Spaces

Creation of spaces

quadratic_space — Method

hermitian_space — Method

hermitian_space(E::NumField, n::Int; cached::Bool = true) -> HermSpace

Create the hermitian space over E with dimension n and Gram matrix equals to the identity matrix. The number field E must be a quadratic extension, that is, $degree(E) == 2$ must hold.

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quadratic_space — Method

hermitian_space — Method

hermitian_space(E::NumField, gram::MatElem; cached::Bool = true) -> HermSpace

Create the hermitian space over E with Gram matrix equals to gram. The matrix gram must be square and hermitian with respect to the non-trivial automorphism of E. The number field E must be a quadratic extension, that is, $degree(E) == 2$ must hold.

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Examples

Here are easy examples to see how these constructors work. We will keep the two following spaces for the rest of this section:

julia> K, a = cyclotomic_real_subfield(7);

julia> Kt, t = K[:t];

julia> E, b = number_field(t^2-a*t+1, :b);

julia> Q = quadratic_space(K, K[0 1; 1 0])
Quadratic space of dimension 2
  over maximal real subfield of cyclotomic field of order 7
with gram matrix
[0   1]
[1   0]

julia> H = hermitian_space(E, 3)
Hermitian space of dimension 3
  over relative number field with defining polynomial t^2 - (z_7 + 1/z_7)*t + 1
    over number field with defining polynomial $^3 + $^2 - 2*$ - 1
      over rational field
with gram matrix
[1   0   0]
[0   1   0]
[0   0   1]

Attributes

Let $(V, \Phi)$ be a space over $E/K$ . We define its dimension to be its dimension as a vector space over its base ring $E$ and its rank to be the rank of its Gram matrix. If these two invariants agree, the space is said to be regular.

While dealing with lattices, one always works with regular ambient spaces.

The determinant $\text{det}(V, \Phi)$ of $(V, \Phi)$ is defined to be the class of the determinant of its Gram matrix in $K^{\times}/N(E^{\times})$ (which is similar to $K^{\times}/(K^{\times})^2$ in the quadratic case). The discriminant $\text{disc}(V, \Phi)$ of $(V, \Phi)$ is defined to be $(-1)^{(m(m-1)/2)}\text{det}(V, \Phi)$ , where $m$ is the rank of $(V, \Phi)$ .

rank — Method

dim — Method

gram_matrix — Method

involution — Method

base_ring — Method

fixed_field — Method

det — Method

discriminant — Method

Examples

So for instance, one could get the following information about the hermitian space $H$ :

julia> K, a = cyclotomic_real_subfield(7);

julia> Kt, t = K[:t];

julia> E, b = number_field(t^2-a*t+1, :b);

julia> H = hermitian_space(E, 3);

julia> rank(H), dim(H)
(3, 3)

julia> gram_matrix(H)
[1   0   0]
[0   1   0]
[0   0   1]

julia> involution(H)
Map
  from relative number field of degree 2 over K
  to relative number field of degree 2 over K
defined by
  b -> -b + (z_7 + 1/z_7)
with trivial map on base field

julia> base_ring(H)
Relative number field with defining polynomial t^2 - (z_7 + 1/z_7)*t + 1
  over number field with defining polynomial $^3 + $^2 - 2*$ - 1
    over rational field

julia> fixed_field(H)
Number field with defining polynomial $^3 + $^2 - 2*$ - 1
  over rational field

julia> det(H), discriminant(H)
(1, -1)

Predicates

Let $(V, \Phi)$ be a hermitian space over $E/K$ (resp. quadratic space $K$ ). We say that $(V, \Phi)$ is definite if $E/K$ is CM (resp. $K$ is totally real) and if there exists an orthogonal basis of $V$ for which the diagonal elements of the associated Gram matrix of $(V, \Phi)$ are either all totally positive or all totally negative. In the former case, $V$ is said to be positive definite, while in the latter case it is negative definite. In all the other cases, we say that $V$ is indefinite.

is_regular — Method

is_quadratic — Method

is_hermitian — Method

is_positive_definite — Method

is_negative_definite — Method

is_definite — Method

Note that the is_hermitian function tests whether the space is non-quadratic.

Examples

julia> K, a = cyclotomic_real_subfield(7);

julia> Kt, t = K[:t];

julia> E, b = number_field(t^2-a*t+1, :b);

julia> Q = quadratic_space(K, K[0 1; 1 0]);

julia> H = hermitian_space(E, 3);

julia> is_regular(Q), is_regular(H)
(true, true)

julia> is_quadratic(Q), is_hermitian(H)
(true, true)

julia> is_definite(Q), is_positive_definite(H)
(false, true)

Inner products and diagonalization

gram_matrix — Method

inner_product — Method

orthogonal_basis — Method

diagonal — Method

diagonal(V::AbstractSpace) -> Vector{FieldElem}

Return a vector of elements $a_1,\dotsc,a_n$ such that the space V is isometric to the diagonal space $\langle a_1,\dotsc,a_n \rangle$ .

