# Creating Multivariate Rings

In this section, we illustrate by examples how to create multivariate polynomial rings and their elements, while at the same time introducing and illustrating a special ring type for modelling multivariate polynomial rings with gradings by finitely presented groups. For more details on multivariate polynomial rings, their coefficient rings (fields), and their elements, we refer to the chapters on rings and fields.

## Types

OSCAR provides types for dense univariate and sparse multivariate polynomials. The univariate ring types belong to the abstract type PolyRing{T}, their elements have abstract type PolyRingElem{T}. The multivariate ring types belong to the abstract type MPolyRing{T}, their elements have abstract type MPolyRingElem{T}. Here, T is the element type of the coefficient ring of the polynomial ring.

## Constructors

The basic constructor below allows one to build multivariate polynomial rings:

PolynomialRing(C::Ring, V::Vector{String}; ordering=:lex, cached = true)

Its return value is a tuple, say R, vars, consisting of a polynomial ring R with coefficient ring C and a vector vars of generators (variables) which print according to the strings in the vector V . The input ordering=:lex refers to the lexicograpical monomial ordering which specifies the default way of storing and displaying polynomials in OSCAR (terms are sorted in descending order). The other possible choices are :deglex and :degrevlex. Gröbner bases, however, can be computed with respect to any monomial ordering. See the section on Gröbner bases.

Note

Caching is used to ensure that a given ring constructed from given parameters is unique in the system. For example, there is only one ring of multivariate polynomials over $\mathbb{Z}$ in the variables x, y, z with ordering=:lex.

###### Examples
julia> R, (x, y, z) = PolynomialRing(ZZ, ["x", "y", "z"])
(Multivariate Polynomial Ring in x, y, z over Integer Ring, fmpz_mpoly[x, y, z])

julia> typeof(R)
FmpzMPolyRing

julia> typeof(x)
fmpz_mpoly

julia> S, (x, y, z) = PolynomialRing(ZZ, ["x", "y", "z"])
(Multivariate Polynomial Ring in x, y, z over Integer Ring, fmpz_mpoly[x, y, z])

julia> R === S
true

julia> R1, x = PolynomialRing(QQ, ["x"])
(Multivariate Polynomial Ring in x over Rational Field, fmpq_mpoly[x])

julia> typeof(x)
Vector{fmpq_mpoly} (alias for Array{fmpq_mpoly, 1})

julia> R2, (x,) = PolynomialRing(QQ, ["x"])
(Multivariate Polynomial Ring in x over Rational Field, fmpq_mpoly[x])

julia> typeof(x)
fmpq_mpoly

julia> R3, x = PolynomialRing(QQ, "x")
(Univariate Polynomial Ring in x over Rational Field, x)

julia> typeof(x)
fmpq_poly

julia> T, x = PolynomialRing(GF(3), ["x", "x"]);

julia> x
2-element Vector{gfp_mpoly}:
x
x


The constructor illustrated below allows for the convenient handling of variables with multi-indices:

julia> R, x, y, z = PolynomialRing(QQ, "x" => (1:3, 1:4), "y" => 1:2, "z" => (1:1, 1:1, 1:1));

julia> x
3×4 Matrix{fmpq_mpoly}:
x[1, 1]  x[1, 2]  x[1, 3]  x[1, 4]
x[2, 1]  x[2, 2]  x[2, 3]  x[2, 4]
x[3, 1]  x[3, 2]  x[3, 3]  x[3, 4]

julia> y
2-element Vector{fmpq_mpoly}:
y
y

julia> z
1×1×1 Array{fmpq_mpoly, 3}:
[:, :, 1] =
z[1, 1, 1]


## Coefficient Rings

Gröbner and standard bases are implemented for multivariate polynomial rings over the fields and rings below:

### The field of rational numbers $\mathbb{Q}$

julia> QQ
Rational Field


### Finite fields $\mathbb{F_p}$, $p$ a prime

julia> GF(3)
Galois field with characteristic 3

julia> GF(ZZ(2)^127 - 1)
Galois field with characteristic 170141183460469231731687303715884105727


### Finite fields $\mathbb{F}_{p^n}$ with $p^n$ elements, $p$ a prime

julia> FiniteField(2, 70, "a")
(Finite field of degree 70 over F_2, a)


### Simple algebraic extensions of $\mathbb{Q}$ or $\mathbb{F}_p$

julia> T, t = PolynomialRing(QQ, "t")
(Univariate Polynomial Ring in t over Rational Field, t)

julia> K, a = NumberField(t^2 + 1, "a")
(Number field over Rational Field with defining polynomial t^2 + 1, a)

julia> F = GF(3)
Galois field with characteristic 3

julia> T, t = PolynomialRing(F, "t")
(Univariate Polynomial Ring in t over Galois field with characteristic 3, t)

julia> K, a = FiniteField(t^2 + 1, "a")
(Finite field of degree 2 over F_3, a)


