Integer Lattices

An integer lattice $L$ is a finitely generated $Z$ -submodule of a quadratic vector space $V = Q^{n}$ over the rational numbers. Integer lattices are also known as quadratic forms over the integers. We will refer to them as $Z$ -lattices.

A $Z$ -lattice $L$ has the type ZZLat. It is given in terms of its ambient quadratic space $V$ together with a basis matrix $B$ whose rows span $L$ , i.e. $L = Z^{r} B$ where $r$ is the ( $Z$ -module) rank of $L$ .

To access $V$ and $B$ see ambient_space(L::ZZLat) and basis_matrix(L::ZZLat).

Creation of integer lattices

From a gram matrix

# integer_lattice — Method.

julia

integer_lattice([B::MatElem]; gram) -> ZZLat

Return the Z-lattice with basis matrix $B$ inside the quadratic space with Gram matrix gram.

If the keyword gram is not specified, the Gram matrix is the identity matrix. If $B$ is not specified, the basis matrix is the identity matrix.

Examples

julia

julia> L = integer_lattice(matrix(QQ, 2, 2, [1//2, 0, 0, 2]));

julia> gram_matrix(L) == matrix(QQ, 2, 2, [1//4, 0, 0, 4])
true

julia> L = integer_lattice(gram = matrix(ZZ, [2 -1; -1 2]));

julia> gram_matrix(L) == matrix(ZZ, [2 -1; -1 2])
true

In a quadratic space

# lattice — Method.

julia

lattice(V::AbstractSpace, basis::MatElem ; check::Bool = true) -> AbstractLat

Given an ambient space V and a matrix basis, return the lattice spanned by the rows of basis inside V. If V is hermitian (resp. quadratic) then the output is a hermitian (resp. quadratic) lattice.

By default, basis is checked to be of full rank. This test can be disabled by setting check to false.

Special lattices

# root_lattice — Method.

julia

root_lattice(R::Symbol, n::Int) -> ZZLat

Return the root lattice of type R given by :A, :D or :E with parameter n.

The type :I with parameter n = 1 is also allowed and denotes the odd unimodular lattice of rank 1.

# hyperbolic_plane_lattice — Method.

julia

hyperbolic_plane_lattice(n::RationalUnion = 1) -> ZZLat

Return the hyperbolic plane with intersection form of scale n, that is, the unique (up to isometry) even unimodular hyperbolic $Z$ -lattice of rank 2, rescaled by n.

Examples

julia

julia> L = hyperbolic_plane_lattice(6);

julia> gram_matrix(L)
[0   6]
[6   0]

julia> L = hyperbolic_plane_lattice(ZZ(-13));

julia> gram_matrix(L)
[  0   -13]
[-13     0]

# integer_lattice — Method.

julia

integer_lattice(S::Symbol, n::RationalUnion = 1) -> ZZlat

Given S = :H or S = :U, return a $Z$ -lattice admitting $n * J_{2}$ as Gram matrix in some basis, where $J_{2}$ is the 2-by-2 matrix with 0's on the main diagonal and 1's elsewhere.

# leech_lattice — Function.

julia

leech_lattice() -> ZZLat

Return the Leech lattice.

julia

leech_lattice(niemeier_lattice::ZZLat) -> ZZLat, QQMatrix, Int

Return a triple L, v, h where L is the Leech lattice.

L is an h-neighbor of the Niemeier lattice N with respect to v. This means that L / L ∩ N ≅ ℤ / h ℤ. Here h is the Coxeter number of the Niemeier lattice.

This implements the 23 holy constructions of the Leech lattice in [5].

Examples

julia

julia> R = integer_lattice(gram=2 * identity_matrix(ZZ, 24));

julia> N = maximal_even_lattice(R) # Some Niemeier lattice
Integer lattice of rank 24 and degree 24
with gram matrix
[2   1   1   1   0   0   0   0   0   0   0   0   0   0   0   0   1   0   1   1   0   0   0   0]
[1   2   1   1   0   0   0   0   0   0   0   0   0   0   0   0   1   1   0   1   0   0   0   0]
[1   1   2   1   0   0   0   0   0   0   0   0   0   0   0   0   1   1   1   0   0   0   0   0]
[1   1   1   2   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0]
[0   0   0   0   2   1   1   1   0   0   0   0   1   0   1   1   0   0   0   0   0   0   0   0]
[0   0   0   0   1   2   1   1   0   0   0   0   1   1   0   1   0   0   0   0   0   0   0   0]
[0   0   0   0   1   1   2   1   0   0   0   0   1   1   1   0   0   0   0   0   0   0   0   0]
[0   0   0   0   1   1   1   2   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0]
[0   0   0   0   0   0   0   0   2   1   1   1   0   0   0   0   0   0   0   0   1   1   1   0]
[0   0   0   0   0   0   0   0   1   2   1   1   0   0   0   0   0   0   0   0   1   0   1   1]
[0   0   0   0   0   0   0   0   1   1   2   1   0   0   0   0   0   0   0   0   1   1   0   1]
[0   0   0   0   0   0   0   0   1   1   1   2   0   0   0   0   0   0   0   0   0   0   0   0]
[0   0   0   0   1   1   1   0   0   0   0   0   2   1   1   1   0   0   0   0   0   0   0   0]
[0   0   0   0   0   1   1   0   0   0   0   0   1   2   0   0   0   0   0   0   0   0   0   0]
[0   0   0   0   1   0   1   0   0   0   0   0   1   0   2   0   0   0   0   0   0   0   0   0]
[0   0   0   0   1   1   0   0   0   0   0   0   1   0   0   2   0   0   0   0   0   0   0   0]
[1   1   1   0   0   0   0   0   0   0   0   0   0   0   0   0   2   1   1   1   0   0   0   0]
[0   1   1   0   0   0   0   0   0   0   0   0   0   0   0   0   1   2   0   0   0   0   0   0]
[1   0   1   0   0   0   0   0   0   0   0   0   0   0   0   0   1   0   2   0   0   0   0   0]
[1   1   0   0   0   0   0   0   0   0   0   0   0   0   0   0   1   0   0   2   0   0   0   0]
[0   0   0   0   0   0   0   0   1   1   1   0   0   0   0   0   0   0   0   0   2   1   1   1]
[0   0   0   0   0   0   0   0   1   0   1   0   0   0   0   0   0   0   0   0   1   2   0   0]
[0   0   0   0   0   0   0   0   1   1   0   0   0   0   0   0   0   0   0   0   1   0   2   0]
[0   0   0   0   0   0   0   0   0   1   1   0   0   0   0   0   0   0   0   0   1   0   0   2]

