# E₈ lattice

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E₈ lattice

In mathematics, the E8 lattice is a special lattice in R8. It can be characterized as the unique positive-definite, even, unimodular lattice of rank 8. The name derives from the fact that it is the root lattice of the E8 root system.

The normIn this article, the "norm" of a vector refers to its length squared (the square of the ordinary norm).] of the E8 lattice (divided by 2) is a positive definite even unimodular quadratic form in 8 variables, and conversely such a quadratic form can be used to construct a positive-definite, even, unimodular lattice of rank 8.The existence of such a form was first shown by H. J. S. Smith in 1867,cite journal | last=Smith | first=H. J. S. | title= On the orders and genera of quadratic forms containing more than three indeterminates | journal=Proceedings of the Royal Society | volume=16 | year=1867 | pages=197–208 | doi= 10.1098/rspl.1867.0036] and the first explicit construction of this quadratic form was given by A. Korkine and G. Zolotareff in 1873 [cite journal | last = Korkine and Zolotareff | title=Sur les formes quadratique positives |journal = Mathematische Annalen | volume = 6 | pages = 366–389 | year = 1877|doi=10.1007/BF01442795] . The E8 lattice is also called the Gosset lattice after Thorold Gosset who was one of the first to study the geometry of the lattice itself around 1900.cite journal | last=Gosset | first=Thorold | title = On the regular and semi-regular figures in space of "n" dimensions | journal = Messenger of Mathematics | volume = 29 | pages = 43–48 | year = 1900]

Lattice points

The E8 lattice is a discrete subgroup of R8 of full rank (i.e. it spans all of R8). It can be given explicitly by the set of points Γ8 &sub; R8 such that
*all the coordinates are integers or all the coordinates are half-integers (a mixture of integers and half-integers is not allowed), and
*the sum of the eight coordinates is an even integer.In symbols,

:$Gamma_8 = left\left\{\left(x_i\right) in mathbb Z^8 cup \left(mathbb Z + frac\left\{1\right\}\left\{2\right\}\right)^8 : \left\{ extstylesum_i\right\} x_i equiv 0;\left(mbox\left\{mod \right\}2\right) ight\right\}.$

It is not hard to check that the sum of two lattice points is another lattice point, so that Γ8 is indeed a subgroup.

An alternative description of the E8 lattice which is sometimes convenient is the set of all points in Γ&prime;8 &sub; R8 such that
*all the coordinates are integers and the sum of the coordinates is even, or
*all the coordinates are half-integers and the sum of the coordinates is odd.In symbols,:$Gamma_8\text{'} = left\left\{\left(x_i\right) in mathbb Z^8 cup \left(mathbb Z + frac\left\{1\right\}\left\{2\right\}\right)^8 : \left\{ extstylesum_i\right\} x_i equiv 2x_1;\left(mbox\left\{mod \right\}2\right) ight\right\}.$

The lattices Γ8 and Γ&prime;8 are isomorphic and one may pass from one to the other by changing the signs of any odd number of coordinates. The lattice Γ8 is sometimes called the "even coordinate system" for E8 while the lattice Γ8' is called the "odd coordinate system". Unless we specify otherwise we shall work in the even coordinate system.

Properties

The E8 lattice Γ8 can be characterized as the unique lattice in R8 with the following properties:
*It is "unimodular", meaning that it can be generated by the columns of a 8&times;8 matrix with determinant &plusmn;1 (i.e. the volume of the fundamental parallelotope of the lattice is 1). Equivalently, Γ8 is "self-dual", meaning it is equal to its dual lattice.
*It is "even", meaning that the norm of any lattice vector is even.Even unimodular lattices can occur only in dimensions divisible by 8. In dimension 16 there are two such lattices: Γ8 &oplus; Γ8 and Γ16 (constructed in an analogous fashion to Γ8). In dimension 24 there are 24 such lattices, called Niemeier lattices. The most important of these is the Leech lattice.

One possible basis for Γ8 is given by the columns of the (upper triangular) matrix:Γ8 is then the integral span of these vectors. All other possible bases are obtained from this one by right multiplication by elements of GL(8,Z).

