# Affine Algebras and Their Ideals

With regard to notation, we use affine algebra as a synonym for quotient of a multivariate polynomial ring by an ideal. More specifically, if $R$ is a multivariate polynomial ring with coefficient ring $C$, and $A=R/I$ is the quotient of $R$ by an ideal $I$ of $R$, we refer to $A$ as an affine algebra over $C$, or an affine $C$-algebra. In this section, we discuss functionality for handling such algebras in OSCAR.

Note

To emphasize this point: In this section, we view $R/I$ together with its ring structure. Realizing $R/I$ as an $R$-module means to implement it as the quotient of a free $R$-module of rank 1. See the section on modules.

Note

Most functions discussed here rely on Gröbner basis techniques. In particular, they typically make use of a Gröbner basis for the modulus of the quotient. Nevertheless, the construction of quotients is lazy in the sense that the computation of such a Gröbner basis is delayed until the user performs an operation that indeed requires it (the Gröbner basis is then computed with respect to the default_ordering on the underlying polynomial ring; see the section on Gröbner/Standard Bases for default orderings in OSCAR). Once computed, the Gröbner basis is cached for later reuse.

Note

Recall that Gröbner basis methods are implemented for multivariate polynomial rings over fields (exact fields supported by OSCAR) and, where not indicated otherwise, for multivariate polynomial rings over the integers.

Note

In OSCAR, elements of a quotient $A = R/I$ are not necessarily represented by polynomials which are reduced with regard to $I$. That is, if $f\in R$ is the internal polynomial representative of an element of $A$, then $f$ may not be the normal form mod $I$ with respect to the default ordering on $R$ (see the section on Gröbner/Standard Bases for normal forms). Operations involving Gröbner basis computations may lead to (partial) reductions. The function simplify discussed in this section computes fully reduced representatives.

Note

Each grading on a multivariate polynomial ring R in OSCAR descends to a grading on the affine algebra A = R/I (recall that OSCAR ideals of graded polynomial rings are required to be homogeneous). Functionality for dealing with such gradings and our notation for describing this functionality descend accordingly. This applies, in particular, to the functions is_graded, is_standard_graded, is_z_graded, is_zm_graded, and is_positively_graded which will not be discussed again here.

## Types

The OSCAR type for quotients of multivariate polynomial rings is of parametrized form MPolyQuoRing{T}, with elements of type MPolyQuoRingElem{T}. Here, T is the element type of the polynomial ring.

## Constructors

quoMethod
quo(R::MPolyRing, I::MPolyIdeal) -> MPolyQuoRing, Map

Create the quotient ring R/I and return the new ring as well as the projection map R $\to$ R/I.

quo(R::MPolyRing, V::Vector{MPolyRingElem}) -> MPolyQuoRing, Map

As above, where I is the ideal of R generated by the polynomials in V.

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> A, p = quo(R, ideal(R, [x^2-y^3, x-y]));

julia> A
Quotient
of multivariate polynomial ring in 2 variables over QQ
by ideal(x^2 - y^3, x - y)

julia> typeof(A)
MPolyQuoRing{QQMPolyRingElem}

julia> typeof(x)
QQMPolyRingElem

julia> p
Map from
Multivariate polynomial ring in 2 variables over QQ to A defined by a julia-function with inverse

julia> p(x)
x

julia> typeof(p(x))
MPolyQuoRingElem{QQMPolyRingElem}
julia> S, (x, y, z) = graded_polynomial_ring(QQ, ["x", "y", "z"]);

julia> B, _ = quo(S, ideal(S, [x^2*z-y^3, x-y]))
(Quotient of multivariate polynomial ring by ideal with 2 generators, Map from
S to B defined by a julia-function with inverse)

julia> typeof(B)
MPolyQuoRing{MPolyDecRingElem{QQFieldElem, QQMPolyRingElem}}
source

## Data Associated to Affine Algebras

### Basic Data

If A=R/I is the quotient of a multivariate polynomial ring R modulo an ideal I of R, then

• base_ring(A) refers to R,
• modulus(A) to I,
• gens(A) to the generators of A,
• ngens(A) to the number of these generators, and
• gen(A, i) as well as A[i] to the i-th such generator.
###### Examples
julia> R, (x, y, z) = polynomial_ring(QQ, ["x", "y", "z"]);

julia> A, _ = quo(R, ideal(R, [y-x^2, z-x^3]));

julia> base_ring(A)
Multivariate polynomial ring in 3 variables x, y, z
over rational field

julia> modulus(A)
ideal(-x^2 + y, -x^3 + z)

julia> gens(A)
3-element Vector{MPolyQuoRingElem{QQMPolyRingElem}}:
x
y
z

julia> ngens(A)
3

julia> gen(A, 2)
y


grading_groupMethod
grading_group(A::MPolyQuoRing{<:MPolyDecRingElem})

If A is, say, G-graded, return G.

Examples

julia> R, (x, y, z) = graded_polynomial_ring(QQ, ["x", "y", "z"]);

julia> A, _ = quo(R, ideal(R, [x^2*z-y^3, x-y]));

GrpAb: Z
source
homogeneous_componentMethod
homogeneous_component(A::MPolyQuoRing{<:MPolyDecRingElem}, g::GrpAbFinGenElem)

Given a graded quotient A of a multivariate polynomial ring over a field, where the grading group is free of type GrpAbFinGen, and given an element g of that group, return the homogeneous component of A of degree g. Additionally, return the embedding of the component into A.

homogeneous_component(A::MPolyQuoRing{<:MPolyDecRingElem}, g::Vector{<:IntegerUnion})

Given a $\mathbb Z^m$-graded quotient A of a multivariate polynomial ring over a field, and given a vector g of $m$ integers, convert g into an element of the grading group of A, and return the homogeneous component of A whose degree is that element. Additionally, return the embedding of the component into A.

homogeneous_component(A::MPolyQuoRing{<:MPolyDecRingElem}, g::IntegerUnion)

Given a $\mathbb Z$-graded quotient A of a multivariate polynomial ring over a field, and given an integer g, convert g into an element of the grading group of A, and return the homogeneous component of A whose degree is that element. Additionally, return the embedding of the component into A.

Note

If the component is not finite dimensional, an error message will be thrown.

Examples

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, ["w", "x", "y", "z"])
(Graded multivariate polynomial ring in 4 variables over QQ, MPolyDecRingElem{QQFieldElem, QQMPolyRingElem}[w, x, y, z])

julia> L = homogeneous_component(R, 2);

julia> HC = gens(L);

julia> EMB = L
Map from
R_ of dim 10 to R defined by a julia-function with inverse

julia> for i in 1:length(HC) println(EMB(HC[i])) end
z^2
y*z
y^2
x*z
x*y
x^2
w*z
w*y
w*x
w^2

julia> PTC = ideal(R, [-x*z + y^2, -w*z + x*y, -w*y + x^2]);

julia> A, _ = quo(R, PTC);

julia> L = homogeneous_component(A, 2);

julia> HC = gens(L);

julia> EMB = L
Map from
Quotient space over:
Rational field with 7 generators and no relations to A defined by a julia-function with inverse

julia> for i in 1:length(HC) println(EMB(HC[i])) end
z^2
y*z
x*z
w*z
w*y
w*x
w^2
julia> R, x, y = polynomial_ring(QQ, "x" => 1:2, "y" => 1:3);

julia> G = abelian_group([0, 0])
GrpAb: Z^2

julia> g = gens(G)
2-element Vector{GrpAbFinGenElem}:
Element of G with components [1 0]
Element of G with components [0 1]

julia> W = [g, g, g, g, g];

julia> S, _ = grade(R, W);

julia> L = homogeneous_component(S, [2,1]);

julia> HC = gens(L);

julia> EMB = L
Map from
homogeneous component of Graded multivariate polynomial ring in 5 variables over QQ of degree Element of G with components [2 1]
to S defined by a julia-function with inverse

julia> for i in 1:length(HC) println(EMB(HC[i])) end
x^2*y
x^2*y
x^2*y
x*x*y
x*x*y
x*x*y
x^2*y
x^2*y
x^2*y

julia> I = ideal(S, [x*y-x*y]);

julia> A, = quo(S, I);

julia> L = homogeneous_component(A, [2,1]);

julia> HC = gens(L);

julia> EMB = L
Map from
Quotient space over:
Rational field with 7 generators and no relations to A defined by a julia-function with inverse

julia> for i in 1:length(HC) println(EMB(HC[i])) end
x^2*y
x^2*y
x^2*y
x*x*y
x*x*y
x^2*y
x^2*y
source

### Dimension

dimMethod
dim(A::MPolyQuoRing)

Return the Krull dimension of A.

