Applications
The vertex algebra of the conformal field theory describing bosonic string theory, compactified on the 24dimensional quotient torus R^{24}/Λ_{24} and orbifolded by a twoelement reflection group, provides an explicit construction of the Griess algebra that has the monster group as its automorphism group. This monster vertex algebra was also used to prove the monstrous moonshine conjectures.
The binary Golay code, independently developed in 1949, is an application in coding theory. More specifically, it is an errorcorrecting code capable of correcting up to three errors in each 24bit word, and detecting a fourth. It was used to communicate with the Voyager probes, as it is much more compact than the previouslyused Hadamard code.
Quantizers, or analogtodigital converters, can use lattices to minimise the average rootmeansquare error. Most quantizers are based on the onedimensional integer lattice, but using multidimensional lattices reduces the RMS error. The Leech lattice is a good solution to this problem, as the Voronoi cells have a low second moment.
See also
References
 Conway, John Horton (1968), "A perfect group of order 8,315,553,613,086,720,000 and the sporadic simple groups", Proceedings of the National Academy of Sciences of the United States of America 61 (2): 398–400, doi:10.1073/pnas.61.2.398, MR0237634
 Conway, John Horton (1983), "The automorphism group of the 26dimensional even unimodular Lorentzian lattice", Journal of Algebra 80 (1): 159–163, doi:10.1016/00218693(83)90025X, ISSN 00218693, MR690711
 Conway, John Horton; Sloane, N. J. A. (1982b), "Laminated lattices", Annals of Mathematics. Second Series 116 (3): 593–620, doi:10.2307/2007025, ISSN 0003486X, MR678483
 Conway, John Horton; Parker, R. A.; Sloane, N. J. A. (1982), "The covering radius of the Leech lattice", Proceedings of the Royal Society. London. Series A. Mathematical and Physical Sciences 380 (1779): 261–290, doi:10.1098/rspa.1982.0042, ISSN 00804630, MR660415
 Conway, John Horton; Sloane, N. J. A. (1982), "Twentythree constructions for the Leech lattice", Proceedings of the Royal Society. London. Series A. Mathematical and Physical Sciences 381 (1781): 275–283, doi:10.1098/rspa.1982.0071, ISSN 00804630, MR661720
 Conway, J. H.; Sloane, N. J. A. (1999). Sphere packings, lattices and groups. (3rd ed.) With additional contributions by E. Bannai, R. E. Borcherds, John Leech, Simon P. Norton, A. M. Odlyzko, Richard A. Parker, L. Queen and B. B. Venkov. Grundlehren der Mathematischen Wissenschaften, 290. New York: SpringerVerlag. ISBN 0387985859.
 Leech, John (1964), "Some sphere packings in higher space", Canadian Journal of Mathematics 16: 657–682, doi:10.4153/CJM19640651, ISSN 0008414X, MR0167901, http://cms.math.ca/10.4153/CJM19640651
 Leech, John (1967), "Notes on sphere packings", Canadian Journal of Mathematics 19: 251–267, doi:10.4153/CJM19670170, ISSN 0008414X, MR0209983, http://cms.math.ca/10.4153/CJM19670170
 O'Connor, R. E.; Pall, G. (1944), "The construction of integral quadratic forms of determinant 1", Duke Mathematical Journal 11: 319–331, doi:10.1215/S0012709444011270, ISSN 00127094, MR0010153
 Thompson, Thomas M.: "From Error Correcting Codes through Sphere Packings to Simple Groups", Carus Mathematical Monographs, Mathematical Association of America, 1983.
 Witt, Ernst (1941), "Eine Identität zwischen Modulformen zweiten Grades", Abhandlungen aus dem Mathematischen Seminar der Universität Hamburg 14: 323–337, doi:10.1007/BF02940750, MR0005508
 Witt, Ernst (1998), Collected papers. Gesammelte Abhandlungen, Berlin, New York: SpringerVerlag, ISBN 9783540570615, MR1643949
 Griess, Robert L.: Twelve Sporadic Groups, SpringerVerlag, 1998.
 Marcus du Sautoy: Finding Moonshine. ISBN 9780007214624.
Categories: Quadratic forms
 Lattice points
 Sporadic groups
 Moonshine theory
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