The elements are contained in the fixed field of V.

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diagonal_with_transform — Method

diagonal_with_transform(V::AbstractSpace) -> Vector{FieldElem},
                                                         MatElem{FieldElem}

Return a vector of elements $a_1,\dotsc,a_n$ such that the space V is isometric to the diagonal space $\langle a_1,\dotsc,a_n \rangle$ . The second output is a matrix U whose rows span an orthogonal basis of V for which the Gram matrix is given by the diagonal matrix of the $a_i$ 's.

The elements are contained in the fixed field of V.

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restrict_scalars — Method

restrict_scalars(V::AbstractSpace, K::QQField,
                                   alpha::FieldElem = one(base_ring(V)))
                                                -> QuadSpace, AbstractSpaceRes

Given a space $(V, \Phi)$ and a subfield K of the base algebra E of V, return the quadratic space W obtained by restricting the scalars of $(V, \alpha\Phi)$ to K, together with the map f for extending the scalars back. The form on the restriction is given by $Tr \circ \Phi$ where $Tr: E \to K$ is the trace form. The rescaling factor $\alpha$ is set to 1 by default.

Note that for now one can only restrict scalars to $\mathbb Q$ .

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Examples

julia> K, a = cyclotomic_real_subfield(7);

julia> Kt, t = K[:t];

julia> E, b = number_field(t^2-a*t+1, :b);

julia> Q = quadratic_space(K, K[0 1; 1 0]);

julia> H = hermitian_space(E, 3);

julia> gram_matrix(Q, K[1 1; 2 0])
[2   2]
[2   0]

julia> gram_matrix(H, E[1 0 0; 0 1 0; 0 0 1])
[1   0   0]
[0   1   0]
[0   0   1]

julia> inner_product(Q, K[1  1], K[0  2])
[2]

julia> orthogonal_basis(H)
[1   0   0]
[0   1   0]
[0   0   1]

julia> diagonal(Q), diagonal(H)
(AbsSimpleNumFieldElem[1, -1], AbsSimpleNumFieldElem[1, 1, 1])

Equivalence

Let $(V, \Phi)$ and $(V', \Phi')$ be spaces over the same extension $E/K$ . A homomorphism of spaces from $V$ to $V'$ is a $E$ -linear mapping $f \colon V \to V'$ such that for all $x,y \in V$ , one has

$\Phi'(f(x), f(y)) = \Phi(x,y).$

An automorphism of spaces is called an isometry and a monomorphism is called an embedding.

hasse_invariant — Method

hasse_invariant(V::QuadSpace, p::Union{InfPlc, AbsNumFieldOrderIdeal{AbsSimpleNumField, AbsSimpleNumFieldElem}}) -> Int

Returns the Hasse invariant of the quadratic space V at p. This is equal to the product of local Hilbert symbols $(a_i, a_j)_p$ , $i < j$ , where $V$ is isometric to $\langle a_1, \dotsc, a_n\rangle$ . If V is degenerate return the hasse invariant of V/radical(V).

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witt_invariant — Method

is_isometric — Method

invariants — Method

Examples

For instance, for the case of $Q$ and the totally ramified prime $\mathfrak p$ of $O_K$ above $7$ , one can get:

julia> K, a = cyclotomic_real_subfield(7);

julia> Q = quadratic_space(K, K[0 1; 1 0]);

julia> OK = maximal_order(K);

julia> p = prime_decomposition(OK, 7)[1][1];

julia> hasse_invariant(Q, p), witt_invariant(Q, p)
(1, 1)

julia> Q2 = quadratic_space(K, K[-1 0; 0 1]);

julia> is_isometric(Q, Q2, p)
true

julia> is_isometric(Q, Q2)
true

julia> invariants(Q2)
(2, 0, -1, Dict{AbsSimpleNumFieldOrderIdeal, Int64}(), Tuple{InfPlc{AbsSimpleNumField, AbsSimpleNumFieldEmbedding}, Int64}[(Infinite place of real embedding with -1.80 of K, 1), (Infinite place of real embedding with -0.45 of K, 1), (Infinite place of real embedding with 1.25 of K, 1)])