The shortest nonzero vectors in Γ8 have norm 2. There are 240 such vectors. These form a root system of type E8. The lattice Γ8 is equal to the E8 root lattice, meaning that it is given by the integral span of the 240 roots. Any choice of 8 simple roots gives a basis for Γ8.

ymmetry group

The automorphism group (or symmetry group) of a lattice in R"n" is defined as the subgroup of the orthogonal group O("n") that preserves the lattice. The symmetry group of the E8 lattice is the Weyl/Coxeter group of type E8. This is the group generated by reflections in the hyperplanes orthogonal to the 240 roots of the lattice. Its order is given by:$|W\left(mathrm\left\{E\right\}_8\right)| = 696729600 = 4!cdot 6!cdot 8!$

The E8 Weyl group contains a subgroup of order 128&middot;8! consisting of all permutations of the coordinates and all even sign changes. This subgroup is the Weyl group of type D8. The full E8 Weyl group is generated by this subgroup and the block diagonal matrix "H"4&oplus;"H"4 where "H"4 is the Hadamard matrix:

Geometry

A hole in a lattice is a point in the ambient Euclidean space whose distance to the nearest lattice point is a local maximum. A deep hole is one whose distance to the lattice is a global maximum. There are two types of holes in the E8 lattice:

*"Deep holes" such as the point (1,0,0,0,0,0,0,0) are at a distance of 1 from the nearest lattice points. There are 16 lattice points at this distance which form the vertices of an 8-dimensional orthoplex centered at the hole (the Delaunay cell of the hole).
*"Shallow holes" such as the point $\left( frac\left\{5\right\}\left\{6\right\}, frac\left\{1\right\}\left\{6\right\}, frac\left\{1\right\}\left\{6\right\}, frac\left\{1\right\}\left\{6\right\}, frac\left\{1\right\}\left\{6\right\}, frac\left\{1\right\}\left\{6\right\}, frac\left\{1\right\}\left\{6\right\}, frac\left\{1\right\}\left\{6\right\}\right)$ are at a distance of $frac\left\{2sqrt 2\right\}\left\{3\right\}$ from the nearest lattice points. There are 9 lattice points at this distance forming the vertices of an 8-dimensional simplex centered at the hole.

8-dimensional space can then by tiled with a semiregular honeycomb composed of regular 8-simplices and 8-orthoplexes.

This honeycomb was first studied by Gosset who called it a "9-ic semi-regular figure" (Gosset regarded honeycombs in "n" dimensions as degenerate "n"+1 polytopes). In Coxeter's notation,cite book | first = H. S. M. | last = Coxeter | authorlink = H. S. M. Coxeter | year = 1973 | title = Regular Polytopes | edition = (3rd ed.) | publisher = Dover Publications | location = New York | id = ISBN 0-486-61480-8] Gosset's honeycomb is denoted by 521 and has the Coxeter-Dynkin diagram::

The vertices of the honeycomb are the points of the E8 lattice. This honeycomb is highly regular in the sense that its symmetry group (the affine E8 Weyl group) acts transitively on the "k"-faces for "k" &le; 6. All of the "k"-faces for "k" &le; 7 are simplices.

The vertex figure of Gosset's honeycomb is the semiregular E8 polytope (421 in Coxeter's notation) given by the convex hull of the 240 roots of the E8 lattice.

Each point of the E8 lattice is surrounded by 2160 8-orthoplexes and 17280 8-simplices. The 2160 deep holes near the origin are exactly the halves of the norm 4 lattice points. The 17520 norm 8 lattice points fall into two classes (two orbits under the action of the E8 automorphism group): 240 are twice the norm 2 lattice points while 17280 are 3 times the shallow holes surrounding the origin.

phere packings and kissing numbers

The E8 lattice is remarkable in that it gives solutions to the lattice packing problem and the kissing number problem in 8 dimensions.