julia> minimum(N)
2

julia> det(N)
1

julia> L, v, h = leech_lattice(N);

julia> minimum(L)
4

julia> det(L)
1

julia> h == index(L, intersect(L, N))
true

We illustrate how the Leech lattice is constructed from N, h and v.

julia

julia> Zmodh, _ = residue_ring(ZZ, h);

julia> V = ambient_space(N);

julia> vG = map_entries(x->Zmodh(ZZ(x)), inner_product(V, v, basis_matrix(N)));

julia> LN = transpose(lift(Hecke.kernel(vG; side = :right)))*basis_matrix(N); # vectors whose inner product with `v` is divisible by `h`.

julia> lattice(V, LN) == intersect(L, N)
true

julia> gensL = vcat(LN, 1//h * v);

julia> lattice(V, gensL, isbasis=false) == L
true

# k3_lattice — Function.

julia

k3_lattice()

Return the integer lattice corresponding to the Beauville-Bogomolov-Fujiki form associated to a K3 surface.

Examples

julia

julia> L = k3_lattice();

julia> is_unimodular(L)
true

julia> signature_tuple(L)
(3, 0, 19)

# mukai_lattice — Method.

julia

mukai_lattice(S::Symbol = :K3; extended::Bool = false)

Return the (extended) Mukai lattice.

If S == :K3, it returns the (extended) Mukai lattice associated to hyperkaehler manifolds which are deformation equivalent to a moduli space of stable sheaves on a K3 surface.

If S == :Ab, it returns the (extended) Mukai lattice associated to hyperkaehler manifolds which are deformation equivalent to a moduli space of stable sheaves on an abelian surface.

Examples

julia

julia> L = mukai_lattice();

julia> genus(L)
Genus symbol for integer lattices
Signatures: (4, 0, 20)
Local symbol:
  Local genus symbol at 2: 1^24

julia> L = mukai_lattice(; extended = true);

julia> genus(L)
Genus symbol for integer lattices
Signatures: (5, 0, 21)
Local symbol:
  Local genus symbol at 2: 1^26

julia> L = mukai_lattice(:Ab);

julia> genus(L)
Genus symbol for integer lattices
Signatures: (4, 0, 4)
Local symbol:
  Local genus symbol at 2: 1^8

julia> L = mukai_lattice(:Ab; extended = true);

julia> genus(L)
Genus symbol for integer lattices
Signatures: (5, 0, 5)
Local symbol:
  Local genus symbol at 2: 1^10

# hyperkaehler_lattice — Method.

julia

hyperkaehler_lattice(S::Symbol; n::Int = 2)

Return the integer lattice corresponding to the Beauville-Bogomolov-Fujiki form on a hyperkaehler manifold whose deformation type is determined by S and n.

If S == :K3 or S == :Kum, then n must be an integer bigger than 2;
If S == :OG6 or S == :OG10, the value of n has no effect.

Examples

julia

julia> L = hyperkaehler_lattice(:Kum; n = 3)
Integer lattice of rank 7 and degree 7
with gram matrix
[0   1   0   0   0   0    0]
[1   0   0   0   0   0    0]
[0   0   0   1   0   0    0]
[0   0   1   0   0   0    0]
[0   0   0   0   0   1    0]
[0   0   0   0   1   0    0]
[0   0   0   0   0   0   -8]

julia> L = hyperkaehler_lattice(:OG6)
Integer lattice of rank 8 and degree 8
with gram matrix
[0   1   0   0   0   0    0    0]
[1   0   0   0   0   0    0    0]
[0   0   0   1   0   0    0    0]
[0   0   1   0   0   0    0    0]
[0   0   0   0   0   1    0    0]
[0   0   0   0   1   0    0    0]
[0   0   0   0   0   0   -2    0]
[0   0   0   0   0   0    0   -2]

julia> L = hyperkaehler_lattice(:OG10);

julia> genus(L)
Genus symbol for integer lattices
Signatures: (3, 0, 21)
Local symbols:
  Local genus symbol at 2: 1^-24
  Local genus symbol at 3: 1^-23 3^1

julia> L = hyperkaehler_lattice(:K3; n = 3);

julia> genus(L)
Genus symbol for integer lattices
Signatures: (3, 0, 20)
Local symbol:
  Local genus symbol at 2: 1^22 4^1_7

From a genus

Integer lattices can be created as representatives of a genus. See (representative(L::ZZGenus))

Rescaling the Quadratic Form

# rescale — Method.

julia

rescale(L::ZZLat, r::RationalUnion) -> ZZLat

Return the lattice L in the quadratic space with form r \Phi.

