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 (multi)gradings. 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.
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[1]", "x[2]"]);
julia> x
2-element Vector{gfp_mpoly}:
x[1]
x[2]
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[1]
y[2]
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
Gradings
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.
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)).
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.
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_graded
— Methodis_graded(R::MPolyRing_dec)
Return true
if R
is graded, false
otherwise.
Constructors for Graded Rings
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.
grade
— Methodgrade(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([0])
GrpAb: Z
julia> g = gen(G, 1)
Element of
GrpAb: Z
with components [1]
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 -> [1]
y -> [1], 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[1], x[2], y[1], y[2], y[3] over Rational Field, fmpq_mpoly[x[1], x[2]], fmpq_mpoly[y[1], y[2], y[3]])
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[1], g[1], g[2], g[2], g[2]];
julia> S, _ = grade(R, W)
(Multivariate Polynomial Ring in x[1], x[2], y[1], y[2], y[3] over Rational Field graded by
x[1] -> [1 0]
x[2] -> [1 0]
y[1] -> [0 1]
y[2] -> [0 1]
y[3] -> [0 1], MPolyElem_dec{fmpq, fmpq_mpoly}[x[1], x[2], y[1], y[2], y[3]])
julia> typeof(x[1])
fmpq_mpoly
julia> x = map(S, x)
2-element Vector{MPolyElem_dec{fmpq, fmpq_mpoly}}:
x[1]
x[2]
julia> y = map(S, y)
3-element Vector{MPolyElem_dec{fmpq, fmpq_mpoly}}:
y[1]
y[2]
y[3]
julia> typeof(x[1])
MPolyElem_dec{fmpq, fmpq_mpoly}
julia> R, x = PolynomialRing(QQ, "x" => 1:5)
(Multivariate Polynomial Ring in x[1], x[2], x[3], x[4], x[5] over Rational Field, fmpq_mpoly[x[1], x[2], x[3], x[4], x[5]])
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[1]+g[3]+g[4], g[2]+g[4], g[1]+g[3], g[2], g[1]+g[2]];
julia> S, x = grade(R, W)
(Multivariate Polynomial Ring in x[1], x[2], x[3], x[4], x[5] over Rational Field graded by
x[1] -> [1 0 1 1]
x[2] -> [0 1 0 1]
x[3] -> [1 0 1 0]
x[4] -> [0 1 0 0]
x[5] -> [1 1 0 0], MPolyElem_dec{fmpq, fmpq_mpoly}[x[1], x[2], x[3], x[4], x[5]])
grade
— Methodgrade(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[1], x[2], y[1], y[2], y[3] over Rational Field, fmpq_mpoly[x[1], x[2]], fmpq_mpoly[y[1], y[2], y[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> grade(R, W)
(Multivariate Polynomial Ring in x[1], x[2], y[1], y[2], y[3] over Rational Field graded by
x[1] -> [1 0]
x[2] -> [1 0]
y[1] -> [0 1]
y[2] -> [0 1]
y[3] -> [0 1], MPolyElem_dec{fmpq, fmpq_mpoly}[x[1], x[2], y[1], y[2], y[3]])
grade
— Methodgrade(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 -> [1]
y -> [2]
z -> [3], 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 -> [1]
y -> [1]
z -> [1], MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])
GradedPolynomialRing
— MethodGradedPolynomialRing(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[1]", "x[2]", "x[3]", "x[4]"], W)
(Multivariate Polynomial Ring in x[1], x[2], x[3], x[4] over Rational Field graded by
x[1] -> [1 0]
x[2] -> [0 1]
x[3] -> [1 0]
x[4] -> [4 1], MPolyElem_dec{fmpq, fmpq_mpoly}[x[1], x[2], x[3], x[4]])
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 -> [1]
y -> [2]
z -> [3], 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 -> [1]
y -> [1]
z -> [1], MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])
Tests on Graded Rings
is_standard_graded
— Methodis_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 -> [1]
y -> [2]
z -> [3], MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])
julia> is_standard_graded(S)
false
is_z_graded
— Methodis_z_graded(R::MPolyRing_dec)
Return true
if R
is $\mathbb Z$-graded, false
otherwise.
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 -> [1]
y -> [2]
z -> [3], MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])
julia> is_z_graded(S)
true
is_zm_graded
— Methodis_zm_graded(R::MPolyRing_dec)
Return true
if R
is $\mathbb Z^m$-graded for some $m$, false
otherwise.