Embeddings

Let $(V, \Phi)$ and $(V', \Phi')$ be two spaces over the same extension $E/K$ , and let $\sigma \colon V \to V'$ be an $E$ -linear morphism. $\sigma$ is called a representation of $V$ into $V'$ if for all $x \in V$

$\Phi'(\sigma(x), \sigma(x)) = \Phi(x,x).$

In such a case, $V$ is said to be represented by $V'$ and $\sigma$ can be seen as an embedding of $V$ into $V'$ . This representation property can be also tested locally with respect to the completions at some finite places. Note that in both quadratic and hermitian cases, completions are taken at finite places of the fixed field $K$ .

is_locally_represented_by — Method

is_locally_represented_by(U::T, V::T, p::AbsNumFieldOrderIdeal{AbsSimpleNumField, AbsSimpleNumFieldElem}) where T <: AbstractSpace -> Bool

Given two spaces U and V over the same algebra E, and a prime ideal p in the maximal order $\mathcal O_K$ of their fixed field K, return whether U is represented by V locally at p, i.e. whether $U_p$ embeds in $V_p$ .

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is_represented_by — Method

Examples

Still using the spaces $Q$ and $H$ , we can decide whether some other spaces embed respectively locally or globally into $Q$ or $H$ :

julia> K, a = cyclotomic_real_subfield(7);

julia> Kt, t = K[:t];

julia> E, b = number_field(t^2-a*t+1, :b);

julia> Q = quadratic_space(K, K[0 1; 1 0]);

julia> H = hermitian_space(E, 3);

julia> OK = maximal_order(K);

julia> p = prime_decomposition(OK, 7)[1][1];

julia> Q2 = quadratic_space(K, K[-1 0; 0 1]);

julia> H2 = hermitian_space(E, E[-1 0 0; 0 1 0; 0 0 -1]);

julia> is_locally_represented_by(Q2, Q, p)
true

julia> is_represented_by(Q2, Q)
true

julia> is_locally_represented_by(H2, H, p)
true

julia> is_represented_by(H2, H)
false

Categorical constructions

One can construct direct sums of spaces of the same kind. Since those are also direct products, they are called biproducts in this context. Depending on the user usage, one of the following three methods can be called to obtain the direct sum of a finite collection of spaces. Note that the corresponding copies of the original spaces in the direct sum are pairwise orthogonal.

direct_sum — Method

direct_sum(x::Vararg{T}) where T <: AbstractSpace -> T, Vector{AbstractSpaceMor}
direct_sum(x::Vector{T}) where T <: AbstractSpace -> T, Vector{AbstractSpaceMor}

Given a collection of quadratic or hermitian spaces $V_1, \ldots, V_n$ , return their direct sum $V := V_1 \oplus \ldots \oplus V_n$ , together with the injections $V_i \to V$ .

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

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direct_sum(M::ModuleFP{T}...; task::Symbol = :sum) where T

Given modules $M_1\dots M_n$ , say, return the direct sum $\bigoplus_{i=1}^n M_i$ .

Additionally, return

a vector containing the canonical injections $M_i\to\bigoplus_{i=1}^n M_i$ if task = :sum (default),
a vector containing the canonical projections $\bigoplus_{i=1}^n M_i\to M_i$ if task = :prod,
two vectors containing the canonical injections and projections, respectively, if task = :both,
none of the above maps if task = :none.

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direct_product — Method

direct_product(algebras::StructureConstantAlgebra...; task::Symbol = :sum)
  -> StructureConstantAlgebra, Vector{AbsAlgAssMor}, Vector{AbsAlgAssMor}
direct_product(algebras::Vector{StructureConstantAlgebra}; task::Symbol = :sum)
  -> StructureConstantAlgebra, Vector{AbsAlgAssMor}, Vector{AbsAlgAssMor}

Returns the algebra $A = A_1 \times \cdots \times A_k$ . task can be ":sum", ":prod", ":both" or ":none" and determines which canonical maps are computed as well: ":sum" for the injections, ":prod" for the projections.

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direct_product(x::Vararg{T}) where T <: AbstractSpace -> T, Vector{AbstractSpaceMor}
direct_product(x::Vector{T}) where T <: AbstractSpace -> T, Vector{AbstractSpaceMor}

Given a collection of quadratic or hermitian spaces $V_1, \ldots, V_n$ , return their direct product $V := V_1 \times \ldots \times V_n$ , together with the projections $V \to V_i$ .

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

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direct_product(F::FreeMod{T}...; task::Symbol = :prod) where T

Given free modules $F_1\dots F_n$ , say, return the direct product $\prod_{i=1}^n F_i$ .