### Purely transcendental extensions of $\mathbb{Q}$ or $\mathbb{F}_p$

julia> T, t = PolynomialRing(QQ, "t")
(Univariate Polynomial Ring in t over Rational Field, t)

julia> QT = FractionField(T)
Fraction field of Univariate Polynomial Ring in t over Rational Field

julia> parent(t)
Univariate Polynomial Ring in t over Rational Field

julia> parent(1//t)
Fraction field of Univariate Polynomial Ring in t over Rational Field

julia> T, (s, t) = PolynomialRing(GF(3), ["s", "t"]);

julia> QT = FractionField(T)
Fraction field of Multivariate Polynomial Ring in s, t over Galois field with characteristic 3


### The ring of integers $\mathbb{Z}$

julia> ZZ
Integer Ring


Given a polynomial ring $R = C[x_1, \dots, x_n]$, we may endow $R$ with various gradings. The standard $\mathbb Z$-grading on $R$ is the decomposition $R=\bigoplus_{d\in \mathbb Z} R_d=\bigoplus_{d\geq 0} R_d$ by the usual degree of polynomials. More general $\mathbb Z$-gradings are obtained by assigning integer weights to the variables and considering the corresponding weighted degrees. Even more generally, we may consider multigradings: Given a finitely generated abelian group $G$, a multigrading on $R$ by $G$, or a $G$-grading, or simply a grading, corresponds to a semigroup homomorphism $\phi: \mathbb N^n \rightarrow G$: Given $\phi$, the degree of a monomial $x^\alpha$ is the image $\deg(x^\alpha):=\phi(\alpha)\in G$; the induced $G$-grading on $R$ is the decomposition $R = \bigoplus_{g\in G} R_g$ satisfying $R_g\cdot R_h\subset R_{g+h}$, where $R_g$ is the free $C$-module generated by the monomials of degree $g$. This grading is determined by assigning the weights $\deg(x_i)$ to the $x_i$. In other words, it is determined by the weight vector $W = (\deg(x_1), \dots, \deg(x_n))\in G^n.$

We refer to the textbooks Ezra Miller, Bernd Sturmfels (2005) and Martin Kreuzer, Lorenzo Robbiano (2005) for details on multigradings. With respect to notation, we follow the former book.

Note

Given a $G$-grading on $R$, we refer to $G$ as the grading group of $R$. Moreover, we then say that $R$ is $G$-graded, or simply that $R$ is graded. If $R$ is a polynomial ring over a field, we say that a $G$-grading on $R$ is positive if $G$ is free and each graded part $R_g$, $g\in G$, has finite dimension. We then also say that $R$ is positively graded (by $G$). Note that the positivity condition can be equivalently expressed by asking that $G$ is free and that the degree zero part consists of the constants only (see Theorem 8.6 in Ezra Miller, Bernd Sturmfels (2005)).

Note

Given a G-grading on R in OSCAR, we say that R is $\mathbb Z^m$-graded if is_free(G) && ngens(G) == rank(G) == m evaluates to true. In this case, conversion routines allow one to switch back and forth between elements of G and integer vectors of length m. Specifically, if R is $\mathbb Z$-graded, that is, is_free(G) && ngens(G) == rank(G) == 1 evaluates to true, elements of G may be converted to integers and vice versa.

### Types

Multivariate rings with gradings are modelled by objects of type MPolyRing_dec{T, S} :< MPolyRing{T}, with elements of type MPolyRingElem_dec{T, S} :< MPolyRingElem{T}. Here, S is the element type of the multivariate ring, and T is the element type of its coefficient ring as above.

Note

The types MPolyRing_dec{T, S} and MPolyRingElem_dec{T, S} are also meant to eventually model multivariate rings with filtrations and their elements.

The following function allows one to distinguish between graded and filtered rings:

is_gradedMethod
is_graded(R::MPolyRing_dec)

Return true if R is graded, false otherwise.

source

There are two basic ways of creating multivariate rings with gradings: While the grade function allows one to assign a grading to a polynomial ring already constructed, the GradedPolynomialRing function is meant to create a graded polynomial ring all at once.

gradeMethod
grade(R::MPolyRing, W::Vector{GrpAbFinGenElem})

Given a vector W of ngens(R) elements of a group G of type GrpAbFinGen, define a G-grading on R by assigning weights to the variables according to the entries of W. Return the graded ring as an object of type MPolyRing_dec, together with the vector of variables.