Examples

julia> R, (x, y, z) = polynomial_ring(QQ, ["x", "y", "z"]);

julia> A, _ = quo(R, ideal(R, [y-x^2, z-x^3]));

julia> dim(A)
1
source
vdimMethod
vdim(A::MPolyQuoRing)

If, say, A = R/I, where R is a multivariate polynomial ring over a field K, and I is an ideal of R, return the dimension of A as a K-vector space if I is zero-dimensional. Return -1, otherwise.

Examples

julia> R, (x, y, z) = polynomial_ring(QQ, ["x", "y", "z"]);

julia> A, _ = quo(R, ideal(R, [x^3+y^3+z^3-1, x^2+y^2+z^2-1, x+y+z-1]));

julia> vdim(A)
6

julia> I = modulus(A)
ideal(x^3 + y^3 + z^3 - 1, x^2 + y^2 + z^2 - 1, x + y + z - 1)

julia> groebner_basis(I, ordering = lex(base_ring(I)))
Gröbner basis with elements
1 -> z^3 - z^2
2 -> y^2 + y*z - y + z^2 - z
3 -> x + y + z - 1
with respect to the ordering
lex([x, y, z])
source

## Elements of Affine Algebras

### Types

The OSCAR type for elements of quotients of multivariate polynomial rings is of parametrized form MPolyQuoRing{T}, where T is the element type of the polynomial ring.

### Creating Elements of Affine Algebras

Elements of an affine algebra A=R/I are created as images of elements of R under the projection map or by directly coercing elements of R into A.

###### Examples
julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> A, p = quo(R, ideal(R, [x^3*y^2-y^3*x^2, x*y^4-x*y^2]));

julia> f = p(x^3*y^2-y^3*x^2+x*y)
x^3*y^2 - x^2*y^3 + x*y

julia> typeof(f)
MPolyQuoRingElem{QQMPolyRingElem}

julia> g = A(x^3*y^2-y^3*x^2+x*y)
x^3*y^2 - x^2*y^3 + x*y

julia> f == g
true


### Reducing Polynomial Representatives

simplifyMethod
simplify(f::MPolyQuoRingElem{T}) where {S<:Union{FieldElem, ZZRingElem}, T<:MPolyRingElem{S}}

If f is an element of the quotient of a multivariate polynomial ring R by an ideal I of R, say, replace the internal polynomial representative of f by its normal form mod I with respect to the default_ordering on R.

Note

Since this method only has a computational backend for quotients of polynomial rings over a field, it is not implemented generically.

Examples

julia> R, (x,) = polynomial_ring(QQ, ["x"]);

julia> A, p = quo(R, ideal(R, [x^4]));

julia> f = p(2*x^6 + x^3 + x)
2*x^6 + x^3 + x

julia> simplify(f)
x^3 + x

julia> f
x^3 + x
source

### Tests on Elements of Affine Algebras

==Method
==(f::MPolyQuoRingElem{T}, g::MPolyQuoRingElem{T}) where T

Return true if f is equal to g, false otherwise.

Examples

julia> R, (x,) = polynomial_ring(QQ, ["x"]);

julia> A, p = quo(R, ideal(R, [x^4]));

julia> f = p(x-x^6)
-x^6 + x

julia> g = p(x)
x

julia> f == g
true
source

is_homogeneousMethod
is_homogeneous(f::MPolyQuoRingElem{<:MPolyDecRingElem})

Given an element f of a graded affine algebra, return true if f is homogeneous, false otherwise.

Examples

julia> R, (x, y, z) = graded_polynomial_ring(QQ, ["x", "y", "z"]);

julia> A, p = quo(R, ideal(R, [y-x, z^3-x^3]));

julia> f = p(y^2-x^2+z^4)
-x^2 + y^2 + z^4

julia> is_homogeneous(f)
true

julia> f
z^4
source

### Data associated to Elements of Affine Algebras

Given an element f of an affine algebra A,

• parent(f) refers to A.

In the graded case, we also have:

homogeneous_componentsMethod
homogeneous_components(f::MPolyQuoRingElem{<:MPolyDecRingElem})

Given an element f of a graded affine algebra, return the homogeneous components of f.

Examples

julia> R, (x, y, z) = graded_polynomial_ring(QQ, ["x", "y", "z"]);

julia> A, p = quo(R, ideal(R, [y-x, z^3-x^3]));

julia> f = p(y^2-x^2+x*y*z+z^4)
-x^2 + x*y*z + y^2 + z^4

julia> homogeneous_components(f)
Dict{GrpAbFinGenElem, MPolyQuoRingElem{MPolyDecRingElem{QQFieldElem, QQMPolyRingElem}}} with 2 entries:
 => z^4
 => y^2*z
source
homogeneous_componentMethod
homogeneous_component(f::MPolyQuoRingElem{<:MPolyDecRingElem}, g::GrpAbFinGenElem)

Given an element f of a graded affine algebra, and given an element g of the grading group of that algebra, return the homogeneous component of f of degree g.

homogeneous_component(f::MPolyQuoRingElem{<:MPolyDecRingElem}, g::Vector{<:IntegerUnion})

Given an element f of a $\mathbb Z^m$-graded affine algebra A, say, and given a vector g of $m$ integers, convert g into an element of the grading group of A, and return the homogeneous component of f whose degree is that element.

homogeneous_component(f::MPolyQuoRingElem{<:MPolyDecRingElem}, g::IntegerUnion)

Given an element f of a $\mathbb Z$-graded affine algebra A, say, and given an integer g, convert g into an element of the grading group of A, and return the homogeneous component of f whose degree is that element.

Examples

julia> R, (x, y, z) = graded_polynomial_ring(QQ, ["x", "y", "z"]);

julia> A, p = quo(R, ideal(R, [y-x, z^3-x^3]));

julia> f = p(y^2-x^2+x*y*z+z^4)
-x^2 + x*y*z + y^2 + z^4

julia> homogeneous_component(f, 4)
z^4
source
degreeMethod
degree(f::MPolyQuoRingElem{<:MPolyDecRingElem})

Given a homogeneous element f of a graded affine algebra, return the degree of f.

degree(::Type{Vector{Int}}, f::MPolyQuoRingElem{<:MPolyDecRingElem})

Given a homogeneous element f of a $\mathbb Z^m$-graded affine algebra, return the degree of f, converted to a vector of integer numbers.

degree(::Type{Int}, f::MPolyQuoRingElem{<:MPolyDecRingElem})

Given a homogeneous element f of a $\mathbb Z$-graded affine algebra, return the degree of f, converted to an integer number.

Examples

julia> R, (x, y, z) = graded_polynomial_ring(QQ, ["x", "y", "z"] );

julia> A, p = quo(R, ideal(R, [y-x, z^3-x^3]))
(Quotient of multivariate polynomial ring by ideal with 2 generators, Map from
R to A defined by a julia-function with inverse)

julia> f = p(y^2-x^2+z^4)
-x^2 + y^2 + z^4

julia> degree(f)

julia> typeof(degree(f))
GrpAbFinGenElem

julia> degree(Int, f)
4

julia> typeof(degree(Int, f))
Int64
source

## Ideals in Affine Algebras

### Constructors

idealMethod
ideal(A::MPolyQuoRing{T}, V::Vector{T}) where T <: MPolyRingElem

Given a (graded) quotient ring A=R/I and a vector V of (homogeneous) polynomials in R, create the ideal of A which is generated by the images of the entries of V.

ideal(A::MPolyQuoRing{T}, V::Vector{MPolyQuoRingElem{T}}) where T <: MPolyRingElem

Given a (graded) quotient ring A and a vector V of (homogeneous) elements of A, create the ideal of A which is generated by the entries of V.