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[1], x[2], x[3], x[4], x[5] over Rational Field, fmpq_mpoly[x[1], x[2], x[3], x[4], x[5]])
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[1]+g[3]+g[4], g[2]+g[4], g[1]+g[3], g[2], g[1]+g[2]];
julia> S, x = grade(R, W)
(Multivariate Polynomial Ring in x[1], x[2], x[3], x[4], x[5] over Rational Field graded by
x[1] -> [1 0 1 1]
x[2] -> [0 1 0 1]
x[3] -> [1 0 1 0]
x[4] -> [0 1 0 0]
x[5] -> [1 1 0 0], MPolyElem_dec{fmpq, fmpq_mpoly}[x[1], x[2], x[3], x[4], x[5]])
julia> is_zm_graded(S)
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
julia> is_zm_graded(R)
false
is_positively_graded
— Methodis_positively_graded(R::MPolyRing_dec)
Return true
if R
is positively graded, false
otherwise.
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([0])
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 -> [1]
y -> [1], MPolyElem_dec{fmpq, fmpq_mpoly}[t, x, y])
julia> is_positively_graded(S)
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])
julia> is_positively_graded(S)
false
Data Associated to Multivariate Rings
Given a multivariate polynomial ring R
with coefficient ring C
,
coefficient_ring(R)
refers toC
,gens(R)
to the generators (variables) ofR
,ngens(R)
to the number of these generators, andgen(R, i)
as well asR[i]
to thei
-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[3]
z
julia> ngens(R)
3
In the graded case, we additionally have:
grading_group
— Methodgrading_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 -> [1]
y -> [2]
z -> [3], MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])
julia> grading_group(R)
GrpAb: Z
homogeneous_component
— Methodhomogeneous_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
.
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);
julia> G = grading_group(S)
GrpAb: Z^2
julia> L = homogeneous_component(S, [1, 1]);
julia> L[1]
homogeneous component of Multivariate Polynomial Ring in x[1], x[2], y[1], y[2], y[3] over Rational Field graded by
x[1] -> [1 0]
x[2] -> [1 0]
y[1] -> [0 1]
y[2] -> [0 1]
y[3] -> [0 1] of degree graded by [1 1]
julia> FG = gens(L[1]);
julia> EMB = L[2]
Map from
homogeneous component of Multivariate Polynomial Ring in x[1], x[2], y[1], y[2], y[3] over Rational Field graded by
x[1] -> [1 0]
x[2] -> [1 0]
y[1] -> [0 1]
y[2] -> [0 1]
y[3] -> [0 1] of degree graded by [1 1]
to Multivariate Polynomial Ring in x[1], x[2], y[1], y[2], y[3] over Rational Field graded by
x[1] -> [1 0]
x[2] -> [1 0]
y[1] -> [0 1]
y[2] -> [0 1]
y[3] -> [0 1] defined by a julia-function with inverse
julia> for i in 1:length(FG) println(EMB(FG[i])) end
x[2]*y[3]
x[2]*y[2]
x[2]*y[1]
x[1]*y[3]
x[1]*y[2]
x[1]*y[1]
julia> T, (x, y, z) = GradedPolynomialRing(QQ, ["x", "y", "z"])
(Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> [1]
y -> [1]
z -> [1], MPolyElem_dec{fmpq, fmpq_mpoly}[x, y, z])
julia> G = grading_group(T)
GrpAb: Z
julia> L = homogeneous_component(T, 2)
(homogeneous component of Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> [1]
y -> [1]
z -> [1] of degree graded by [2]
, Map from
homogeneous component of Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> [1]
y -> [1]
z -> [1] of degree graded by [2]
to Multivariate Polynomial Ring in x, y, z over Rational Field graded by
x -> [1]
y -> [1]
z -> [1] defined by a julia-function with inverse)
julia> FG = gens(L[1]);
julia> EMB = L[2];
julia> for i in 1:length(FG) println(EMB(FG[i])) end
z^2
y*z
y^2
x*z
x*y
x^2
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 -> [1]
y -> [1]
z -> [1], 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 toR
,total_degree(f)
to the total degree off
,monomial(f, i)
to thei
-th monomial off
,term(f, i)
to thei
-th term off
,coeff(f, i)
to the coefficient of thei
-th term off
, andexponent_vector(f, i)
to the exponent vector of thei
-th term off
.