Examples

julia> R, (t, x, y) = PolynomialRing(QQ, ["t", "x", "y"])
(Multivariate Polynomial Ring in t, x, y over Rational Field, fmpq_mpoly[t, x, y])

julia> typeof(R)
FmpqMPolyRing

julia>  typeof(x)
fmpq_mpoly

julia> G = abelian_group()
GrpAb: Z

julia> g = gen(G, 1)
Element of
GrpAb: Z
with components 

julia> S, (t, x, y) = grade(R, [-g, g, g])
(Multivariate Polynomial Ring in t, x, y over Rational Field graded by
t -> [-1]
x -> 
y -> , MPolyElem_dec{fmpq, fmpq_mpoly}[t, x, y])

julia> typeof(S)
MPolyRing_dec{fmpq, FmpqMPolyRing}

julia> S isa MPolyRing
true

julia> typeof(x)
MPolyElem_dec{fmpq, fmpq_mpoly}

julia> R, x, y = PolynomialRing(QQ, "x" => 1:2, "y" => 1:3)
(Multivariate Polynomial Ring in x, x, y, y, y over Rational Field, fmpq_mpoly[x, x], fmpq_mpoly[y, y, y])

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

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

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

julia> S, _ = grade(R, W)
(Multivariate Polynomial Ring in x, x, y, y, y over Rational Field graded by
x -> [1 0]
x -> [1 0]
y -> [0 1]
y -> [0 1]
y -> [0 1], MPolyElem_dec{fmpq, fmpq_mpoly}[x, x, y, y, y])

julia> typeof(x)
fmpq_mpoly

julia> x = map(S, x)
2-element Vector{MPolyElem_dec{fmpq, fmpq_mpoly}}:
x
x

julia> y = map(S, y)
3-element Vector{MPolyElem_dec{fmpq, fmpq_mpoly}}:
y
y
y

julia> typeof(x)
MPolyElem_dec{fmpq, fmpq_mpoly}

julia> R, x = PolynomialRing(QQ, "x" => 1:5)
(Multivariate Polynomial Ring in x, x, x, x, x over Rational Field, fmpq_mpoly[x, x, x, x, x])

julia> G = abelian_group([0, 0, 2, 2])
(General) abelian group with relation matrix
[0 0 0 0; 0 0 0 0; 0 0 2 0; 0 0 0 2]

julia> g = gens(G)
4-element Vector{GrpAbFinGenElem}:
Element of
(General) abelian group with relation matrix
[0 0 0 0; 0 0 0 0; 0 0 2 0; 0 0 0 2]
with components [1 0 0 0]
Element of
(General) abelian group with relation matrix
[0 0 0 0; 0 0 0 0; 0 0 2 0; 0 0 0 2]
with components [0 1 0 0]
Element of
(General) abelian group with relation matrix
[0 0 0 0; 0 0 0 0; 0 0 2 0; 0 0 0 2]
with components [0 0 1 0]
Element of
(General) abelian group with relation matrix
[0 0 0 0; 0 0 0 0; 0 0 2 0; 0 0 0 2]
with components [0 0 0 1]

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

julia> S, x = grade(R, W)
(Multivariate Polynomial Ring in x, x, x, x, x over Rational Field graded by
x -> [1 0 1 1]
x -> [0 1 0 1]
x -> [1 0 1 0]
x -> [0 1 0 0]
x -> [1 1 0 0], MPolyElem_dec{fmpq, fmpq_mpoly}[x, x, x, x, x])
source
gradeMethod
grade(R::MPolyRing, W::Vector{<:Vector{<:IntegerUnion}})

Given a vector W of ngens(R) integer vectors of the same size m, say, define a $\mathbb Z^m$-grading on R by creating a free abelian group of type GrpAbFinGen given by m free generators, converting the vectors in W to elements of that group, and assigning these elements as weights to the variables. Return the graded ring as an object of type MPolyRing_dec, together with the vector of variables.

grade(R::MPolyRing, W::Union{fmpz_mat, Matrix{<:IntegerUnion}})

As above, converting the columns of W.

Examples

julia> R, x, y = PolynomialRing(QQ, "x" => 1:2, "y" => 1:3)
(Multivariate Polynomial Ring in x, x, y, y, y over Rational Field, fmpq_mpoly[x, x], fmpq_mpoly[y, y, y])

julia> W = [1 1 0 0 0; 0 0 1 1 1]
2×5 Matrix{Int64}:
1  1  0  0  0
0  0  1  1  1

(Multivariate Polynomial Ring in x, x, y, y, y over Rational Field graded by
x -> [1 0]
x -> [1 0]
y -> [0 1]
y -> [0 1]
y -> [0 1], MPolyElem_dec{fmpq, fmpq_mpoly}[x, x, y, y, y])
source
gradeMethod
grade(R::MPolyRing, W::Vector{<:IntegerUnion})

Given a vector W of ngens(R) integers, define a $\mathbb Z$-grading on R by creating a free abelian group of type GrpAbFinGen given by one free generator, converting the entries of W to elements of that group, and assigning these elements as weights to the variables. Return the graded ring as an object of type MPolyRing_dec, together with the vector of variables.

grade(R::MPolyRing)

As above, where the grading is the standard $\mathbb Z$-grading on R.