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> A, _ = quo(R, ideal(R, [x^2-y^3, x-y]));

julia> I = ideal(A, [x^2-y])
ideal(x^2 - y)
julia> S, (x, y, z) = graded_polynomial_ring(QQ, ["x", "y", "z"]);

julia> B, _ = quo(S, ideal(S, [x^2*z-y^3, x-y]));

julia> J = ideal(B, [x^2-y^2])
ideal(x^2 - y^2)
source

### Reducing Polynomial Representatives of Generators

simplifyMethod
simplify(a::MPolyQuoIdeal)

If a is an ideal of the quotient of a multivariate polynomial ring R by an ideal I of R, say, replace the internal polynomial representative of each generator of a by its normal form mod I with respect to the default_ordering on R.

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> A, _ = quo(R, ideal(R, [x^3*y^2-y^3*x^2, x*y^4-x*y^2]));

julia> a = ideal(A, [x^3*y^4-x+y, x*y+y^2*x])
ideal(x^3*y^4 - x + y, x*y^2 + x*y)

julia> gens(a)
2-element Vector{MPolyQuoRingElem{QQMPolyRingElem}}:
x^3*y^4 - x + y
x*y^2 + x*y

julia> simplify(a)
ideal(x^2*y^3 - x + y, x*y^2 + x*y)

julia> gens(a)
2-element Vector{MPolyQuoRingElem{QQMPolyRingElem}}:
x^2*y^3 - x + y
x*y^2 + x*y
source

### Data Associated to Ideals in Affine Algebras

#### Basic Data

If a is an ideal of the affine algebra A, then

• base_ring(a) refers to A,
• gens(a) to the generators of a,
• ngens(a) to the number of these generators, and
• gen(a, i) as well as a[i] to the i-th such generator.
###### Examples
julia> R, (x, y, z) = polynomial_ring(QQ, ["x", "y", "z"]);

julia> A, _ = quo(R, ideal(R, [y-x^2, z-x^3]));

julia> a = ideal(A, [x-y, z^4])
ideal(x - y, z^4)

julia> base_ring(a)
Quotient
of multivariate polynomial ring in 3 variables over QQ
by ideal(-x^2 + y, -x^3 + z)

julia> gens(a)
2-element Vector{MPolyQuoRingElem{QQMPolyRingElem}}:
x - y
z^4

julia> ngens(a)
2

julia> gen(a, 2)
z^4


#### Dimension of Ideals in Affine Algebras

dimMethod
dim(a::MPolyQuoIdeal)

Return the Krull dimension of a.

Examples

julia> R, (x, y, z) = polynomial_ring(QQ, ["x", "y", "z"]);

julia> A, _ = quo(R, ideal(R, [y-x^2, z-x^3]));

julia> a = ideal(A, [x-y])
ideal(x - y)

julia> dim(a)
0
source

#### Minimal Sets of Generators

In the graded case, we have:

minimal_generating_setMethod
minimal_generating_set(I::MPolyQuoIdeal{<:MPolyDecRingElem})

Given a homogeneous ideal I of a graded affine algebra over a field, return an array containing a minimal set of generators of I. If I is the zero ideal an empty list is returned.

Examples

julia> R, (x, y, z) = graded_polynomial_ring(QQ, ["x", "y", "z"]);

julia> A, p = quo(R, ideal(R, [x-y]));

julia> V = [x, z^2, x^3+y^3, y^4, y*z^5];

julia> a = ideal(A, V);

julia> minimal_generating_set(a)
2-element Vector{MPolyQuoRingElem{MPolyDecRingElem{QQFieldElem, QQMPolyRingElem}}}:
y
z^2

julia> a = ideal(A, [x-y])
ideal(x - y)

julia> minimal_generating_set(a)
MPolyQuoRingElem{MPolyDecRingElem{QQFieldElem, QQMPolyRingElem}}[]
source

### Operations on Ideals in Affine Algebras

#### Simple Ideal Operations in Affine Algebras

##### Powers of Ideal
^Method
:^(a::MPolyQuoIdeal, m::Int)

Return the m-th power of a.

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> A, _ = quo(R, [x^2-y, y^2-x+y]);

julia> a = ideal(A, [x+y])
ideal(x + y)

julia> a^2
ideal(x^2 + 2*x*y + y^2)
source
##### Sum of Ideals
+Method
:+(a::MPolyQuoIdeal{T}, b::MPolyQuoIdeal{T}) where T

Return the sum of a and b.

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> A, _ = quo(R, [x^2-y, y^2-x+y]);

julia> a = ideal(A, [x+y])
ideal(x + y)

julia> b = ideal(A, [x^2+y^2, x+y])
ideal(x^2 + y^2, x + y)

julia> a+b
ideal(x + y, x^2 + y^2)
source
##### Product of Ideals
*Method
:*(a::MPolyQuoIdeal{T}, b::MPolyQuoIdeal{T}) where T

Return the product of a and b.

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> A, _ = quo(R, [x^2-y, y^2-x+y]);

julia> a = ideal(A, [x+y])
ideal(x + y)

julia> b = ideal(A, [x^2+y^2, x+y])
ideal(x^2 + y^2, x + y)

julia> a*b
ideal(x^3 + x^2*y + x*y^2 + y^3, x^2 + 2*x*y + y^2)
source

#### Intersection of Ideals

intersectMethod
intersect(a::MPolyQuoIdeal{T}, bs::MPolyQuoIdeal{T}...) where T
intersect(V::Vector{MPolyQuoIdeal{T}}) where T

Return the intersection of two or more ideals.

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> A, _ = quo(R, ideal(R, [x^2-y^3, x-y]));

julia> a = ideal(A, [y^2])
ideal(y^2)

julia> b = ideal(A, [x])
ideal(x)

julia> intersect(a,b)
ideal(x*y)

julia> intersect([a,b])
ideal(x*y)
source
intersectMethod
intersect(I::MPolyIdeal{T}, Js::MPolyIdeal{T}...) where T
intersect(V::Vector{MPolyIdeal{T}}) where T

Return the intersection of two or more ideals.

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"])
(Multivariate polynomial ring in 2 variables over QQ, QQMPolyRingElem[x, y])

julia> I = ideal(R, [x, y])^2;

julia> J = ideal(R, [y^2-x^3+x]);

julia> intersect(I, J)
ideal(x^3*y - x*y - y^3, x^4 - x^2 - x*y^2)

julia> intersect([I, J])
ideal(x^3*y - x*y - y^3, x^4 - x^2 - x*y^2)
source
intersect(a::MPolyQuoIdeal{T}, bs::MPolyQuoIdeal{T}...) where T
intersect(V::Vector{MPolyQuoIdeal{T}}) where T

Return the intersection of two or more ideals.

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> A, _ = quo(R, ideal(R, [x^2-y^3, x-y]));

julia> a = ideal(A, [y^2])
ideal(y^2)

julia> b = ideal(A, [x])
ideal(x)

julia> intersect(a,b)
ideal(x*y)

julia> intersect([a,b])
ideal(x*y)
source
intersect(I::PBWAlgIdeal{D, T, S}, Js::PBWAlgIdeal{D, T, S}...) where {D, T, S}
intersect(V::Vector{PBWAlgIdeal{D, T, S}}) where {D, T, S}

Return the intersection of two or more ideals.

Examples

julia> D, (x, y, dx, dy) = weyl_algebra(QQ, ["x", "y"]);

julia> I = intersect(left_ideal(D, [x^2, x*dy, dy^2])+left_ideal(D, [dx]), left_ideal(D, [dy^2-x^3+x]))
left_ideal(-x^3 + dy^2 + x)
source
intersect(M::SubquoModule{T}, N::SubquoModule{T}) where T

Given subquotients M and N such that ambient_module(M) == ambient_module(N), return the intersection of M and N regarded as submodules of the common ambient module.

Additionally, return the inclusion maps M $\cap$ N $\to$ M and M $\cap$ N $\to$ N.