Examples

This can be useful to apply methods intended for positive definite lattices.

julia

julia> L = integer_lattice(gram=ZZ[-1 0; 0 -1])
Integer lattice of rank 2 and degree 2
with gram matrix
[-1    0]
[ 0   -1]

julia> shortest_vectors(rescale(L, -1))
2-element Vector{Vector{ZZRingElem}}:
 [0, 1]
 [1, 0]

Attributes

# ambient_space — Method.

julia

ambient_space(L::AbstractLat) -> AbstractSpace

Return the ambient space of the lattice L. If the ambient space is not known, an error is raised.

# basis_matrix — Method.

julia

basis_matrix(L::ZZLat) -> QQMatrix

Return the basis matrix $B$ of the integer lattice $L$ .

The lattice is given by the row span of $B$ seen inside of the ambient quadratic space of $L$ .

# gram_matrix — Method.

julia

gram_matrix(L::ZZLat) -> QQMatrix

Return the gram matrix of $L$ .

Examples

julia

julia> L = integer_lattice(matrix(ZZ, [2 0; -1 2]));

julia> gram_matrix(L)
[ 4   -2]
[-2    5]

# rational_span — Method.

julia

rational_span(L::ZZLat) -> QuadSpace

Return the rational span of $L$ , which is the quadratic space with Gram matrix equal to gram_matrix(L).

Examples

julia

julia> L = integer_lattice(matrix(ZZ, [2 0; -1 2]));

julia> rational_span(L)
Quadratic space of dimension 2
  over rational field
with gram matrix
[ 4   -2]
[-2    5]

Invariants

# rank — Method.

julia

rank(L::AbstractLat) -> Int

Return the rank of the underlying module of the lattice L.

# det — Method.

julia

det(L::ZZLat) -> QQFieldElem

Return the determinant of the gram matrix of L.

# scale — Method.

julia

scale(L::ZZLat) -> QQFieldElem

Return the scale of L.

The scale of L is defined as the positive generator of the $Z$ -ideal generated by ${Φ (x, y) : x, y \in L}$ .

# norm — Method.

julia

norm(L::ZZLat) -> QQFieldElem

Return the norm of L.

The norm of L is defined as the positive generator of the $Z$ - ideal generated by ${Φ (x, x) : x \in L}$ .

# iseven — Method.

julia

iseven(L::ZZLat) -> Bool

Return whether L is even.

An integer lattice L in the rational quadratic space $(V, Φ)$ is called even if $Φ (x, x) \in 2 Z$ for all $x i n L$ .

# is_integral — Method.

julia

is_integral(L::AbstractLat) -> Bool

Return whether the lattice L is integral.

# is_primary_with_prime — Method.

julia

is_primary_with_prime(L::ZZLat) -> Bool, ZZRingElem

Given a $Z$ -lattice L, return whether L is primary, that is whether L is integral and its discriminant group (see discriminant_group) is a p-group for some prime number p. In case it is, p is also returned as second output.

Note that for unimodular lattices, this function returns (true, 1). If the lattice is not primary, the second return value is -1 by default.

# is_primary — Method.

julia

is_primary(L::ZZLat, p::Union{Integer, ZZRingElem}) -> Bool

Given an integral $Z$ -lattice L and a prime number p, return whether L is p-primary, that is whether its discriminant group (see discriminant_group) is a p-group.

# is_elementary_with_prime — Method.

julia

is_elementary_with_prime(L::ZZLat) -> Bool, ZZRingElem

Given a $Z$ -lattice L, return whether L is elementary, that is whether L is integral and its discriminant group (see discriminant_group) is an elemenentary p-group for some prime number p. In case it is, p is also returned as second output.

Note that for unimodular lattices, this function returns (true, 1). If the lattice is not elementary, the second return value is -1 by default.

# is_elementary — Method.

julia

is_elementary(L::ZZLat, p::Union{Integer, ZZRingElem}) -> Bool

Given an integral $Z$ -lattice L and a prime number p, return whether L is p-elementary, that is whether its discriminant group (see discriminant_group) is an elementary p-group.

The Genus

For an integral lattice The genus of an integer lattice collects its local invariants. genus(::ZZLat)

# mass — Method.

julia

mass(L::ZZLat) -> QQFieldElem

Return the mass of the genus of L.

# genus_representatives — Method.

julia

genus_representatives(L::ZZLat) -> Vector{ZZLat}

Return representatives for the isometry classes in the genus of L.

Real invariants

# signature_tuple — Method.

julia

signature_tuple(L::ZZLat) -> Tuple{Int,Int,Int}

Return the number of (positive, zero, negative) inertia of L.