Examples

julia> R, (x, y, z) = PolynomialRing(QQ, ["x", "y", "z"])
(Multivariate Polynomial Ring in x, y, z over Rational Field, fmpq_mpoly[x, y, z])

julia> W = [1, 2, 3];

julia> S, (x, y, z) = grade(R, W)
(Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> 
y -> 
z -> , MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])

julia> T, (x, y, z) = grade(R)
(Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> 
y -> 
z -> , MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])
source
GradedPolynomialRingMethod
GradedPolynomialRing(C::Ring, V::Vector{String}, W; ordering=:lex)

Create a multivariate polynomial ring with coefficient ring C and variables which print according to the strings in V, and grade this ring according to the data provided by W (see the documentation of the grade-function for what is possible). Return the graded ring as an object of type MPolyRing_dec, together with the vector of variables.

GradedPolynomialRing(C::Ring, V::Vector{String}; ordering=:lex)

As above, where the grading is the standard $\mathbb Z$-grading.

Examples

julia> W = [[1, 0], [0, 1], [1, 0], [4, 1]]
4-element Vector{Vector{Int64}}:
[1, 0]
[0, 1]
[1, 0]
[4, 1]

julia> R, x = GradedPolynomialRing(QQ, ["x", "x", "x", "x"], W)
(Multivariate Polynomial Ring in x, x, x, x over Rational Field graded by
x -> [1 0]
x -> [0 1]
x -> [1 0]
x -> [4 1], MPolyElem_dec{fmpq, fmpq_mpoly}[x, x, x, x])

julia> S, (x, y, z) = GradedPolynomialRing(QQ, ["x", "y", "z"], [1, 2, 3])
(Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> 
y -> 
z -> , MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])

julia> T, (x, y, z) = GradedPolynomialRing(QQ, ["x", "y", "z"])
(Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> 
y -> 
z -> , MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])
source

is_standard_gradedMethod
is_standard_graded(R::MPolyRing_dec)

Return true if R is standard $\mathbb Z$-graded, false otherwise.

Examples

julia> R, (x, y, z) = PolynomialRing(QQ, ["x", "y", "z"])
(Multivariate Polynomial Ring in x, y, z over Rational Field, fmpq_mpoly[x, y, z])

julia> W = [1, 2, 3];

julia> S, (x, y, z) = grade(R, W)
(Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> 
y -> 
z -> , MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])

false
source
is_z_gradedMethod
is_z_graded(R::MPolyRing_dec)

Return true if R is $\mathbb Z$-graded, false otherwise.

Note

Writing G = grading_group(R), we say that R is $\mathbb Z$-graded if is_free(G) && ngens(G) == rank(G) == 1 evaluates to true.

Examples

julia> R, (x, y, z) = PolynomialRing(QQ, ["x", "y", "z"])
(Multivariate Polynomial Ring in x, y, z over Rational Field, fmpq_mpoly[x, y, z])

julia> W = [1, 2, 3];

julia> S, (x, y, z) = grade(R, W)
(Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> 
y -> 
z -> , MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])

true
source
is_zm_gradedMethod
is_zm_graded(R::MPolyRing_dec)

Return true if R is $\mathbb Z^m$-graded for some $m$, false otherwise.

Note

Writing G = grading_group(R), we say that R is $\mathbb Z^m$-graded if is_free(G) && ngens(G) == rank(G) == m evaluates to true.

Examples

julia> R, x = PolynomialRing(QQ, "x" => 1:5)
(Multivariate Polynomial Ring in x, x, x, x, x over Rational Field, fmpq_mpoly[x, x, x, x, x])

julia> G = abelian_group([0, 0, 2, 2])
(General) abelian group with relation matrix
[0 0 0 0; 0 0 0 0; 0 0 2 0; 0 0 0 2]

julia> g = gens(G);

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

julia> S, x = grade(R, W)
(Multivariate Polynomial Ring in x, x, x, x, x over Rational Field graded by
x -> [1 0 1 1]
x -> [0 1 0 1]
x -> [1 0 1 0]
x -> [0 1 0 0]
x -> [1 1 0 0], MPolyElem_dec{fmpq, fmpq_mpoly}[x, x, x, x, x])

false

julia> G = abelian_group(fmpz_mat([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) = GradedPolynomialRing(QQ, ["w", "x", "y", "z"], W);

julia> is_free(G)
true

false
source
is_positively_gradedMethod
is_positively_graded(R::MPolyRing_dec)

Return true if R is positively graded, false otherwise.