Examples

julia> R, (x, y, z) = polynomial_ring(QQ, ["x", "y", "z"])
(Multivariate polynomial ring in 3 variables over QQ, QQMPolyRingElem[x, y, z])

julia> F = free_module(R, 1)
Free module of rank 1 over Multivariate polynomial ring in 3 variables over QQ

julia> AM = R[x;]
[x]

julia> BM = R[x^2; y^3; z^4]
[x^2]
[y^3]
[z^4]

julia> M = SubquoModule(F, AM, BM)
Subquotient of Submodule with 1 generator
1 -> x*e
by Submodule with 3 generators
1 -> x^2*e
2 -> y^3*e
3 -> z^4*e

julia> AN = R[y;]
[y]

julia> BN = R[x^2; y^3; z^4]
[x^2]
[y^3]
[z^4]

julia> N = SubquoModule(F, AN, BN)
Subquotient of Submodule with 1 generator
1 -> y*e
by Submodule with 3 generators
1 -> x^2*e
2 -> y^3*e
3 -> z^4*e

julia> intersect(M, N)
(Subquotient of Submodule with 2 generators
1 -> -x*y*e
2 -> x*z^4*e
by Submodule with 3 generators
1 -> x^2*e
2 -> y^3*e
3 -> z^4*e, Map with following data
Domain:
=======
Subquotient of Submodule with 2 generators
1 -> -x*y*e
2 -> x*z^4*e
by Submodule with 3 generators
1 -> x^2*e
2 -> y^3*e
3 -> z^4*e
Codomain:
=========
Subquotient of Submodule with 1 generator
1 -> x*e
by Submodule with 3 generators
1 -> x^2*e
2 -> y^3*e
3 -> z^4*e
, Map with following data
Domain:
=======
Subquotient of Submodule with 2 generators
1 -> -x*y*e
2 -> x*z^4*e
by Submodule with 3 generators
1 -> x^2*e
2 -> y^3*e
3 -> z^4*e
Codomain:
=========
Subquotient of Submodule with 1 generator
1 -> y*e
by Submodule with 3 generators
1 -> x^2*e
2 -> y^3*e
3 -> z^4*e
)
julia> R, _ = polynomial_ring(QQ, ["x", "y", "z"]);

julia> Z = abelian_group(0);

julia> Rg, (x, y, z) = grade(R, [Z,Z,Z]);

julia> AM = Rg[x;];

julia> BM = Rg[x^2; y^3; z^4];

julia> M = SubquoModule(F, AM, BM)
Graded subquotient of submodule of F generated by
1 -> x*e
by submodule of F generated by
1 -> x^2*e
2 -> y^3*e
3 -> z^4*e

julia> AN = Rg[y;];

julia> BN = Rg[x^2; y^3; z^4];

julia> N = SubquoModule(F, AN, BN)
Graded subquotient of submodule of F generated by
1 -> y*e
by submodule of F generated by
1 -> x^2*e
2 -> y^3*e
3 -> z^4*e

julia> intersect(M, N)
(Graded subquotient of submodule of F generated by
1 -> -x*y*e
2 -> x*z^4*e
by submodule of F generated by
1 -> x^2*e
2 -> y^3*e
3 -> z^4*e, Graded subquotient of submodule of F generated by
1 -> -x*y*e
2 -> x*z^4*e
by submodule of F generated by
1 -> x^2*e
2 -> y^3*e
3 -> z^4*e -> M
-x*y*e -> -x*y*e
x*z^4*e -> x*z^4*e
Homogeneous module homomorphism, Graded subquotient of submodule of F generated by
1 -> -x*y*e
2 -> x*z^4*e
by submodule of F generated by
1 -> x^2*e
2 -> y^3*e
3 -> z^4*e -> N
-x*y*e -> x*y*e
x*z^4*e -> 0
Homogeneous module homomorphism)

source
intersect(T1, T2)

Intersect two tropical varieties.

Examples

julia> RR = TropicalSemiring(min)
Tropical semiring (min)

julia> S,(x,y) = RR["x","y"]
(Multivariate polynomial ring in 2 variables over tropical semiring (min), AbstractAlgebra.Generic.MPoly{Oscar.TropicalSemiringElem{typeof(min)}}[x, y])

julia> f1 = x+y+1
x + y + (1)

julia> f2 = x^2+y^2+RR(-6)
x^2 + y^2 + (-6)

julia> hyp1 = TropicalHypersurface(f1)
min tropical hypersurface embedded in 2-dimensional Euclidean space

julia> hyp2 = TropicalHypersurface(f2)
min tropical hypersurface embedded in 2-dimensional Euclidean space

julia> tv12 = intersect(hyp1, hyp2)
min tropical variety of dimension 1 embedded in 2-dimensional Euclidean space
source
intersect(a::AlgAssAbsOrdIdl, b::AlgAssAbsOrdIdl) -> AlgAssAbsOrdIdl

Returns $a \cap b$.

intersect(a::AlgAssRelOrdIdl, b::AlgAssRelOrdIdl) -> AlgAssRelOrdIdl

Returns $a \cap b$.

#### Ideal Quotients

quotientMethod
quotient(a::MPolyQuoIdeal{T}, b::MPolyQuoIdeal{T}) where T

Return the ideal quotient of a by b. Alternatively, use a:b.

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> A, _ = quo(R, ideal(R, [x^2-y^3, x-y]));

julia> a = ideal(A, [y^2])
ideal(y^2)

julia> b = ideal(A, [x])
ideal(x)

julia> a:b
ideal(y)
source

### Tests on Ideals in Affine Algebras

#### Basic Tests

is_zeroMethod
is_zero(a::MPolyQuoIdeal)

Return true if a is the zero ideal, false otherwise.

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> A, _ = quo(R, [x^2-y, y^2-x+y]);

julia> a = ideal(A, [x^2+y^2, x+y])
ideal(x^2 + y^2, x + y)

julia> is_zero(a)
false

julia> b = ideal(A, [x^2-y])
ideal(x^2 - y)

julia> is_zero(b)
true
source

#### Containment of Ideals in Affine Algebras

is_subsetMethod
is_subset(a::MPolyQuoIdeal{T}, b::MPolyQuoIdeal{T}) where T

Return true if a is contained in b, false otherwise.

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> A, _ = quo(R, ideal(R, [x^3*y^2-y^3*x^2, x*y^4-x*y^2]));

julia> a = ideal(A, [x^3*y^4-x+y, x*y+y^2*x])
ideal(x^3*y^4 - x + y, x*y^2 + x*y)

julia> b = ideal(A, [x^3*y^3-x+y, x^2*y+y^2*x])
ideal(x^3*y^3 - x + y, x^2*y + x*y^2)

julia> is_subset(a,b)
false

julia> is_subset(b,a)
true
source

#### Equality of Ideals in Affine Algebras

==Method
==(a::MPolyQuoIdeal{T}, b::MPolyQuoIdeal{T}) where T

Return true if a is equal to b, false otherwise.

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> A, _ = quo(R, ideal(R, [x^3*y^2-y^3*x^2, x*y^4-x*y^2]));

julia> a = ideal(A, [x^3*y^4-x+y, x*y+y^2*x])
ideal(x^3*y^4 - x + y, x*y^2 + x*y)

julia> b = ideal(A, [x^3*y^3-x+y, x^2*y+y^2*x])
ideal(x^3*y^3 - x + y, x^2*y + x*y^2)

julia> a == b
false
source

#### Ideal Membership

ideal_membershipMethod
ideal_membership(f::MPolyQuoRingElem{T}, a::MPolyQuoIdeal{T}) where T

Return true if f is contained in a, false otherwise. Alternatively, use f in a.

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> A, _ = quo(R, ideal(R, [x^3*y^2-y^3*x^2, x*y^4-x*y^2]));

julia> a = ideal(A, [x^3*y^4-x+y, x*y+y^2*x])
ideal(x^3*y^4 - x + y, x*y^2 + x*y)

julia> f = A(x^2*y^3-x+y)
x^2*y^3 - x + y

julia> f in a
true
source

## Homomorphisms From Affine Algebras

If $A=R/I$ is an affine $C$-algebra, and $S$ is any ring, then defining a ring homomorphism $\overline{\phi}: A \to S$ means to define a ring homomorphism $\phi: R \to S$ such that $I\subset \ker(\phi)$. Thus, $\overline{\phi}$ is determined by specifying its restriction to $C$, and by assigning an image to each generator of $A$. In OSCAR, such homomorphisms are created as follows:

homMethod
hom(A::MPolyQuoRing, S::NCRing, coeff_map, images::Vector; check::Bool = true)

hom(A::MPolyQuoRing, S::NCRing, images::Vector; check::Bool = true)

Given a homomorphism coeff_map from C to S, where C is the coefficient ring of the base ring of A, and given a vector images of ngens(A) elements of S, return the homomorphism A $\to$ S whose restriction to C is coeff_map, and which sends the i-th generator of A to the i-th entry of images.