# is_positive_definite — Method.

julia

is_positive_definite(L::AbstractLat) -> Bool

Return whether the rational span of the lattice L is positive definite.

# is_negative_definite — Method.

julia

is_negative_definite(L::AbstractLat) -> Bool

Return whether the rational span of the lattice L is negative definite.

# is_definite — Method.

julia

is_definite(L::AbstractLat) -> Bool

Return whether the rational span of the lattice L is definite.

Isometries

# automorphism_group_generators — Method.

julia

automorphism_group_generators(E::EllipticCurve) -> Vector{EllCrvIso}

Return generators of the automorphism group of $E$ .

julia

automorphism_group_generators(L::AbstractLat; ambient_representation::Bool = true,
                                              depth::Int = -1, bacher_depth::Int = 0)
                                                      -> Vector{MatElem}

Given a definite lattice L, return generators for the automorphism group of L. If ambient_representation == true (the default), the transformations are represented with respect to the ambient space of L. Otherwise, the transformations are represented with respect to the (pseudo-)basis of L.

Setting the parameters depth and bacher_depth to a positive value may improve performance. If set to -1 (default), the used value of depth is chosen heuristically depending on the rank of L. By default, bacher_depth is set to 0.

# automorphism_group_order — Method.

julia

automorphism_group_order(L::AbstractLat; depth::Int = -1, bacher_depth::Int = 0) -> Int

Given a definite lattice L, return the order of the automorphism group of L.

Setting the parameters depth and bacher_depth to a positive value may improve performance. If set to -1 (default), the used value of depth is chosen heuristically depending on the rank of L. By default, bacher_depth is set to 0.

# is_isometric — Method.

julia

is_isometric(L::AbstractLat, M::AbstractLat; depth::Int = -1, bacher_depth::Int = 0) -> Bool

Return whether the lattices L and M are isometric.

Setting the parameters depth and bacher_depth to a positive value may improve performance. If set to -1 (default), the used value of depth is chosen heuristically depending on the rank of L. By default, bacher_depth is set to 0.

# is_locally_isometric — Method.

julia

is_locally_isometric(L::ZZLat, M::ZZLat, p::Int) -> Bool

Return whether L and M are isometric over the p-adic integers.

i.e. whether $L \otimes Z_{p} ≅ M \otimes Z_{p}$ .

Root lattices

# root_lattice_recognition — Method.

julia

root_lattice_recognition(L::ZZLat)

Return the ADE type of the root sublattice of L.

The root sublattice is the lattice spanned by the vectors of squared length $1$ and $2$ . The odd lattice of rank 1 and determinant $1$ is denoted by (:I, 1).

Input:

L – a definite and integral $Z$ -lattice.

Output:

Two lists, the first one containing the ADE types and the second one the irreducible root sublattices.

For more recognizable gram matrices use root_lattice_recognition_fundamental.

Examples

julia

julia> L = integer_lattice(gram=ZZ[4  0 0  0 3  0 3  0;
                            0 16 8 12 2 12 6 10;
                            0  8 8  6 2  8 4  5;
                            0 12 6 10 2  9 5  8;
                            3  2 2  2 4  2 4  2;
                            0 12 8  9 2 12 6  9;
                            3  6 4  5 4  6 6  5;
                            0 10 5  8 2  9 5  8])
Integer lattice of rank 8 and degree 8
with gram matrix
[4    0   0    0   3    0   3    0]
[0   16   8   12   2   12   6   10]
[0    8   8    6   2    8   4    5]
[0   12   6   10   2    9   5    8]
[3    2   2    2   4    2   4    2]
[0   12   8    9   2   12   6    9]
[3    6   4    5   4    6   6    5]
[0   10   5    8   2    9   5    8]

julia> R = root_lattice_recognition(L)
([(:A, 1), (:D, 6)], ZZLat[Integer lattice of rank 1 and degree 8, Integer lattice of rank 6 and degree 8])

julia> L = integer_lattice(; gram = QQ[1 0 0  0;
                                       0 9 3  3;
                                       0 3 2  1;
                                       0 3 1 11])
Integer lattice of rank 4 and degree 4
with gram matrix
[1   0   0    0]
[0   9   3    3]
[0   3   2    1]
[0   3   1   11]

julia> root_lattice_recognition(L)
([(:A, 1), (:I, 1)], ZZLat[Integer lattice of rank 1 and degree 4, Integer lattice of rank 1 and degree 4])

# root_lattice_recognition_fundamental — Method.

julia

root_lattice_recognition_fundamental(L::ZZLat)

Return the ADE type of the root sublattice of L as well as the corresponding irreducible root sublattices with basis given by a fundamental root system.

The type (:I, 1) corresponds to the odd unimodular root lattice of rank 1.

Input:

L – a definite and integral $Z$ -lattice.

Output:

the root sublattice, with basis given by a fundamental root system
the ADE types
a Vector consisting of the irreducible root sublattices.