Note

We say that R is positively graded by a finitely generated abelian group $G$ if the coefficient ring of R is a field, $G$ is free, and each graded part $R_g$, $g\in G$, has finite dimension.

Examples

julia> R, (t, x, y) = PolynomialRing(QQ, ["t", "x", "y"])
(Multivariate Polynomial Ring in t, x, y over Rational Field, fmpq_mpoly[t, x, y])

julia> G = abelian_group()
GrpAb: Z

julia> S, (t, x, y) = grade(R, [-1, 1, 1])
(Multivariate Polynomial Ring in t, x, y over Rational Field graded by
t -> [-1]
x -> 
y -> , MPolyElem_dec{fmpq, fmpq_mpoly}[t, x, y])

false

julia> R, (x, y) = PolynomialRing(QQ, ["x", "y"])
(Multivariate Polynomial Ring in x, y over Rational Field, fmpq_mpoly[x, y])

julia> G = abelian_group([0, 2])
(General) abelian group with relation matrix
[0 0; 0 2]

julia> W = [gen(G, 1)+gen(G, 2), gen(G, 1)]
2-element Vector{GrpAbFinGenElem}:
Element of
(General) abelian group with relation matrix
[0 0; 0 2]
with components [1 1]
Element of
(General) abelian group with relation matrix
[0 0; 0 2]
with components [1 0]

julia> S, (x, y) = grade(R, W)
(Multivariate Polynomial Ring in x, y over Rational Field graded by
x -> [1 1]
y -> [1 0], MPolyElem_dec{fmpq, fmpq_mpoly}[x, y])

false
source

## Data Associated to Multivariate Rings

Given a multivariate polynomial ring R with coefficient ring C,

• coefficient_ring(R) refers to C,
• gens(R) to the generators (variables) of R,
• ngens(R) to the number of these generators, and
• gen(R, i) as well as R[i] to the i-th such generator.
###### Examples
julia> R, (x, y, z) = PolynomialRing(QQ, ["x", "y", "z"])
(Multivariate Polynomial Ring in x, y, z over Rational Field, fmpq_mpoly[x, y, z])

julia> coefficient_ring(R)
Rational Field

julia> gens(R)
3-element Vector{fmpq_mpoly}:
x
y
z

julia> gen(R, 2)
y

julia> R
z

julia> ngens(R)
3


grading_groupMethod
grading_group(R::MPolyRing_dec)

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

Examples

julia> R, (x, y, z) = GradedPolynomialRing(QQ, ["x", "y", "z"], [1, 2, 3])
(Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> 
y -> 
z -> , MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])

GrpAb: Z
source
homogeneous_componentMethod
homogeneous_component(R::MPolyRing_dec, g::GrpAbFinGenElem)

Given a polynomial ring R over a field which is graded by a free group of type GrpAbFinGen, and given an element g of that group, return the homogeneous component of R of degree g. Additionally, return the embedding of the component into R.

homogeneous_component(R::MPolyRing_dec, g::Vector{<:IntegerUnion})

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

homogeneous_component(R::MPolyRing_dec, g::IntegerUnion)

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

Note

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

Examples

julia> R, x, y = PolynomialRing(QQ, "x" => 1:2, "y" => 1:3);

julia> W = [1 1 0 0 0; 0 0 1 1 1]
2×5 Matrix{Int64}:
1  1  0  0  0
0  0  1  1  1

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

GrpAb: Z^2

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

julia> L
homogeneous component of Multivariate Polynomial Ring in x, x, y, y, y over Rational Field graded by
x -> [1 0]
x -> [1 0]
y -> [0 1]
y -> [0 1]
y -> [0 1] of degree graded by [1 1]

julia> FG = gens(L);

julia> EMB = L
Map from
homogeneous component of Multivariate Polynomial Ring in x, x, y, y, y over Rational Field graded by
x -> [1 0]
x -> [1 0]
y -> [0 1]
y -> [0 1]
y -> [0 1] of degree graded by [1 1]
to Multivariate Polynomial Ring in x, x, y, y, y over Rational Field graded by
x -> [1 0]
x -> [1 0]
y -> [0 1]
y -> [0 1]
y -> [0 1] defined by a julia-function with inverse

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

julia> T, (x, y, z) = GradedPolynomialRing(QQ, ["x", "y", "z"])
(Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> 
y -> 
z -> , MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])