If no coefficient map is entered, invoke a canonical homomorphism of C to S, if such a homomorphism exists, and throw an error, otherwise.

Note

The function returns a well-defined homomorphism A $\to$ S iff the given data defines a homomorphism from the base ring of A to S whose kernel contains the modulus of A. This condition is checked by the function in case check = true (default).

Note

In case check = true (default), the function also checks the conditions below:

• If S is graded, the assigned images must be homogeneous with respect to the given grading.
• If S is noncommutative, the assigned images must pairwise commute.

Examples

julia> R, (x, y, z) = polynomial_ring(QQ, ["x", "y", "z"] );

julia> A, _ = quo(R, ideal(R, [y-x^2, z-x^3]));

julia> S, (s, t) = polynomial_ring(QQ, ["s", "t"]);

julia> F = hom(A, S, [s, s^2, s^3])
Map with following data
Domain:
=======
A
Codomain:
=========
Multivariate polynomial ring in 2 variables over QQ
source

Given a ring homomorphism F : R $\to$ S as above, domain(F) and codomain(F) refer to R and S, respectively. Given ring homomorphisms F : R $\to$ S and G : S $\to$ T as above, compose(F, G) refers to their composition.

## Homomorphisms of Affine Algebras

The OSCAR homomorphism type AffAlgHom models ring homomorphisms R $\to$ S such that the type of both R and S is a subtype of Union{MPolyRing{T}, MPolyQuoRing{U}}, where T <: FieldElem and U <: MPolyRingElem{T}. Functionality for these homomorphism is discussed in what follows.

### Data Associated to Homomorphisms of Affine Algebras

preimageMethod
preimage(F::AffAlgHom, I::U) where U <: Union{MPolyIdeal, MPolyQuoIdeal}

Return the preimage of the ideal I under F.

source
###### Examples
julia> D1, (w, x, y, z) = graded_polynomial_ring(QQ, ["w", "x", "y", "z"]);

julia> C1, (s,t) = graded_polynomial_ring(QQ, ["s", "t"]);

julia> V1 = [s^3, s^2*t, s*t^2, t^3];

julia> para = hom(D1, C1, V1)
Map with following data
Domain:
=======
Graded multivariate polynomial ring in 4 variables over QQ
Codomain:
=========
Graded multivariate polynomial ring in 2 variables over QQ

julia> twistedCubic = kernel(para)
ideal(-x*z + y^2, -w*z + x*y, -w*y + x^2)

julia> C2, p2 = quo(D1, twistedCubic);

julia> D2, (a, b, c) = graded_polynomial_ring(QQ, ["a", "b", "c"]);

julia> V2 = [p2(w-y), p2(x), p2(z)];

julia> proj = hom(D2, C2, V2)
Map with following data
Domain:
=======
Graded multivariate polynomial ring in 3 variables over QQ
Codomain:
=========
Quotient of multivariate polynomial ring by ideal with 3 generators

julia> nodalCubic = kernel(proj)
ideal(-a^2*c + b^3 - 2*b^2*c + b*c^2)

julia> D3,y = polynomial_ring(QQ, "y" => 1:3);

julia> C3, x = polynomial_ring(QQ, "x" => 1:3);

julia> V3 = [x*x, x*x, x*x];

julia> F3 = hom(D3, C3, V3)
Map with following data
Domain:
=======
Multivariate polynomial ring in 3 variables over QQ
Codomain:
=========
Multivariate polynomial ring in 3 variables over QQ

julia> sphere = ideal(C3, [x^3 + x^3  + x^3 - 1])
ideal(x^3 + x^3 + x^3 - 1)

julia> steinerRomanSurface = preimage(F3, sphere)
ideal(y^6*y^6 + 2*y^6*y^3*y^3 + y^6*y^6 + 2*y^3*y^6*y^3 + 2*y^3*y^3*y^6 - y^3*y^3*y^3 + y^6*y^6)


### Tests on Homomorphisms of Affine Algebras

is_injectiveMethod
is_injective(F::AffAlgHom)

Return true if F is injective, false otherwise.

source
is_surjectiveMethod
is_surjective(F::AffAlgHom)

Return true if F is surjective, false otherwise.

source
is_bijectiveMethod
is_bijective(F::AffAlgHom)

Return true if F is bijective, false otherwise.

source
is_finiteMethod
is_finite(F::AffAlgHom)

Return true if F is finite, false otherwise.

source
###### Examples
julia> D, (x, y, z) = polynomial_ring(QQ, ["x", "y", "z"]);

julia> S, (a, b, c) = polynomial_ring(QQ, ["a", "b", "c"]);

julia> C, p = quo(S, ideal(S, [c-b^3]));

julia> V = [p(2*a + b^6), p(7*b - a^2), p(c^2)];

julia> F = hom(D, C, V)
Map with following data
Domain:
=======
Multivariate polynomial ring in 3 variables over QQ
Codomain:
=========
Quotient of multivariate polynomial ring by ideal with 1 generator

julia> is_surjective(F)
true

julia> D1, _ = quo(D, kernel(F));

julia> F1 = hom(D1, C, V);

julia> is_bijective(F1)
true

julia> R, (x, y, z) = polynomial_ring(QQ, [ "x", "y", "z"]);

julia> C, (s, t) = polynomial_ring(QQ, ["s", "t"]);

julia> V = [s*t, t, s^2];

julia> paraWhitneyUmbrella = hom(R, C, V)
Map with following data
Domain:
=======
Multivariate polynomial ring in 3 variables over QQ
Codomain:
=========
Multivariate polynomial ring in 2 variables over QQ

julia> D, _ = quo(R, kernel(paraWhitneyUmbrella));

julia> is_finite(hom(D, C, V))
true

### Inverting Homomorphisms of Affine Algebras

inverseMethod
inverse(F::AffAlgHom)

If F is bijective, return its inverse.

Examples

julia> D1, (x, y, z) = polynomial_ring(QQ, ["x", "y", "z"]);

julia> D, _ = quo(D1, [y-x^2, z-x^3]);

julia> C, (t,) = polynomial_ring(QQ, ["t"]);

julia> F = hom(D, C, [t, t^2, t^3]);

julia> is_bijective(F)
true

julia> G = inverse(F)
Map with following data
Domain:
=======
Multivariate polynomial ring in 1 variable over QQ
Codomain:
=========
D

julia> G(t)
x
source

## Subalgebras

### Subalgebra Membership

subalgebra_membershipMethod
subalgebra_membership(f::T, V::Vector{T}) where T <: Union{MPolyRingElem, MPolyQuoRingElem}

Given an element f of a multivariate polynomial ring over a field, or of a quotient of such a ring, and given a vector V of further elements of that ring, consider the subalgebra generated by the entries of V in the given ring. If f is contained in the subalgebra, return (true, h), where h is giving the polynomial relation. Return, (false, 0), otherwise.

Examples

julia> R, x = polynomial_ring(QQ, "x" => 1:3);

julia> f = x^6*x^6-x^6*x^6;

julia> V = [x^3*x^3-x^3*x^3, x^3*x^3+x^3*x^3]
2-element Vector{QQMPolyRingElem}:
x^3*x^3 - x^3*x^3
x^3*x^3 + x^3*x^3

julia> subalgebra_membership(f, V)
(true, t1*t2)
source

### Minimal Subalgebra Generators

minimal_subalgebra_generatorsMethod
minimal_subalgebra_generators(V::Vector{T}; check::Bool = true) where T <: Union{MPolyRingElem, MPolyQuoRingElem}

Given a vector V of homogeneous elements of a positively graded multivariate polynomial ring, or of a quotient of such a ring, return a minimal subset of the elements in V which, in the given ring, generate the same subalgebra as all elements in V.

If check is true (default), the conditions on V and the given ring are checked.

