The Leech lattice, the 24-dimensional even unimodular lattice with no roots.
Properties
A lattice is unimodular if and only if its dual lattice is integral. Unimodular lattices are equal to their dual lattices, and for this reason, unimodular lattices are also known as self-dual. Given a pair of nonnegative integers, an even unimodular lattice of signature exists if and only if m-n is divisible by 8, but an odd unimodular lattice of signature always exists. In particular, even unimodular definite lattices only exist in dimension divisible by 8. Examples in all admissible signatures are given by the IIm,n and Im,n constructions, respectively. The theta function of a unimodular positive definite lattice is a modular form whose weight is one half the rank. If the lattice is even, the form has level 1, and if the lattice is odd the form has Γ0 structure. Due to the dimension bound on spaces of modular forms, the minimum norm of a nonzero vector of an even unimodular lattice is no greater than ⎣n/24⎦ + 1. An even unimodular lattice that achieves this bound is called extremal. Extremal even unimodular lattices are known in relevant dimensions up to 80, and their non-existence has been proven for dimensions above 163,264.
Classification
For indefinite lattices, the classification is easy to describe. Write Rm,n for the m + n dimensional vector space Rm+n with the inner product of and given by In Rm,n there is one odd indefinite unimodular lattice up to isomorphism, denoted by which is given by all vectors in Rm,n with all the ai integers. There are no indefinite even unimodular lattices unless in which case there is a unique example up to isomorphism, denoted by This is given by all vectors in Rm,n such that either all the ai are integers or they are all integers plus 1/2, and their sum is even. The lattice II8,0 is the same as the E8 lattice. Positive definite unimodular lattices have been classified up to dimension 25. There is a unique example In,0 in each dimension n less than 8, and two examples in dimension 8. The number of lattices increases moderately up to dimension 25, but beyond dimension 25 the Smith-Minkowski-Siegel mass formula implies that the number increases very rapidly with the dimension; for example, there are more than 80,000,000,000,000,000 in dimension 32. In some sense unimodular lattices up to dimension 9 are controlled by E8, and up to dimension 25 they are controlled by the Leech lattice, and this accounts for their unusually good behavior in these dimensions. For example, the Dynkin diagram of the norm-2 vectors of unimodular lattices in dimension up to 25 can be naturally identified with a configuration of vectors in the Leech lattice. The wild increase in numbers beyond 25 dimensions might be attributed to the fact that these lattices are no longer controlled by the Leech lattice. Even positive definite unimodular lattice exist only in dimensions divisible by 8. There is one in dimension 8, two in dimension 16, and 24 in dimension 24, called the Niemeier lattices. Beyond 24 dimensions the number increases very rapidly; in 32 dimensions there are more than a billion of them. Unimodular lattices with no roots have been classified up to dimension 28. There are none of dimension less than 23. There is one in dimension 23, two in dimension 24, and showed that there are 0, 1, 3, 38 in dimensions 25, 26, 27, 28. Beyond this the number increases very rapidly; there are at least 8000 in dimension 29. In sufficiently high dimensions most unimodular lattices have no roots. The only non-zero example of even positive definite unimodular lattices with no roots in dimension less than 32 is the Leech lattice in dimension 24. In dimension 32 there are more than ten million examples, and above dimension 32 the number increases very rapidly. The following table from gives the numbers of even or odd unimodular lattices in various dimensions, and shows the very rapid growth starting shortly after dimension 24.
Dimension
Odd lattices
Odd lattices no roots
Even lattices
Even lattices no roots
0
0
0
1
1
1
1
0
2
1
0
3
1
0
4
1
0
5
1
0
6
1
0
7
1
0
8
1
0
1
0
9
2
0
10
2
0
11
2
0
12
3
0
13
3
0
14
4
0
15
5
0
16
6
0
2
0
17
9
0
18
13
0
19
16
0
20
28
0
21
40
0
22
68
0
23
117
1
24
273
1
24
1
25
665
0
26
≥ 2307
1
27
≥ 14179
3
28
≥ 327972
38
29
≥ 37938009
≥ 8900
30
≥ 20169641025
≥ 82000000
31
≥ 5000000000000
≥ 800000000000
32
≥ 80000000000000000
≥ 10000000000000000
≥ 1160000000
≥ 10900000
Beyond 32 dimensions, the numbers increase even more rapidly.
Applications
The second cohomology group of a closed simply connectedoriented topological 4-manifold is a unimodular lattice. Michael Freedman showed that this lattice almost determines the manifold: there is a unique such manifold for each even unimodular lattice, and exactly two for each odd unimodular lattice. In particular if we take the lattice to be 0, this implies the Poincaré conjecture for 4-dimensional topological manifolds. Donaldson's theorem states that if the manifold is smooth and the lattice is positive definite, then it must be a sum of copies of Z, so most of these manifolds have no smooth structure. One such example is the E8 manifold.