GrpAb: Z

julia> L = homogeneous_component(T, 2)
(homogeneous component of Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> 
y -> 
z ->  of degree graded by 
, Map from
homogeneous component of Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> 
y -> 
z ->  of degree graded by 
to Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> 
y -> 
z ->  defined by a julia-function with inverse)

julia> FG = gens(L);

julia> EMB = L;

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

## Elements of Multivariate Rings

### Constructors

One way to create elements of a multivariate polynomial ring is to build up polynomials from the generators (variables) of the ring using basic arithmetic as shown below:

###### Examples
julia> R, (x, y, z) = PolynomialRing(QQ, ["x", "y", "z"])
(Multivariate Polynomial Ring in x, y, z over Rational Field, fmpq_mpoly[x, y, z])

julia> f = 3*x^2+y*z
3*x^2 + y*z

julia> typeof(f)
fmpq_mpoly

julia> S, (x, y, z) = grade(R)
(Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> 
y -> 
z -> , MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])

julia> g = 3*x^2+y*z
3*x^2 + y*z

julia> typeof(g)
MPolyElem_dec{fmpq, fmpq_mpoly}

julia> g == S(f)
true


Alternatively, there is the following constructor:

(R::MPolyRing{T})(c::Vector{T}, e::Vector{Vector{Int}}) where T <: RingElem

Its return value is the element of R whose nonzero coefficients are specified by the elements of c, with exponent vectors given by the elements of e.

###### Examples
julia> R, (x, y, z) = PolynomialRing(QQ, ["x", "y", "z"])
(Multivariate Polynomial Ring in x, y, z over Rational Field, fmpq_mpoly[x, y, z])

julia> f = 3*x^2+y*z
3*x^2 + y*z

julia> g = R(QQ.([3, 1]), [[2, 0, 0], [0, 1, 1]])
3*x^2 + y*z

julia> f == g
true


An often more effective way to create polynomials is to use the MPoly build context as indicated below:

julia> R, (x, y) = PolynomialRing(QQ, ["x", "y"])
(Multivariate Polynomial Ring in x, y over Rational Field, fmpq_mpoly[x, y])

julia> B = MPolyBuildCtx(R)
Builder for an element of Multivariate Polynomial Ring in x, y over Rational Field

julia> for i = 1:5 push_term!(B, QQ(i), [i, i-1]) end

julia> finish(B)
5*x^5*y^4 + 4*x^4*y^3 + 3*x^3*y^2 + 2*x^2*y + x


### Special Elements

Given a multivariate polynomial ring R, zero(R) and one(R) refer to the additive and multiplicative identity of R, respectively. Relevant test calls on an element f of R are iszero(f) and isone(f).

### Data Associated to Elements of Multivariate Rings

Given an element f of a multivariate polynomial ring R or a graded version of such a ring,

• parent(f) refers to R,
• total_degree(f) to the total degree of f,
• monomial(f, i) to the i-th monomial of f,
• term(f, i) to the i-th term of f,
• coeff(f, i) to the coefficient of the i-th term of f, and
• exponent_vector(f, i) to the exponent vector of the i-th term of f.
###### Examples
julia> R, (x, y) = PolynomialRing(GF(5), ["x", "y"])
(Multivariate Polynomial Ring in x, y over Galois field with characteristic 5, gfp_mpoly[x, y])

julia> c = map(GF(5), [1, 2, 3])
3-element Vector{gfp_elem}:
1
2
3

julia> e = [[3, 2], [1, 0], [0, 1]]
3-element Vector{Vector{Int64}}:
[3, 2]
[1, 0]
[0, 1]

julia> f = R(c, e)
x^3*y^2 + 2*x + 3*y

julia> parent(f)
Multivariate Polynomial Ring in x, y over Galois field with characteristic 5

julia> total_degree(f)
5

julia> coeff(f, 2)
2

julia> exponent_vector(f, 2)
2-element Vector{Int64}:
1
0

julia> monomial(f, 2)
x

julia> term(f, 2)
2*x


Further functionality is available in the graded case:

homogeneous_componentsMethod
homogeneous_components(f::MPolyElem_dec{T, S}) where {T, S}

Given an element f of a graded multivariate ring, return the homogeneous components of f.