Examples

julia> R, (x, y) = graded_polynomial_ring(QQ, ["x", "y"]);

julia> V = [x, y, x^2+y^2]
3-element Vector{MPolyDecRingElem{QQFieldElem, QQMPolyRingElem}}:
x
y
x^2 + y^2

julia> minimal_subalgebra_generators(V)
2-element Vector{MPolyDecRingElem{QQFieldElem, QQMPolyRingElem}}:
x
y
source

## Noether Normalization

noether_normalizationMethod
noether_normalization(A::MPolyQuoRing)

Given an affine algebra $A=R/I$ over a field $K$, return a triple $(V,F,G)$ such that:

• $V$ is a vector of $d=\dim A$ elements of $A$, represented by linear forms $l_i\in R$, and such that $K[V]\hookrightarrow A$ is a Noether normalization for $A$;
• $F: A=R/I \to B = R/\phi(I)$ is an isomorphism, induced by a linear change $\phi$ of coordinates of $R$ which maps the $l_i$ to the the last $d$ variables of $R$;
• $G = F^{-1}.$
Warning

The algorithm may not terminate over a small finite field. If it terminates, the result is correct.

source
###### Examples
julia> R, (x, y, z) = polynomial_ring(QQ, ["x", "y", "z"]);

julia> A, _ = quo(R, ideal(R, [x*y, x*z]));

julia> L = noether_normalization(A);

julia> L
2-element Vector{MPolyQuoRingElem{QQMPolyRingElem}}:
-2*x + y
-5*y + z

julia> L
Map with following data
Domain:
=======
Quotient of multivariate polynomial ring by ideal with 2 generators
Codomain:
=========
Quotient of multivariate polynomial ring by ideal with 2 generators

julia> L
Map with following data
Domain:
=======
Quotient of multivariate polynomial ring by ideal with 2 generators
Codomain:
=========
Quotient of multivariate polynomial ring by ideal with 2 generators


## Normalization

normalizationMethod
normalization(A::MPolyQuoRing; algorithm = :equidimDec)

Find the normalization of a reduced affine algebra over a perfect field $K$. That is, given the quotient $A=R/I$ of a multivariate polynomial ring $R$ over $K$ modulo a radical ideal $I$, compute the integral closure $\overline{A}$ of $A$ in its total ring of fractions $Q(A)$, together with the embedding $f: A \to \overline{A}$.

Implemented Algorithms and how to Read the Output

The function relies on the algorithm of Greuel, Laplagne, and Seelisch which proceeds by finding a suitable decomposition $I=I_1\cap\dots\cap I_r$ into radical ideals $I_k$, together with maps $A = R/I \to A_k=\overline{R/I_k}$ which give rise to the normalization map of $A$:

$$$A\hookrightarrow A_1\times \dots\times A_r=\overline{A}$$$

For each $k$, the function specifies two representations of $A_k$: It returns an array of triples $(A_k, f_k, \mathfrak a_k)$, where $A_k$ is represented as an affine $K$-algebra, and $f_k$ as a map of affine $K$-algebras. The third entry $\mathfrak a_k$ is a tuple $(d_k, J_k)$, consisting of an element $d_k\in A$ and an ideal $J_k\subset A$, such that $\frac{1}{d_k}J_k = A_k$ as $A$-submodules of the total ring of fractions of $A$.

By default (algorithm = :equidimDec), as a first step on its way to find the decomposition $I=I_1\cap\dots\cap I_r$, the algorithm computes an equidimensional decomposition of the radical ideal $I$. Alternatively, if specified by algorithm = :primeDec, the algorithm computes $I=I_1\cap\dots\cap I_r$ as the prime decomposition of the radical ideal $I$.

Warning

The function does not check whether $A$ is reduced. Use is_reduced(A) in case you are unsure (this may take some time).

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> A, _ = quo(R, ideal(R, [(x^2-y^3)*(x^2+y^2)*x]));

julia> L = normalization(A);

julia> size(L)
(2,)

julia> LL = normalization(A, algorithm = :primeDec);

julia> size(LL)
(3,)

julia> LL
Quotient
of multivariate polynomial ring in 3 variables over QQ
by ideal(-T(1)*y + x, -T(1)*x + y^2, T(1)^2 - y, -x^2 + y^3)

julia> LL
Map with following data
Domain:
=======
A
Codomain:
=========
Quotient of multivariate polynomial ring by ideal with 4 generators

julia> LL
(y, ideal(x, y))
source
normalization_with_deltaMethod
normalization_with_delta(A::MPolyQuoRing; algorithm::Symbol = :equidimDec)

Compute the normalization

$$$A\hookrightarrow A_1\times \dots\times A_r=\overline{A}$$$

of $A$ as does normalize(A), but return additionally the delta invariant of $A$, that is, the dimension

$$$\dim_K(\overline{A}/A)$$$

.

The return value is a tuple whose first element is normalize(A), whose second element is an array containing the delta invariants of the $A_k$, and whose third element is the (total) delta invariant of $A$. The return value -1 in the third element indicates that the delta invariant is infinite.

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> A, _ = quo(R, ideal(R, [(x^2-y^3)*(x^2+y^2)*x]));

julia> L = normalization_with_delta(A);

julia> L
3-element Vector{Int64}:
1
1
0

julia> L
13
julia> R, (x, y, z) = polynomial_ring(QQ, ["x", "y", "z"]);

julia> A, _ = quo(R, ideal(R, [z^3-x*y^4]));

julia> L = normalization_with_delta(A);

julia> L
-1
source

## Integral Bases

integral_basisMethod
integral_basis(f::MPolyRingElem, i::Int; algorithm::Symbol = :normal_local)

Given a polynomial $f$ in two variables with coefficients in a perfect field $K$, and given an integer $i\in\{1,2\}$ specifying one of the variables, $f$ must be irreducible and monic in the specified variable: Say, $f\in\mathbb K[x,y]$ is monic in $y$. Then the normalization of $A = K[x,y]/\langle f \rangle$, that is, the integral closure $\overline{A}$ of $A$ in its quotient field, is a free module over $K[x]$ of finite rank, and any set of free generators for $\overline{A}$ over $K[x]$ is called an integral basis for $\overline{A}$ over $K[x]$. The function returns a pair $(d, V)$, where $d$ is an element of $A$, and $V$ is a vector of elements in $A$, such that the fractions $v/d, v\in V$, form an integral basis for $\overline{A}$ over $K[x]$.

By default (algorithm = :normal_local), the function relies on the local-to-global approach to normalization presented in Janko Böhm, Wolfram Decker, Santiago Laplagne, Gerhard Pfister, Andreas Steenpaß, Stefan Steidel (2013). Alternatively, if specified by algorithm = :normal_global, the global normalization algorithm in Gert-Martin Greuel, Santiago Laplagne, Frank Seelisch (2010) is used. If $K = \mathbb Q$, it is recommended to apply the algorithm in Janko Böhm, Wolfram Decker, Santiago Laplagne, Gerhard Pfister (2019), which makes use of Puiseux expansions and Hensel lifting (algorithm = :hensel).

Note

The conditions on $f$ are automatically checked.

Examples

julia> R, (x, y) = polynomial_ring(QQ, ["x", "y"]);

julia> f = (y^2-2)^2 + x^5
x^5 + y^4 - 4*y^2 + 4

julia> integral_basis(f, 2)
(x^2, MPolyQuoRingElem{QQMPolyRingElem}[x^2, x^2*y, y^2 - 2, y^3 - 2*y])
source

## Tests on Affine Algebras

### Reducedness Test

is_reducedMethod
is_reduced(A::MPolyQuoRing)

Given an affine algebra A, return true if A is reduced, false otherwise.

Warning

The function computes the radical of the modulus of A. This may take some time.

Examples

julia> R, (x,) = polynomial_ring(QQ, ["x"]);

julia> A, _ = quo(R, ideal(R, [x^4]));

julia> is_reduced(A)
false
source

### Normality Test

is_normalMethod
is_normal(A::MPolyQuoRing)

Given an affine algebra A over a perfect field, return true if A is normal, false otherwise.

Note

This function performs the first step of the normalization algorithm of Greuel, Laplagne, and Seelisch Gert-Martin Greuel, Santiago Laplagne, Frank Seelisch (2010) and may, thus, be more efficient than computing the full normalization of A.