The general sphere packing problem asks what is the densest way to pack "n"-dimensional (solid) spheres in R"n" so that no two spheres overlap. Lattice packings are special types of sphere packings where the spheres are centered at the points of a lattice. Placing spheres of radius 1/&radic;2 at the points of the E8 lattice gives a lattice packing in R8 with a density of:$frac\left\{pi^4\right\}\left\{384\right\} cong 0.25367.$It is known that this is the maximum density that can be achieved by a lattice packing in 8 dimensions. [cite journal | first = H. F. | last = Blichfeldt | year = 1935 | title = The minimum values of positive quadratic forms in six, seven and eight variables | journal = Mathematische Zeitschrift | volume = 39 | pages = 1–15 | doi = 10.1007/BF01201341] Furthermore, the E8 lattice is the unique lattice (up to isometries and rescalings) with this density. [cite conference | first = N. M. | last = Vetčinkin | title = Uniqueness of classes of positive quadratic forms on which values of the Hermite constant are attained for 6 &le; "n" &le; 8 | booktitle = Geometry of positive quadratic forms | publisher = Trudy Math. Inst. Steklov | volume = 152 | year = 1980 | pages = 34&ndash;86] It is conjectured the this density is, in fact, optimal (even among irregular packings). Researchers have recently shown that no irregular packing density can exceed that of the E8 lattice by a factor of more than 1 + 10−14. [cite journal | first = Henry | last = Cohn | coauthors = Kumar, Abhinav | year = 2004 | title = Optimality and uniqueness of the Leech lattice among lattices | id = arxiv|math. DG|0403263]

The kissing number problem asks what is the maximum number of spheres of a fixed radius that can touch (or "kiss") a central sphere of the same radius. In the E8 lattice packing mentioned above any given sphere touches 240 neighboring spheres. This is because there are 240 lattice vectors of minimum nonzero norm (the roots of the E8 lattice). It was shown in 1979 that this is the maximum possible number in 8-dimensions. [cite journal | first = V. I. | last = Levenshtein | title = On bounds for packing in "n"-dimensional Euclidean space | journal = Soviet Mathematics Doklady | volume = 20 | year = 1979 | pages = 417–421] [cite journal | first = A. M. | last = Odlyzko | coauthors = Sloane, N. J. A. | title = New bounds on the number of unit spheres that can touch a unit sphere in "n" dimensions | journal = Journal of Combinatorial Theory | volume = A26 | year = 1979 | pages = 210–214 This is also Chapter 13 of Conway and Sloane (1998).]

The kissing number problem is remarkably difficult and solutions are only known in 1, 2, 3, 4, 8, and 24 dimensions. Perhaps surprisingly, it is easier to find the solution in 8 (and 24) dimensions than in 3 or 4. This follows from the special properties of the E8 lattice (and its 24-dimensional cousin, the Leech lattice).

Theta function

One can associate to any (positive-definite) lattice &Lambda; a theta function given by:$Theta_Lambda\left( au\right) = sum_\left\{xinLambda\right\}e^\left\{ipi au|x|^2\right\}qquadmathrm\left\{Im\right\}, au > 0.$The theta function of a lattice is then a holomorphic function on the upper half-plane. 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 $q = e^\left\{ipi au\right\}$ so that the coefficient of "q""n" gives the number of lattice vectors of norm "n".

Up to normalization, there is a unique modular form of weight 4: the Eisenstein series "G"4(&tau;). The theta function for the E8 lattice must then be proportional to "G"4(&tau;). The normalization can be fixed by noting that there is a unique vector of norm 0. This gives:$Theta_\left\{Gamma_8\right\}\left( au\right) = 1 + 240sum_\left\{n=1\right\}^infty sigma_3\left(n\right) q^\left\{2n\right\}$where &sigma;3("n") is the divisor function. It follows that the number of E8 lattice vectors of norm 2"n" is 240 times the sum of the cubes of the divisors of "n". The first few terms of this series are given by OEIS|id=A004009::$Theta_\left\{Gamma_8\right\}\left( au\right) = 1 + 240,q^2 + 2160,q^4 + 6720,q^6 + 17520,q^8 + 30240, q^\left\{10\right\} + 60480,q^\left\{12\right\} + O\left(q^\left\{14\right\}\right).$The E8 theta function may be written in terms of the Jacobi theta functions as follows::$Theta_\left\{Gamma_8\right\}\left( au\right) = frac\left\{1\right\}\left\{2\right\}left\left( heta_2\left(q\right)^8 + heta_3\left(q\right)^8 + heta_4\left(q\right)^8 ight\right)$where:$heta_2\left(q\right) = sum_\left\{n=-infty\right\}^\left\{infty\right\}q^\left\{\left(n+frac\left\{1\right\}\left\{2\right\}\right)^2\right\}qquad heta_3\left(q\right) = sum_\left\{n=-infty\right\}^\left\{infty\right\}q^\left\{n^2\right\}qquad heta_4\left(q\right) = sum_\left\{n=-infty\right\}^\left\{infty\right\}\left(-1\right)^n q^\left\{n^2\right\}.$