Examples

julia> R, (x, y, z) = GradedPolynomialRing(QQ, ["x", "y", "z"], [1, 2, 3])
(Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> 
y -> 
z -> , MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])

julia> f = x^2+y+z
x^2 + y + z

julia> homogeneous_components(f)
Dict{GrpAbFinGenElem, MPolyElem_dec{fmpq, fmpq_mpoly}} with 2 entries:
 => x^2 + y
 => z

julia> R, x = PolynomialRing(QQ, "x" => 1:5)
(Multivariate Polynomial Ring in x, x, x, x, x over Rational Field, fmpq_mpoly[x, x, x, x, x])

julia> G = abelian_group([0, 0, 2, 2])
(General) abelian group with relation matrix
[0 0 0 0; 0 0 0 0; 0 0 2 0; 0 0 0 2]

julia> g = gens(G);

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

julia> S, x = grade(R, W)
(Multivariate Polynomial Ring in x, x, x, x, x over Rational Field graded by
x -> [1 0 1 1]
x -> [0 1 0 1]
x -> [1 0 1 0]
x -> [0 1 0 0]
x -> [1 1 0 0], MPolyElem_dec{fmpq, fmpq_mpoly}[x, x, x, x, x])

julia> f = x^2+x^2+x^2
x^2 + x^2 + x^2

julia> homogeneous_components(f)
Dict{GrpAbFinGenElem, MPolyElem_dec{fmpq, fmpq_mpoly}} with 2 entries:
[2 2 0 0] => x^2
[2 0 0 0] => x^2 + x^2
source
homogeneous_componentMethod
homogeneous_component(f::MPolyElem_dec, g::GrpAbFinGenElem)

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

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

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

homogeneous_component(f::MPolyElem_dec, g::IntegerUnion)

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

Examples

julia> R, x = PolynomialRing(QQ, "x" => 1:5)
(Multivariate Polynomial Ring in x, x, x, x, x over Rational Field, fmpq_mpoly[x, x, x, x, x])

julia> G = abelian_group([0, 0, 2, 2])
(General) abelian group with relation matrix
[0 0 0 0; 0 0 0 0; 0 0 2 0; 0 0 0 2]

julia> g = gens(G);

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

julia> S, x = grade(R, W)
(Multivariate Polynomial Ring in x, x, x, x, x over Rational Field graded by
x -> [1 0 1 1]
x -> [0 1 0 1]
x -> [1 0 1 0]
x -> [0 1 0 0]
x -> [1 1 0 0], MPolyElem_dec{fmpq, fmpq_mpoly}[x, x, x, x, x])

julia> f = x^2+x^2+x^2
x^2 + x^2 + x^2

julia> homogeneous_component(f, 2*g)
x^2 + x^2

julia> W = [[1, 0], [0, 1], [1, 0], [4, 1]]
4-element Vector{Vector{Int64}}:
[1, 0]
[0, 1]
[1, 0]
[4, 1]

julia> R, x = GradedPolynomialRing(QQ, ["x", "x", "x", "x"], W)
(Multivariate Polynomial Ring in x, x, x, x over Rational Field graded by
x -> [1 0]
x -> [0 1]
x -> [1 0]
x -> [4 1], MPolyElem_dec{fmpq, fmpq_mpoly}[x, x, x, x])

julia> f = x^2*x+x
x^2*x + x

julia> homogeneous_component(f, [2, 1])
x^2*x

julia> R, (x, y, z) = GradedPolynomialRing(QQ, ["x", "y", "z"], [1, 2, 3])
(Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> 
y -> 
z -> , MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])

julia> f = x^2+y+z
x^2 + y + z

julia> homogeneous_component(f, 1)
0

julia> homogeneous_component(f, 2)
x^2 + y

julia> homogeneous_component(f, 3)
z
source
is_homogeneousMethod
is_homogeneous(f::MPolyElem_dec)

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

Examples

julia> R, (x, y, z) = GradedPolynomialRing(QQ, ["x", "y", "z"], [1, 2, 3])
(Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> 
y -> 
z -> , MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])

julia> f = x^2+y*z
x^2 + y*z

julia> is_homogeneous(f)
false

julia> W = [1 2 1 0; 3 4 0 1]
2×4 Matrix{Int64}:
1  2  1  0
3  4  0  1

julia> S, (w, x, y, z) = GradedPolynomialRing(QQ, ["w", "x", "y", "z"], W)
(Multivariate Polynomial Ring in w, x, y, z over Rational Field graded by
w -> [1 3]
x -> [2 4]
y -> [1 0]
z -> [0 1], MPolyElem_dec{fmpq, fmpq_mpoly}[w, x, y, z])

julia> F = w^3*y^3*z^3 + w^2*x*y^2*z^2 + w*x^2*y*z + x^3
w^3*y^3*z^3 + w^2*x*y^2*z^2 + w*x^2*y*z + x^3

julia> is_homogeneous(F)
true
source
degreeMethod
degree(f::MPolyElem_dec)

Given a homogeneous element f of a graded multivariate ring, return the degree of f.

degree(::Type{Vector{Int}}, f::MPolyElem_dec)

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

degree(::Type{Int}, f::MPolyElem_dec)