Examples

julia> R, (x, y, z) = polynomial_ring(QQ, ["x", "y", "z"]);

julia> A, _ = quo(R, ideal(R, [z^2-x*y]));

julia> is_normal(A)
true
source

### Cohen-Macaulayness Test

is_cohen_macaulayMethod
 is_cohen_macaulay(A::MPolyQuoRing)

Given a $\mathbb Z$-graded affine algebra A = R/I over a field, say, K, where the grading is inherited from the standard $\mathbb Z$-grading on the polynomial ring R, return true if A is a Cohen-Macaulay ring, false otherwise.

Examples

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, ["w", "x", "y", "z"]);

julia> I = ideal(R, [x*z-y^2, w*z-x*y, w*y-x^2]);

julia> A, _ = quo(R, I);

julia> is_cohen_macaulay(A)
true
julia> R, (x, y, z) = graded_polynomial_ring(QQ, ["x", "y", "z"]);

julia> I = ideal(R, [x*z, y*z]);

julia> A, _ = quo(R, I);

julia> is_cohen_macaulay(A)
false
source

## Hilbert Series and Hilbert Polynomial

Given a multivariate polynomial ring $R$ over a field $K$ together with a (multi)grading on $R$ by a finitely generated abelian group $G$, let $I$ be an ideal of $R$ which is homogeneous with respect to this grading. Then the affine $K-$algebra $A=R/I$ inherits the grading: $A = \bigoplus_{g\in G} A_g$. Suppose now that $R$ is positively graded by $G$. That is, $G$ is free and each graded piece $R_g$ has finite dimension. Then also $A_g$ is a finite dimensional $K$-vector space for each $g$, and we have the well-defined Hilbert function of $A$,

$$$H(A, \underline{\phantom{d}}): G \to \N, \; g\mapsto \dim_K(A_g).$$$

The Hilbert series of $A$ is the generating function

$$$H_A(\mathbb t)=\sum_{g\in G} H(A, g) \mathbb t^g$$$

(see Section 8.2 in Ezra Miller, Bernd Sturmfels (2005) for a formal discussion extending the classical case of $\mathbb Z$-gradings with positive weights to the more general case of multigradings). As in the classical case, the infinitely many values of the Hilbert function can be expressed in finite terms by representing the Hilbert series as a rational function (see Theorem 8.20 in Ezra Miller, Bernd Sturmfels (2005) for a precise statement).

By a result of Macaulay, if $A = R/I$ is an affine algebra, and $L_{>}(I)$ is the leading ideal of $I$ with respect to a global monomial ordering $>$, then the Hilbert function of $A$ equals that of $R/L_{>}(I)$ (see Theorem 15.26 in David Eisenbud (1995)). Thus, using Gröbner bases, the computation of Hilbert series can be reduced to the case where the modulus of the affine algebra is a monomial ideal. In the latter case, we face a problem of combinatorial nature, and there are various strategies of how to proceed (see Martin Kreuzer, Lorenzo Robbiano (2005)). The functions hilbert_series, hilbert_series_reduced, hilbert_series_expanded, hilbert_function, hilbert_polynomial, and degree address the case of $\mathbb Z$-gradings with positive weights, relying on corresponding Singular functionality. The functions multi_hilbert_series, multi_hilbert_series_reduced, and multi_hilbert_function offer a variety of different strategies and allow one to handle positive gradings in general.

### $\mathbb Z$-Gradings With Positive Weights

Let $R=K[x_1, \dots x_n]$ be a polynomial ring in $n$ variables over a field $K$. Assign positive integer weights $w_i$ to the variables $x_i$, and grade $R=\bigoplus_{d\in \mathbb Z} R_d=\bigoplus_{d\geq 0} R_d$ according to the corresponding weighted degree. Let $I$ be an ideal of $R$ which is homogeneous with respect to this grading. Then the affine $K$-algebra $A=R/I$ inherits the grading: $A = \bigoplus_{d\geq 0} A_d$, where each graded piece $A_d$ is a finite dimensional $K$-vector space. In this situation, the Hilbert function of $A$ is of type

$$$H(A, \underline{\phantom{d}}): \N \to \N, \;d \mapsto \dim_K(d),$$$

and the Hilbert series of $A$ is the formal power series

$$$H_A(t)=\sum_{d\geq 0} H(A, d) t^d\in\mathbb Z[[t]].$$$

The Hilbert series can be written as a rational function $p(t)/q(t)$, with denominator

$$$q(t) = (1-t^{w_1})\cdots (1-t^{w_n}).$$$

In the standard $\mathbb Z$-graded case, where the weights on the variables are all 1, the Hilbert function is of polynomial nature: There exists a unique polynomial $P_A(t)\in\mathbb{Q}[t]$, the Hilbert polynomial, which satisfies $H(M,d)=P_M(d)$ for all $d \gg 0$. Furthermore, the degree of $A$ is defined as the dimension of $A$ over $K$ if this dimension is finite, and as the integer $d$ such that the leading term of the Hilbert polynomial has the form $d t^e/e!$, otherwise.

hilbert_seriesMethod
hilbert_series(A::MPolyQuoRing)

Given a $\mathbb Z$-graded affine algebra $A = R/I$ over a field $K$, where the grading is inherited from a $\mathbb Z$-grading on the polynomial ring $R$ defined by assigning positive integer weights to the variables, return a pair $(p,q)$, say, of univariate polynomials $p, q\in\mathbb Z[t]$ such that $p/q$ represents the Hilbert series of $A$ as a rational function with denominator

$$$q = (1-t^{w_1})\cdots (1-t^{w_n}),$$$

where $n$ is the number of variables of $R$, and $w_1, \dots, w_n$ are the assigned weights.

See also hilbert_series_reduced.

Examples

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, ["w", "x", "y", "z"]);

julia> A, _ = quo(R, ideal(R, [w*y-x^2, w*z-x*y, x*z-y^2]));

julia> hilbert_series(A)
(2*t^3 - 3*t^2 + 1, t^4 - 4*t^3 + 6*t^2 - 4*t + 1)

julia> R, (x, y, z) = graded_polynomial_ring(QQ, ["x", "y", "z"], [1, 2, 3]);

julia> A, _ = quo(R, ideal(R, [x*y*z]));

julia> hilbert_series(A)
(-t^6 + 1, -t^6 + t^5 + t^4 - t^2 - t + 1)
source
hilbert_series_reducedMethod
hilbert_series_reduced(A::MPolyQuoRing)

Given a $\mathbb Z$-graded affine algebra $A = R/I$ over a field $K$, where the grading is inherited from a $\mathbb Z$-grading on the polynomial ring $R$ defined by assigning positive integer weights to the variables, return a pair $(p,q)$, say, of univariate polynomials $p, q\in\mathbb Z[t]$ such that $p/q$ represents the Hilbert series of $A$ as a rational function written in lowest terms.

See also hilbert_series.

Examples

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, ["w", "x", "y", "z"]);

julia> A, _ = quo(R, ideal(R, [w*y-x^2, w*z-x*y, x*z-y^2]));

julia> hilbert_series_reduced(A)
(2*t + 1, t^2 - 2*t + 1)

julia> R, (x, y, z) = graded_polynomial_ring(QQ, ["x", "y", "z"], [1, 2, 3]);

julia> A, _ = quo(R, ideal(R, [x*y*z]));

julia> hilbert_series(A)
(-t^6 + 1, -t^6 + t^5 + t^4 - t^2 - t + 1)

julia> hilbert_series_reduced(A)
(t^2 - t + 1, t^2 - 2*t + 1)
source
hilbert_series_expandedMethod
hilbert_series_expanded(A::MPolyQuoRing, d::Int)

Given a $\mathbb Z$-graded affine algebra $A = R/I$ over a field $K$, where the grading is inherited from a $\mathbb Z$-grading on the polynomial ring $R$ defined by assigning positive integer weights to the variables, return the Hilbert series of $A$ to precision $d$.