Examples

julia

julia> L = integer_lattice(gram=ZZ[4  0 0  0 3  0 3  0;
                            0 16 8 12 2 12 6 10;
                            0  8 8  6 2  8 4  5;
                            0 12 6 10 2  9 5  8;
                            3  2 2  2 4  2 4  2;
                            0 12 8  9 2 12 6  9;
                            3  6 4  5 4  6 6  5;
                            0 10 5  8 2  9 5  8])
Integer lattice of rank 8 and degree 8
with gram matrix
[4    0   0    0   3    0   3    0]
[0   16   8   12   2   12   6   10]
[0    8   8    6   2    8   4    5]
[0   12   6   10   2    9   5    8]
[3    2   2    2   4    2   4    2]
[0   12   8    9   2   12   6    9]
[3    6   4    5   4    6   6    5]
[0   10   5    8   2    9   5    8]

julia> R = root_lattice_recognition_fundamental(L);

julia> gram_matrix(R[1])
[2    0    0    0    0    0    0]
[0    2    0   -1    0    0    0]
[0    0    2   -1    0    0    0]
[0   -1   -1    2   -1    0    0]
[0    0    0   -1    2   -1    0]
[0    0    0    0   -1    2   -1]
[0    0    0    0    0   -1    2]

# ADE_type — Method.

julia

ADE_type(G::MatrixElem) -> Tuple{Symbol,Int64}

Return the type of the irreducible root lattice with gram matrix G.

See also root_lattice_recognition.

Examples

julia

julia> Hecke.ADE_type(gram_matrix(root_lattice(:A,3)))
(:A, 3)

# coxeter_number — Method.

julia

coxeter_number(ADE::Symbol, n) -> Int

Return the Coxeter number of the corresponding ADE root lattice.

If $L$ is a root lattice and $R$ its set of roots, then the Coxeter number $h$ is $| R | / n$ where n is the rank of $L$ .

Examples

julia

julia> coxeter_number(:D, 4)
6

# highest_root — Method.

julia

highest_root(ADE::Symbol, n) -> ZZMatrix

Return coordinates of the highest root of root_lattice(ADE, n).

Examples

julia

julia> highest_root(:E, 6)
[1   2   3   2   1   2]

Module operations

Most module operations assume that the lattices live in the same ambient space. For instance only lattices in the same ambient space compare.

# == — Method.

Return true if both lattices have the same ambient quadratic space and the same underlying module.

# is_sublattice — Method.

julia

is_sublattice(L::AbstractLat, M::AbstractLat) -> Bool

Return whether M is a sublattice of the lattice L.

# is_sublattice_with_relations — Method.

julia

is_sublattice_with_relations(M::ZZLat, N::ZZLat) -> Bool, QQMatrix

Returns whether $N$ is a sublattice of $M$ . In this case, the second return value is a matrix $B$ such that $B B_{M} = B_{N}$ , where $B_{M}$ and $B_{N}$ are the basis matrices of $M$ and $N$ respectively.

# + — Method.

julia

+(L::AbstractLat, M::AbstractLat) -> AbstractLat

Return the sum of the lattices L and M.

The lattices L and M must have the same ambient space.

# * — Method.

julia

*(a::RationalUnion, L::ZZLat) -> ZZLat

Return the lattice $a M$ inside the ambient space of $M$ .

# intersect — Method.

julia

intersect(L::AbstractLat, M::AbstractLat) -> AbstractLat

Return the intersection of the lattices L and M.

The lattices L and M must have the same ambient space.

# in — Method.

julia

Base.in(v::Vector, L::ZZLat) -> Bool

Return whether the vector v lies in the lattice L.

# in — Method.

julia

Base.in(v::QQMatrix, L::ZZLat) -> Bool

Return whether the row span of v lies in the lattice L.

# primitive_closure — Method.

julia

primitive_closure(M::ZZLat, N::ZZLat) -> ZZLat

Given two $Z$ -lattices M and N with $N \subseteq Q M$ , return the primitive closure $M \cap Q N$ of N in M.

Examples

julia

julia> M = root_lattice(:D, 6);

julia> N = lattice_in_same_ambient_space(M, 3*basis_matrix(M)[1:1,:]);

julia> basis_matrix(N)
[3   0   0   0   0   0]

julia> N2 = primitive_closure(M, N)
Integer lattice of rank 1 and degree 6
with gram matrix
[2]

julia> basis_matrix(N2)
[1   0   0   0   0   0]

julia> M2 = primitive_closure(dual(M), M);

julia> is_integral(M2)
false

# is_primitive — Method.

julia

is_primitive(M::ZZLat, N::ZZLat) -> Bool

Given two $Z$ -lattices $N \subseteq M$ , return whether N is a primitive sublattice of M.

Examples

julia

julia> U = hyperbolic_plane_lattice(3);

julia> bU = basis_matrix(U);

julia> e1, e2 = bU[1:1,:], bU[2:2,:]
([1 0], [0 1])

julia> N = lattice_in_same_ambient_space(U, e1 + e2)
Integer lattice of rank 1 and degree 2
with gram matrix
[6]

julia> is_primitive(U, N)
true

julia> M = root_lattice(:A, 3);

julia> f = matrix(QQ, 3, 3, [0 1 1; -1 -1 -1; 1 1 0]);

julia> N = kernel_lattice(M, f+1)
Integer lattice of rank 1 and degree 3
with gram matrix
[4]

julia> is_primitive(M, N)
true

# is_primitive — Method.

julia

is_primitive(L::ZZLat, v::Union{Vector, QQMatrix}) -> Bool

Return whether the vector v is primitive in L.

A vector v in a $Z$ -lattice L is called primitive if for all w in L such that $v = d w$ for some integer d, then $d = \pm 1$ .