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

Examples

julia> R, x = PolynomialRing(QQ, "x" => 1:5)
(Multivariate Polynomial Ring in x, x, x, x, x over Rational Field, fmpq_mpoly[x, x, x, x, x])

julia> G = abelian_group([0, 0, 2, 2])
(General) abelian group with relation matrix
[0 0 0 0; 0 0 0 0; 0 0 2 0; 0 0 0 2]

julia> g = gens(G);

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

julia> S, x = grade(R, W)
(Multivariate Polynomial Ring in x, x, x, x, x over Rational Field graded by
x -> [1 0 1 1]
x -> [0 1 0 1]
x -> [1 0 1 0]
x -> [0 1 0 0]
x -> [1 1 0 0], MPolyElem_dec{fmpq, fmpq_mpoly}[x, x, x, x, x])

julia> f = x^2+2*x^2
x^2 + 2*x^2

julia> degree(f)
Element of
(General) abelian group with relation matrix
[0 0 0 0; 0 0 0 0; 0 0 2 0; 0 0 0 2]
with components [0 2 0 0]

julia> W = [[1, 0], [0, 1], [1, 0], [4, 1]]
4-element Vector{Vector{Int64}}:
[1, 0]
[0, 1]
[1, 0]
[4, 1]

julia> R, x = GradedPolynomialRing(QQ, ["x", "x", "x", "x"], W)
(Multivariate Polynomial Ring in x, x, x, x over Rational Field graded by
x -> [1 0]
x -> [0 1]
x -> [1 0]
x -> [4 1], MPolyElem_dec{fmpq, fmpq_mpoly}[x, x, x, x])

julia> f = x^4*x+x
x^4*x + x

julia> degree(f)

julia> degree(Vector{Int}, f)
2-element Vector{Int64}:
4
1

julia>  R, (x, y, z) = GradedPolynomialRing(QQ, ["x", "y", "z"], [1, 2, 3])
(Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> 
y -> 
z -> , MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])

julia> f = x^6+y^3+z^2
x^6 + y^3 + z^2

julia> degree(f)

julia> typeof(degree(f))
GrpAbFinGenElem

julia> degree(Int, f)
6

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

## Homomorphisms From Multivariate Rings

If $R$ is a multivariate polynomial ring, and $S$ is any ring, then a ring homomorphism $R \rightarrow S$ is determined by specifying its restriction to the coefficient ring of $R$, and by assigning an image to each variable of $R$. In OSCAR, such homomorphisms are created by using the following constructor:

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

hom(R::MPolyRing, S::NCRing, images::Vector; check::Bool = true)

Given a homomorphism coeff_map from C to S, where C is the coefficient ring of R, and given a vector images of nvars(R) elements of S, return the homomorphism R $\to$ S whose restriction to C is coeff_map, and which sends the i-th variable of R 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

In case check = true (default), the function 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> K, a = FiniteField(2, 2, "a");

julia> R, (x, y) = PolynomialRing(K, ["x", "y"]);

julia> F = hom(R, R, z -> z^2, [y, x])
Map with following data
Domain:
=======
Multivariate Polynomial Ring in x, y over Finite field of degree 2 over F_2
Codomain:
=========
Multivariate Polynomial Ring in x, y over Finite field of degree 2 over F_2

julia> F(a * y)
(a + 1)*x

(Imaginary quadratic field defined by x^2 + 1, sqrt(-1))

julia> S, (x, y) = PolynomialRing(Qi, ["x", "y"]);

julia> G = hom(S, S, hom(Qi, Qi, -i), [x^2, y^2])
Map with following data
Domain:
=======
Multivariate Polynomial Ring in x, y over Imaginary quadratic field defined by x^2 + 1
Codomain:
=========
Multivariate Polynomial Ring in x, y over Imaginary quadratic field defined by x^2 + 1

julia> G(x+i*y)
x^2 - sqrt(-1)*y^2

julia> R, (x, y) = PolynomialRing(ZZ, ["x", "y"]);

julia> f = 3*x^2+2*x+1;

julia> S, (x, y) = PolynomialRing(GF(2), ["x", "y"]);

julia> H = hom(R, S, gens(S))
Map with following data
Domain:
=======
Multivariate Polynomial Ring in x, y over Integer Ring
Codomain:
=========
Multivariate Polynomial Ring in x, y over Galois field with characteristic 2

julia> H(f)
x^2 + 1
source

Given a ring homomorphism F from R to S as above, domain(F) and codomain(F) refer to R and S, respectively.

Note

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}, MPolyQuo{U}}, where T <: FieldElem and U <: MPolyElem{T}. Functionality for these homomorphism is discussed in the section on affine algebras.