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_components
— Methodhomogeneous_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 -> [1]
y -> [2]
z -> [3], 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:
[2] => x^2 + y
[3] => z
julia> R, x = PolynomialRing(QQ, "x" => 1:5)
(Multivariate Polynomial Ring in x[1], x[2], x[3], x[4], x[5] over Rational Field, fmpq_mpoly[x[1], x[2], x[3], x[4], x[5]])
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[1]+g[3]+g[4], g[2]+g[4], g[1]+g[3], g[2], g[1]+g[2]];
julia> S, x = grade(R, W)
(Multivariate Polynomial Ring in x[1], x[2], x[3], x[4], x[5] over Rational Field graded by
x[1] -> [1 0 1 1]
x[2] -> [0 1 0 1]
x[3] -> [1 0 1 0]
x[4] -> [0 1 0 0]
x[5] -> [1 1 0 0], MPolyElem_dec{fmpq, fmpq_mpoly}[x[1], x[2], x[3], x[4], x[5]])
julia> f = x[1]^2+x[3]^2+x[5]^2
x[1]^2 + x[3]^2 + x[5]^2
julia> homogeneous_components(f)
Dict{GrpAbFinGenElem, MPolyElem_dec{fmpq, fmpq_mpoly}} with 2 entries:
[2 2 0 0] => x[5]^2
[2 0 0 0] => x[1]^2 + x[3]^2
homogeneous_component
— Methodhomogeneous_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[1], x[2], x[3], x[4], x[5] over Rational Field, fmpq_mpoly[x[1], x[2], x[3], x[4], x[5]])
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[1]+g[3]+g[4], g[2]+g[4], g[1]+g[3], g[2], g[1]+g[2]];
julia> S, x = grade(R, W)
(Multivariate Polynomial Ring in x[1], x[2], x[3], x[4], x[5] over Rational Field graded by
x[1] -> [1 0 1 1]
x[2] -> [0 1 0 1]
x[3] -> [1 0 1 0]
x[4] -> [0 1 0 0]
x[5] -> [1 1 0 0], MPolyElem_dec{fmpq, fmpq_mpoly}[x[1], x[2], x[3], x[4], x[5]])
julia> f = x[1]^2+x[3]^2+x[5]^2
x[1]^2 + x[3]^2 + x[5]^2
julia> homogeneous_component(f, 2*g[1])
x[1]^2 + x[3]^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[1]", "x[2]", "x[3]", "x[4]"], W)
(Multivariate Polynomial Ring in x[1], x[2], x[3], x[4] over Rational Field graded by
x[1] -> [1 0]
x[2] -> [0 1]
x[3] -> [1 0]
x[4] -> [4 1], MPolyElem_dec{fmpq, fmpq_mpoly}[x[1], x[2], x[3], x[4]])
julia> f = x[1]^2*x[2]+x[4]
x[1]^2*x[2] + x[4]
julia> homogeneous_component(f, [2, 1])
x[1]^2*x[2]
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 -> [1]
y -> [2]
z -> [3], 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
is_homogeneous
— Methodis_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 -> [1]
y -> [2]
z -> [3], 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
degree
— Methoddegree(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[1], x[2], x[3], x[4], x[5] over Rational Field, fmpq_mpoly[x[1], x[2], x[3], x[4], x[5]])
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[1]+g[3]+g[4], g[2]+g[4], g[1]+g[3], g[2], g[1]+g[2]];
julia> S, x = grade(R, W)
(Multivariate Polynomial Ring in x[1], x[2], x[3], x[4], x[5] over Rational Field graded by
x[1] -> [1 0 1 1]
x[2] -> [0 1 0 1]
x[3] -> [1 0 1 0]
x[4] -> [0 1 0 0]
x[5] -> [1 1 0 0], MPolyElem_dec{fmpq, fmpq_mpoly}[x[1], x[2], x[3], x[4], x[5]])
julia> f = x[2]^2+2*x[4]^2
x[2]^2 + 2*x[4]^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[1]", "x[2]", "x[3]", "x[4]"], W)
(Multivariate Polynomial Ring in x[1], x[2], x[3], x[4] over Rational Field graded by
x[1] -> [1 0]
x[2] -> [0 1]
x[3] -> [1 0]
x[4] -> [4 1], MPolyElem_dec{fmpq, fmpq_mpoly}[x[1], x[2], x[3], x[4]])
julia> f = x[1]^4*x[2]+x[4]
x[1]^4*x[2] + x[4]
julia> degree(f)
graded by [4 1]
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 -> [1]
y -> [2]
z -> [3], 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)
graded by [6]
julia> typeof(degree(f))
GrpAbFinGenElem
julia> degree(Int, f)
6
julia> typeof(degree(Int, f))
Int64
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:
hom
— Methodhom(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.
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
julia> Qi, i = quadratic_field(-1)
(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
Given a ring homomorphism F
from R
to S
as above, domain(F)
and codomain(F)
refer to R
and S
, respectively.
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.