# divisibility — Method.

julia

divisibility(L::ZZLat, v::Union{Vector, QQMatrix}) -> QQFieldElem

Return the divisibility of v with respect to L.

For a vector v in the ambient quadratic space $(V, Φ)$ of L, we call the divisibility of v with the respect to L the non-negative generator of the fractional $Z$ -ideal $Φ (v, L)$ .

Embeddings

Categorical constructions

# direct_sum — Method.

julia

direct_sum(x::Vararg{ZZLat}) -> ZZLat, Vector{AbstractSpaceMor}
direct_sum(x::Vector{ZZLat}) -> ZZLat, Vector{AbstractSpaceMor}

Given a collection of $Z$ -lattices $L_{1}, \dots, L_{n}$ , return their direct sum $L := L_{1} \oplus \dots \oplus L_{n}$ , together with the injections $L_{i} \to L$ . (seen as maps between the corresponding ambient spaces).

For objects of type ZZLat, finite direct sums and finite direct products agree and they are therefore called biproducts. If one wants to obtain L as a direct product with the projections $L \to L_{i}$ , one should call direct_product(x). If one wants to obtain L as a biproduct with the injections $L_{i} \to L$ and the projections $L \to L_{i}$ , one should call biproduct(x).

# direct_product — Method.

julia

direct_product(x::Vararg{ZZLat}) -> ZZLat, Vector{AbstractSpaceMor}
direct_product(x::Vector{ZZLat}) -> ZZLat, Vector{AbstractSpaceMor}

Given a collection of $Z$ -lattices $L_{1}, \dots, L_{n}$ , return their direct product $L := L_{1} \times \dots \times L_{n}$ , together with the projections $L \to L_{i}$ . (seen as maps between the corresponding ambient spaces).

For objects of type ZZLat, finite direct sums and finite direct products agree and they are therefore called biproducts. If one wants to obtain L as a direct sum with the injections $L_{i} \to L$ , one should call direct_sum(x). If one wants to obtain L as a biproduct with the injections $L_{i} \to L$ and the projections $L \to L_{i}$ , one should call biproduct(x).

# biproduct — Method.

julia

biproduct(x::Vararg{ZZLat}) -> ZZLat, Vector{AbstractSpaceMor}, Vector{AbstractSpaceMor}
biproduct(x::Vector{ZZLat}) -> ZZLat, Vector{AbstractSpaceMor}, Vector{AbstractSpaceMor}

Given a collection of $Z$ -lattices $L_{1}, \dots, L_{n}$ , return their biproduct $L := L_{1} \oplus \dots \oplus L_{n}$ , together with the injections $L_{i} \to L$ and the projections $L \to L_{i}$ . (seen as maps between the corresponding ambient spaces).

For objects of type ZZLat, finite direct sums and finite direct products agree and they are therefore called biproducts. If one wants to obtain L as a direct sum with the injections $L_{i} \to L$ , one should call direct_sum(x). If one wants to obtain L as a direct product with the projections $L \to L_{i}$ , one should call direct_product(x).

Orthogonal sublattices

# orthogonal_submodule — Method.

julia

orthogonal_submodule(L::ZZLat, S::ZZLat) -> ZZLat

Return the largest submodule of $L$ orthogonal to $S$ .

# irreducible_components — Method.

julia

irreducible_components(L::ZZLat) -> Vector{ZZLat}

Return the irreducible components $L_{i}$ of the positive definite lattice $L$ .

This yields a maximal orthogonal splitting of L as

L = ⨁_{i} L_{i} .

Dual lattice

# dual — Method.

julia

dual(L::AbstractLat) -> AbstractLat

Return the dual lattice of the lattice L.

Discriminant group

See discriminant_group(L::ZZLat).

Overlattices

# glue_map — Method.

julia

glue_map(L::ZZLat, S::ZZLat, R::ZZLat; check=true)
                       -> Tuple{TorQuadModuleMap, TorQuadModuleMap, TorQuadModuleMap}

Given three integral $Z$ -lattices L, S and R, with S and R primitive sublattices of L and such that the sum of the ranks of S and R is equal to the rank of L, return the glue map $γ$ of the primitive extension $S + R \subseteq L$ , as well as the inclusion maps of the domain and codomain of $γ$ into the respective discriminant groups of S and R.

Example

julia

julia> M = root_lattice(:E,8);

julia> f = matrix(QQ, 8, 8, [-1 -1  0  0  0  0  0  0;
                              1  0  0  0  0  0  0  0;
                              0  1  1  0  0  0  0  0;
                              0  0  0  1  0  0  0  0;
                              0  0  0  0  1  0  0  0;
                              0  0  0  0  0  1  1  0;
                             -2 -4 -6 -5 -4 -3 -2 -3;
                              0  0  0  0  0  0  0  1]);

julia> S = kernel_lattice(M ,f-1)
Integer lattice of rank 4 and degree 8
with gram matrix
[12   -3    0   -3]
[-3    2   -1    0]
[ 0   -1    2    0]
[-3    0    0    2]

julia> R = kernel_lattice(M , f^2+f+1)
Integer lattice of rank 4 and degree 8
with gram matrix
[ 2   -1    0    0]
[-1    2   -6    0]
[ 0   -6   30   -3]
[ 0    0   -3    2]

julia> glue, iS, iR = glue_map(M, S, R)
(Map: finite quadratic module -> finite quadratic module, Map: finite quadratic module -> finite quadratic module, Map: finite quadratic module -> finite quadratic module)

julia> is_bijective(glue)
true

# overlattice — Method.

julia

overlattice(glue_map::TorQuadModuleMap) -> ZZLat

Given the glue map of a primitive extension of $Z$ -lattices $S + R \subseteq L$ , return L.