Additionally, return

a vector containing the canonical projections $\prod_{i=1}^n F_i\to F_i$ if task = :prod (default),
a vector containing the canonical injections $F_i\to\prod_{i=1}^n F_i$ if task = :sum,
two vectors containing the canonical projections and injections, respectively, if task = :both,
none of the above maps if task = :none.

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direct_product(M::ModuleFP{T}...; task::Symbol = :prod) where T

Given modules $M_1\dots M_n$ , say, return the direct product $\prod_{i=1}^n M_i$ .

Additionally, return

a vector containing the canonical projections $\prod_{i=1}^n M_i\to M_i$ if task = :prod (default),
a vector containing the canonical injections $M_i\to\prod_{i=1}^n M_i$ if task = :sum,
two vectors containing the canonical projections and injections, respectively, if task = :both,
none of the above maps if task = :none.

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biproduct — Method

biproduct(x::Vararg{T}) where T <: AbstractSpace -> T, Vector{AbstractSpaceMor}, Vector{AbstractSpaceMor}
biproduct(x::Vector{T}) where T <: AbstractSpace -> T, Vector{AbstractSpaceMor}, Vector{AbstractSpaceMor}

Given a collection of quadratic or hermitian spaces $V_1, \ldots, V_n$ , return their biproduct $V := V_1 \oplus \ldots \oplus V_n$ , together with the injections $V_i \to V$ and the projections $V \to V_i$ .

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

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Example

julia> E, b = cyclotomic_field_as_cm_extension(7);

julia> H = hermitian_space(E, 3);

julia> H2 = hermitian_space(E, E[-1 0 0; 0 1 0; 0 0 -1]);

julia> H3, inj, proj = biproduct(H, H2)
(Hermitian space of dimension 6, AbstractSpaceMor[Map: hermitian space -> hermitian space, Map: hermitian space -> hermitian space], AbstractSpaceMor[Map: hermitian space -> hermitian space, Map: hermitian space -> hermitian space])

julia> is_one(matrix(compose(inj[1], proj[1])))
true

julia> is_zero(matrix(compose(inj[1], proj[2])))
true

Orthogonality operations

orthogonal_complement — Method

orthogonal_projection — Method

Example

julia> K, a = cyclotomic_real_subfield(7);

julia> Kt, t = K[:t];

julia> Q = quadratic_space(K, K[0 1; 1 0]);

julia> orthogonal_complement(Q, matrix(K, 1, 2, [1 0]))
[1   0]

Isotropic spaces

Let $(V, \Phi)$ be a space over $E/K$ and let $\mathfrak p$ be a place in $K$ . $V$ is said to be isotropic locally at $\mathfrak p$ if there exists an element $x \in V_{\mathfrak p}$ such that $\Phi_{\mathfrak p}(x,x) = 0$ , where $\Phi_{\mathfrak p}$ is the continuous extension of $\Phi$ to $V_{\mathfrak p} \times V_{\mathfrak p}$ .

is_isotropic — Method

Example

julia> K, a = cyclotomic_real_subfield(7);

julia> Kt, t = K[:t];

julia> E, b = number_field(t^2-a*t+1, :b);

julia> H = hermitian_space(E, 3);

julia> OK = maximal_order(K);

julia> p = prime_decomposition(OK, 7)[1][1];

julia> is_isotropic(H, p)
true

Hyperbolic spaces

Let $(V, \Phi)$ be a space over $E/K$ and let $\mathfrak p$ be a prime ideal of $\mathcal O_K$ . $V$ is said to be hyperbolic locally at $\mathfrak p$ if the completion $V_{\mathfrak p}$ of $V$ can be decomposed as an orthogonal sum of hyperbolic planes. The hyperbolic plane is the space $(H, \Psi)$ of rank 2 over $E/K$ such that there exists a basis $e_1, e_2$ of $H$ such that $\Psi(e_1, e_1) = \Psi(e_2, e_2) = 0$ and $\Psi(e_1, e_2) = 1$ .

is_locally_hyperbolic — Method

is_locally_hyperbolic(V::Hermspace, p::AbsNumFieldOrderIdeal{AbsSimpleNumField, AbsSimpleNumFieldElem}) -> Bool

Return whether the completion of the hermitian space V over $E/K$ at the prime ideal p of $\mathcal O_K$ is hyperbolic.

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Example

julia> K, a = cyclotomic_real_subfield(7);

julia> Kt, t = K[:t];

julia> E, b = number_field(t^2-a*t+1, :b);

julia> H = hermitian_space(E, 3);

julia> OK = maximal_order(K);

julia> p = prime_decomposition(OK, 7)[1][1];

julia> is_locally_hyperbolic(H, p)
false