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 norm of the E8 lattice 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, and the first explicit construction of this quadratic form was given by A. Korkin and G. Zolotarev in 1873.
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.

Lattice points

The E8 lattice is a discrete subgroup of R8 of full rank. It can be given explicitly by the set of points Γ8 ⊂ R8 such that
In symbols,
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′8 ⊂ R8 such that
In symbols,
The lattices Γ8 and Γ′8 are isomorphic and one may pass from one to the other by changing the signs of any odd number of half-integer 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:
Even unimodular lattices can occur only in dimensions divisible by 8. In dimension 16 there are two such lattices: Γ8 ⊕ Γ8 and Γ16. 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 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.
The shortest nonzero vectors in Γ8 have norm squared 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.

Symmetry group

The automorphism group of a lattice in Rn is defined as the subgroup of the orthogonal group O 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
The E8 Weyl group contains a subgroup of order 128·8! 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 H4⊕H4 where H4 is the Hadamard matrix

Geometry

The E8 lattice points are the vertices of the 521 honeycomb, which is composed of regular 8-simplex and 8-orthoplex facets. This honeycomb was first studied by Gosset who called it a 9-ic semi-regular figure. In Coxeter's notation, Gosset's honeycomb is denoted by 521 and has the Coxeter-Dynkin diagram:
This honeycomb is highly regular in the sense that its symmetry group acts transitively on the k-faces for k ≤ 6. All of the k-faces for k ≤ 7 are simplices.
The vertex figure of Gosset's honeycomb is the semiregular E8 polytope 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 : 240 are twice the norm 2 lattice points while 17280 are 3 times the shallow holes surrounding the origin.
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:
The E8 lattice is remarkable in that it gives optimal solutions to the sphere packing problem and the kissing number problem in 8 dimensions.
The sphere packing problem asks what is the densest way to pack n-dimensional spheres of a fixed radius in Rn 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/ at the points of the E8 lattice gives a lattice packing in R8 with a density of
It has long been known that this is the maximum density that can be achieved by a lattice packing in 8 dimensions. Furthermore, the E8 lattice is the unique lattice with this density. The mathematician Maryna Viazovska proved in 2016 that this density is, in fact, optimal even among irregular packings.
The kissing number problem asks what is the maximum number of spheres of a fixed radius that can touch 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. It was shown in 1979 that this is the maximum possible number in 8 dimensions.
The sphere packing problem and the kissing number problem are remarkably difficult and optimal solutions are only known in 1, 2, 3, 8, and 24 dimensions. The fact that solutions are known in dimensions 8 and 24 follows in part from the special properties of the E8 lattice and its 24-dimensional cousin, the Leech lattice.

Theta function

One can associate to any lattice Λ a theta function given by
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 so that the coefficient of qn gives the number of lattice vectors of norm n.
Up to normalization, there is a unique modular form of weight 4: the Eisenstein series G4. The theta function for the E8 lattice must then be proportional to G4. The normalization can be fixed by noting that there is a unique vector of norm 0. This gives
where σ3 is the divisor function. It follows that the number of E8 lattice vectors of norm 2n is 240 times the sum of the cubes of the divisors of n. The first few terms of this series are given by :
The E8 theta function may be written in terms of the Jacobi theta functions as follows:
where

Other constructions

Hamming code

The E8 lattice is very closely related to the Hamming code H and can, in fact, be constructed from it. The Hamming code H is a binary code of length 8 and rank 4; that is, it is a 4-dimensional subspace of the finite vector space 8. Writing elements of 8 as 8-bit integers in hexadecimal, the code H can by given explicitly as the set
The code H 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 Λ from a binary code C of length n by taking the set of all vectors x in Zn such that x is congruent to a codeword of C. It is often convenient to rescale Λ by a factor of 1/,
Applying this construction a Type II self-dual code gives an even, unimodular lattice. In particular, applying it to the Hamming code H 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.. Embedded in the octonions in this manner the E8 lattice takes on the structure of a nonassociative ring.
Fixing a basis of unit octonions,
one can define the integral octonions as a maximal order containing this basis.. This amounts to finding the largest subring of O containing the units on which the expressions x*x and x + 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.
A detailed account of the integral octonions and their relation to the E8 lattice can be found in Conway and Smith.

Example definition of integral octonions

Consider octonion multiplication defined by triads: 137, 267, 457, 125, 243, 416, 356. Then integral octonions form vectors:
1), i=0, 1,..., 7
2), indexes abc run through the seven triads 124, 235, 346, 457, 561, 672, 713
3), indexes pqrs run through the seven tetrads 3567, 1467, 1257, 1236, 2347, 1345, 2456.
Imaginary octonions in this set, namely 14 from 1) and 7*16=112 from 3), form the roots of the Lie algebra. Along with the remaining 2+112 vectors we obtain 240 vectors that form roots of Lie algebra. See the Koca work on this subject.

Applications

In 1982 Michael Freedman produced an 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⊕Γ8 and Γ16. These lead to two version of the heterotic string known as the E8×E8 heterotic string and the SO heterotic string.