Example

julia

julia> M = root_lattice(:E,8);

julia> f = matrix(QQ, 8, 8, [ 1  0  0  0  0  0  0  0;
                              0  1  0  0  0  0  0  0;
                              1  2  4  4  3  2  1  2;
                             -2 -4 -6 -5 -4 -3 -2 -3;
                              2  4  6  4  3  2  1  3;
                             -1 -2 -3 -2 -1  0  0 -2;
                              0  0  0  0  0 -1  0  0;
                             -1 -2 -3 -3 -2 -1  0 -1]);

julia> S = kernel_lattice(M ,f-1)
Integer lattice of rank 4 and degree 8
with gram matrix
[ 2   -1     0     0]
[-1    2    -1     0]
[ 0   -1    12   -15]
[ 0    0   -15    20]

julia> R = kernel_lattice(M , f^4+f^3+f^2+f+1)
Integer lattice of rank 4 and degree 8
with gram matrix
[10   -4    0    1]
[-4    2   -1    0]
[ 0   -1    4   -3]
[ 1    0   -3    4]

julia> glue, iS, iR = glue_map(M, S, R);

julia> overlattice(glue) == M
true

# local_modification — Method.

julia

local_modification(M::ZZLat, L::ZZLat, p)

Return a local modification of M that matches L at p.

INPUT:

$M$ – a \mathbb{Z}_p-maximal lattice
$L$ – the a lattice isomorphic to M over \QQ_p
$p$ – a prime number

OUTPUT:

an integral lattice M' in the ambient space of M such that M and M' are locally equal at all completions except at p where M' is locally isometric to the lattice L.

# maximal_integral_lattice — Method.

julia

maximal_integral_lattice(L::AbstractLat) -> AbstractLat

Given a lattice L with integral norm, return a maximal integral overlattice M of L.

Sublattices defined by endomorphisms

# kernel_lattice — Method.

julia

kernel_lattice(L::ZZLat, f::MatElem;
               ambient_representation::Bool = true) -> ZZLat

Given a $Z$ -lattice $L$ and a matrix $f$ inducing an endomorphism of $L$ , return $\ker (f)$ is a sublattice of $L$ .

If ambient_representation is true (the default), the endomorphism is represented with respect to the ambient space of $L$ . Otherwise, the endomorphism is represented with respect to the basis of $L$ .

# invariant_lattice — Method.

julia

invariant_lattice(L::ZZLat, G::Vector{MatElem};
                  ambient_representation::Bool = true) -> ZZLat
invariant_lattice(L::ZZLat, G::MatElem;
                  ambient_representation::Bool = true) -> ZZLat

Given a $Z$ -lattice $L$ and a list of matrices $G$ inducing endomorphisms of $L$ (or just one matrix $G$ ), return the lattice $L^{G}$ , consisting on elements fixed by $G$ .

If ambient_representation is true (the default), the endomorphism is represented with respect to the ambient space of $L$ . Otherwise, the endomorphism is represented with respect to the basis of $L$ .

# coinvariant_lattice — Method.

julia

coinvariant_lattice(L::ZZLat, G::Vector{MatElem};
                    ambient_representation::Bool = true) -> ZZLat
coinvariant_lattice(L::ZZLat, G::MatElem;
                    ambient_representation::Bool = true) -> ZZLat

Given a $Z$ -lattice $L$ and a list of matrices $G$ inducing endomorphisms of $L$ (or just one matrix $G$ ), return the orthogonal complement $L_{G}$ in $L$ of the fixed lattice $L^{G}$ (see invariant_lattice).

If ambient_representation is true (the default), the endomorphism is represented with respect to the ambient space of $L$ . Otherwise, the endomorphism is represented with respect to the basis of $L$ .

Computing embeddings

# embed — Method.

julia

embed(S::ZZLat, G::Genus, primitive::Bool=true) -> Bool, embedding

Return a (primitive) embedding of the integral lattice S into some lattice in the genus of G.

julia

julia> G = integer_genera((8,0), 1, even=true)[1];

julia> L, S, i = embed(root_lattice(:A,5), G);

# embed_in_unimodular — Method.

julia

embed_in_unimodular(S::ZZLat, pos::Int, neg::Int, primitive=true, even=true) -> Bool, L, S', iS, iR

Return a (primitive) embedding of the integral lattice S into some (even) unimodular lattice of signature (pos, neg).