Examples

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, ["w", "x", "y", "z"]);

julia> A, _ = quo(R, ideal(R, [w*y-x^2, w*z-x*y, x*z-y^2]));

julia> hilbert_series_expanded(A, 7)
1 + 4*t + 7*t^2 + 10*t^3 + 13*t^4 + 16*t^5 + 19*t^6 + 22*t^7 + O(t^8)

julia> R, (x, y, z) = graded_polynomial_ring(QQ, ["x", "y", "z"], [1, 2, 3]);

julia> A, _ = quo(R, ideal(R, [x*y*z]));

julia> hilbert_series_expanded(A, 5)
1 + t + 2*t^2 + 3*t^3 + 4*t^4 + 5*t^5 + O(t^6)
source
hilbert_functionMethod
hilbert_function(A::MPolyQuoRing, d::Int)

Given a $\mathbb Z$-graded affine algebra $A = R/I$ over a field $K$, where the grading is inherited from a $\mathbb Z$-grading on the polynomial ring $R$ defined by assigning positive integer weights to the variables, return the value $H(A, d),$ where

$$$H(A, \underline{\phantom{d}}): \N \to \N, \; d \mapsto \dim_K A_d,$$$

is the Hilbert function of $A$.

Examples

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, ["w", "x", "y", "z"]);

julia> A, _ = quo(R, ideal(R, [w*y-x^2, w*z-x*y, x*z-y^2]));

julia> hilbert_function(A,7)
22

julia> R, (x, y, z) = graded_polynomial_ring(QQ, ["x", "y", "z"], [1, 2, 3]);

julia> A, _ = quo(R, ideal(R, [x*y*z]));

julia> hilbert_function(A, 5)
5
source
hilbert_polynomialMethod
 hilbert_polynomial(A::MPolyQuoRing)

Given a $\mathbb Z$-graded affine algebra $A = R/I$ over a field $K$, where the grading is inherited from the standard $\mathbb Z$-grading on the polynomial ring $R$, return the Hilbert polynomial of $A$.

Examples

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, ["w", "x", "y", "z"]);

julia> A, _ = quo(R, ideal(R, [w*y-x^2, w*z-x*y, x*z-y^2]));

julia> hilbert_polynomial(A)
3*t + 1
source
degreeMethod
degree(A::MPolyQuoRing)

Given a $\mathbb Z$-graded affine algebra $A = R/I$ over a field $K$, where the grading is inherited from the standard $\mathbb Z$-grading on the polynomial ring $R$, return the degree of $A$.

Examples

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, ["w", "x", "y", "z"]);

julia> A, _ = quo(R, ideal(R, [w*y-x^2, w*z-x*y, x*z-y^2]));

julia> degree(A)
3
source

multi_hilbert_seriesMethod
multi_hilbert_series(A::MPolyQuoRing; algorithm::Symbol=:BayerStillmanA)

Return the Hilbert series of the positively graded affine algebra A.

Note

The advanced user can select a algorithm for the computation; see the code for details.

Examples

julia> W = [1 1 1; 0 0 -1];

julia> R, x = graded_polynomial_ring(QQ, ["x", "x", "x"], W)
(Graded multivariate polynomial ring in 3 variables over QQ, MPolyDecRingElem{QQFieldElem, QQMPolyRingElem}[x, x, x])

julia> I = ideal(R, [x^3*x, x*x^2, x^2*x, x^4]);

julia> A, _ = quo(R, I);

julia> H = multi_hilbert_series(A);

julia> H
-t^7*t^-2 + t^6*t^-1 + t^6*t^-2 + t^5*t^-4 - t^4 + t^4*t^-2 - t^4*t^-4 - t^3*t^-1 - t^3*t^-2 + 1

julia> H
-t^3*t^-1 + t^2 + 2*t^2*t^-1 - 2*t - t*t^-1 + 1

julia> H
GrpAb: Z^2

julia> H
Identity map with

Domain:
=======
GrpAb: Z^2

julia> G = abelian_group(ZZMatrix([1 -1]));

julia> g = gen(G, 1)
Element of
(General) abelian group with relation matrix
[1 -1]
with components [0 1]

julia> W = [g, g, g, g];

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, ["w", "x", "y", "z"], W);

julia> A, _ = quo(R, ideal(R, [w*y-x^2, w*z-x*y, x*z-y^2]));

julia> H = multi_hilbert_series(A);

julia> H
2*t^3 - 3*t^2 + 1

julia> H
t^4 - 4*t^3 + 6*t^2 - 4*t + 1

julia> H
GrpAb: Z

julia> H
Map with following data
Domain:
=======
Abelian group with structure: Z
Codomain:
=========
G
source
multi_hilbert_series_reducedMethod
multi_hilbert_series_reduced(A::MPolyQuoRing; algorithm::Symbol=:BayerStillmanA)

Return the reduced Hilbert series of the positively graded affine algebra A.

Note

The advanced user can select a algorithm for the computation; see the code for details.

Examples

julia> W = [1 1 1; 0 0 -1];

julia> R, x = graded_polynomial_ring(QQ, ["x", "x", "x"], W)
(Graded multivariate polynomial ring in 3 variables over QQ, MPolyDecRingElem{QQFieldElem, QQMPolyRingElem}[x, x, x])

julia> I = ideal(R, [x^3*x, x*x^2, x^2*x, x^4]);

julia> A, _ = quo(R, I);

julia> H = multi_hilbert_series_reduced(A);

julia> H
-t^5*t^-1 + t^3 + t^3*t^-3 + t^2 + t^2*t^-1 + t^2*t^-2 + t + t*t^-1 + 1

julia> H
-t + 1

julia> H
GrpAb: Z^2

julia> H
Identity map with

Domain:
=======
GrpAb: Z^2

julia> G = abelian_group(ZZMatrix([1 -1]));

julia> g = gen(G, 1)
Element of
(General) abelian group with relation matrix
[1 -1]
with components [0 1]

julia> W = [g, g, g, g];

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, ["w", "x", "y", "z"], W);

julia> A, _ = quo(R, ideal(R, [w*y-x^2, w*z-x*y, x*z-y^2]));

julia> H = multi_hilbert_series_reduced(A);

julia> H
2*t + 1

julia> H
t^2 - 2*t + 1

julia> H
GrpAb: Z

julia> H
Map with following data
Domain:
=======
Abelian group with structure: Z
Codomain:
=========
G
source
multi_hilbert_functionMethod
multi_hilbert_function(A::MPolyQuoRing, g::GrpAbFinGenElem)

Given a positively graded affine algebra $A$ over a field $K$ with grading group $G$, say, and given an element $g$ of $G$, return the value $H(A, g)$ of the Hilbert function

$$$H(A, \underline{\phantom{d}}): G \to \N, \; g\mapsto \dim_K(A_g).$$$
multi_hilbert_function(A::MPolyQuoRing, g::Vector{<:IntegerUnion})

Given a positively $\mathbb Z^m$-graded affine algebra $A$ over a field $K$, and given a vector $g$ of $m$ integers, convert $g$ into an element of the grading group of $A$, and return the value $H(A, g)$ as above.

multi_hilbert_function(A::MPolyQuoRing, g::IntegerUnion)

Given a positively $\mathbb Z$-graded affine algebra $A$ over a field $K$, and given an integer $g$, convert $g$ into an element of the grading group of $A$, and return the value $H(A, g)$ as above.

Examples

julia> W = [1 1 1; 0 0 -1];

julia> R, x = graded_polynomial_ring(QQ, ["x", "x", "x"], W)
(Graded multivariate polynomial ring in 3 variables over QQ, MPolyDecRingElem{QQFieldElem, QQMPolyRingElem}[x, x, x])

julia> I = ideal(R, [x^3*x, x*x^2, x^2*x, x^4]);

julia> A, _ = quo(R, I);

julia> multi_hilbert_function(A::MPolyQuoRing, [1, 0])
2

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, ["w", "x", "y", "z"], [-1, -1, -1, -1]);

julia> A, _ = quo(R, ideal(R, [w*y-x^2, w*z-x*y, x*z-y^2]));

julia> multi_hilbert_function(A, -7)
22

julia> G = abelian_group(ZZMatrix([1 -1]));

julia> g = gen(G, 1);

julia> W = [g, g, g, g];

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, ["w", "x", "y", "z"], W);

julia> A, _ = quo(R, ideal(R, [w*y-x^2, w*z-x*y, x*z-y^2]));

julia> multi_hilbert_function(A, 7*g)
22
source