Other constructions

Hamming code

The E8 lattice is very closely related to the Hamming code "H"(8,4) and can, in fact, be constructed from it. The Hamming code "H"(8,4) is a binary code of length 8 and rank 4; that is, it is a 4-dimensional subspace of the finite vector space (F2)8. Writing elements of (F2)8 as 8-bit integers in hexadecimal, the code "H"(8,4) can by given explicitly as the set:{00, 0F, 33, 3C, 55, 5A, 66, 69, 96, 99, A5, AA, C3, CC, F0, FF}The code "H"(8,4) is significant partly because it is a Type II self-dual code. It has a minimum Hamming weight 4, meaning that any two codewords differ by at least 4 bits. It is the largest length 8 binary code with this property.

One can construct a lattice &Lambda; from a binary code "C" of length "n" by taking the set of all vectors "x" in Z"n" such that "x" is congruent (modulo 2) to a codeword of "C". [This is the so-called "Construction A" in Conway and Sloane (1998). See &sect;2 of Ch. 5.] It is often convenient to rescale &Lambda; by a factor of 1/&radic;2,

:

Applying this construction a Type II self-dual code gives an even, unimodular lattice. In particular, applying it to the Hamming code "H"(8,4) gives an E8 lattice. It is not entirely trivial, however, to find an explicit isomorphism between this lattice and the lattice Γ8 defined above.

Integral octonions

The E8 lattice is also closely related to the nonassociative algebra of real octonions O. It is possible to define the concept of an integral octonion analogous to that of an integral quaternion. The integral octonions naturally form a lattice inside O. This lattice is just a rescaled E8 lattice. (The minimum norm in the integral octonion lattice is 1 rather than 2). Embedded in the octonions in this manner the E8 lattice takes on the structure of a nonassociative ring.

Fixing a basis (1, "i", "j", "k", ℓ, ℓ"i", ℓ"j", ℓ"k") of unit octonions,one can define the integral octonions as a maximal order containing this basis. (One must, of course, extend the definitions of "order" and "ring" to include the nonassociative case). This amounts to finding the largest subring of O containing the units on which the expressions "x"*"x" (the norm of "x") and "x" + "x"* (twice the real part of "x") are integer-valued. There are actually seven such maximal orders, one corresponding to each of the seven imaginary units. However, all seven maximal orders are isomorphic. One such maximal order is generated by the octonions "i", "j", and ½ ("i" + "j" + "k" + ℓ).

A detailed account of the integral octonions and their relation to the E8 lattice can be found in Conway and Smith (2003).

Applications

In 1982 Michael Freedman produced a bizarre example of a topological 4-manifold, called the E8 manifold, whose intersection form is given by the E8 lattice. This manifold is an example of a topological manifold which admits no smooth structure and is not even triangulable.

In string theory, the heterotic string is a peculiar hybrid of a 26-dimensional bosonic string and a 10-dimensional superstring. In order for the theory to work correctly, the 16 mismatched dimensions must be compactified on an even, unimodular lattice of rank 16. There are two such lattices: Γ8&oplus;Γ8 and Γ16 (constructed in a fashion analogous to that of Γ8). These lead to two version of the heterotic string known as the E8&times;E8 heterotic string and the SO(32) heterotic string.

ee also

*Leech lattice
*E8 (mathematics)
*Semiregular E-polytope

Notes

References

*cite book | first = John H. | last = Conway | authorlink = John Horton Conway | coauthors = Sloane, Neil J. A. | year = 1998 | title = Sphere Packings, Lattices and Groups | edition = (3rd ed.) | publisher = Springer-Verlag | location = New York | id = ISBN 0-387-98585-9
*cite book | first = John H. | last = Conway | coauthors = Smith, Derek A. | title = On Quaternions and Octonions | publisher = AK Peters, Ltd | location = Natick, Massachusetts | year = 2003 | isbn = 1-56881-134-9 Chapter 9 contains a discussion of the integral octonions and the E8 lattice.

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