For now this works only for even lattices.

julia

julia> NS = direct_sum(integer_lattice(:U), rescale(root_lattice(:A, 16), -1))[1];

julia> LK3, iNS, i = embed_in_unimodular(NS, 3, 19);

julia> genus(LK3)
Genus symbol for integer lattices
Signatures: (3, 0, 19)
Local symbol:
  Local genus symbol at 2: 1^22

julia> iNS
Integer lattice of rank 18 and degree 22
with gram matrix
[0   1    0    0    0    0    0    0    0    0    0    0    0    0    0    0    0    0]
[1   0    0    0    0    0    0    0    0    0    0    0    0    0    0    0    0    0]
[0   0   -2    1    0    0    0    0    0    0    0    0    0    0    0    0    0    0]
[0   0    1   -2    1    0    0    0    0    0    0    0    0    0    0    0    0    0]
[0   0    0    1   -2    1    0    0    0    0    0    0    0    0    0    0    0    0]
[0   0    0    0    1   -2    1    0    0    0    0    0    0    0    0    0    0    0]
[0   0    0    0    0    1   -2    1    0    0    0    0    0    0    0    0    0    0]
[0   0    0    0    0    0    1   -2    1    0    0    0    0    0    0    0    0    0]
[0   0    0    0    0    0    0    1   -2    1    0    0    0    0    0    0    0    0]
[0   0    0    0    0    0    0    0    1   -2    1    0    0    0    0    0    0    0]
[0   0    0    0    0    0    0    0    0    1   -2    1    0    0    0    0    0    0]
[0   0    0    0    0    0    0    0    0    0    1   -2    1    0    0    0    0    0]
[0   0    0    0    0    0    0    0    0    0    0    1   -2    1    0    0    0    0]
[0   0    0    0    0    0    0    0    0    0    0    0    1   -2    1    0    0    0]
[0   0    0    0    0    0    0    0    0    0    0    0    0    1   -2    1    0    0]
[0   0    0    0    0    0    0    0    0    0    0    0    0    0    1   -2    1    0]
[0   0    0    0    0    0    0    0    0    0    0    0    0    0    0    1   -2    1]
[0   0    0    0    0    0    0    0    0    0    0    0    0    0    0    0    1   -2]

julia> is_primitive(LK3, iNS)
true

LLL, Short and Close Vectors

LLL and indefinite LLL

# lll — Method.

julia

lll(L::ZZLat, same_ambient::Bool = true) -> ZZLat

Given an integral $Z$ -lattice L with basis matrix B, compute a basis C of L such that the gram matrix $G_{C}$ of L with respect to C is LLL-reduced.

By default, it creates the lattice in the same ambient space as L. This can be disabled by setting same_ambient = false. Works with both definite and indefinite lattices.

Short Vectors

# short_vectors — Function.

julia

short_vectors(L::ZZLat, [lb = 0], ub, [elem_type = ZZRingElem]; check::Bool = true)
                                   -> Vector{Tuple{Vector{elem_type}, QQFieldElem}}

Return all tuples (v, n) such that $n = | v G v^{t} |$ satisfies lb <= n <= ub, where G is the Gram matrix of L and v is non-zero.

Note that the vectors are computed up to sign (so only one of v and -v appears).

It is assumed and checked that L is definite.

See also short_vectors_iterator for an iterator version.

# shortest_vectors — Function.

julia

shortest_vectors(L::ZZLat, [elem_type = ZZRingElem]; check::Bool = true)
                                           -> QQFieldElem, Vector{elem_type}, QQFieldElem}

Return the list of shortest non-zero vectors in absolute value. Note that the vectors are computed up to sign (so only one of v and -v appears).

It is assumed and checked that L is definite.

See also minimum.

# short_vectors_iterator — Function.

julia

short_vectors_iterator(L::ZZLat, [lb = 0], ub,
                       [elem_type = ZZRingElem]; check::Bool = true)
                                -> Tuple{Vector{elem_type}, QQFieldElem} (iterator)

Return an iterator for all tuples (v, n) such that $n = | v G v^{t} |$ satisfies lb <= n <= ub, where G is the Gram matrix of L and v is non-zero.

Note that the vectors are computed up to sign (so only one of v and -v appears).

It is assumed and checked that L is definite.

See also short_vectors.

# minimum — Method.

julia

minimum(L::ZZLat) -> QQFieldElem

Return the minimum absolute squared length among the non-zero vectors in L.

# kissing_number — Method.

julia

kissing_number(L::ZZLat) -> Int

Return the Kissing number of the sphere packing defined by L.

This is the number of non-overlapping spheres touching any other given sphere.

Close Vectors

# close_vectors — Method.

julia

close_vectors(L:ZZLat, v:Vector, [lb,], ub; check::Bool = false)
                                        -> Vector{Tuple{Vector{Int}}, QQFieldElem}

Return all tuples (x, d) where x is an element of L such that d = b(v - x, v - x) <= ub. If lb is provided, then also lb <= d.

If filter is not nothing, then only those x with filter(x) evaluating to true are returned.

By default, it will be checked whether L is positive definite. This can be disabled setting check = false.

Both input and output are with respect to the basis matrix of L.

Examples

julia

julia> L = integer_lattice(matrix(QQ, 2, 2, [1, 0, 0, 2]));

julia> close_vectors(L, [1, 1], 1)
3-element Vector{Tuple{Vector{ZZRingElem}, QQFieldElem}}:
 ([2, 1], 1)
 ([0, 1], 1)
 ([1, 1], 0)

julia> close_vectors(L, [1, 1], 1, 1)
2-element Vector{Tuple{Vector{ZZRingElem}, QQFieldElem}}:
 ([2, 1], 1)
 ([0, 1], 1)