Rational function fields

AbstractAlgebra.jl provides a module, implemented in src/generic/RationalFunctionField.jl for rational function fields $k(x)$ over a field $k$.

Generic rational function field type

Rational functions in $k(x)$ have type Generic.Rat{T} where T is the type of elements of the coefficient field $k$. See the file src/generic/GenericTypes.jl for details.

Parent objects corresponding to the rational function field $k$ have type Generic.RationalFunctionField{T}.

Abstract types

The rational function types belong to the abstract type Field and the rational function field types belong to the abstract type FieldElem.

Rational function field constructors

In order to construct rational functions in AbstractAlgebra.jl, one can first construct the function field itself. This is accomplished with the following constructor.

RationalFunctionField(k::Field, s::AbstractString; cached::Bool = true)

Given a coefficient field k return a tuple (S, x) consisting of the parent object of the rational function field over $k$ and the generator x. By default the parent object S will depend only on R and s and will be cached. Setting the optional argument cached to false will prevent the parent object S from being cached.

Here are some examples of creating rational function fields and making use of the resulting parent objects to coerce various elements into the function field.

Examples

julia> S, x = RationalFunctionField(QQ, "x")
(Rational function field over Rationals, x)

julia> f = S()
0

julia> g = S(123)
123

julia> h = S(BigInt(1234))
1234

julia> k = S(x + 1)
x + 1

julia> m = S(numerator(x + 1, false), numerator(x + 2, false))
(x + 1)//(x + 2)

Basic rational function field functionality

Fraction fields in AbstractAlgebra.jl implement the full Field interface and the entire fraction field interface.

We give some examples of such functionality.

Examples

julia> S, x = RationalFunctionField(QQ, "x")
(Rational function field over Rationals, x)

julia> f = S(x + 1)
x + 1

julia> g = (x^2 + x + 1)//(x^3 + 3x + 1)
(x^2 + x + 1)//(x^3 + 3*x + 1)

julia> h = zero(S)
0

julia> k = one(S)
1

julia> isone(k)
true

julia> iszero(f)
false

julia> m = characteristic(S)
0

julia> U = base_ring(S)
Rationals

julia> V = base_ring(f)
Rationals

julia> T = parent(f)
Rational function field over Rationals

julia> r = deepcopy(f)
x + 1

julia> n = numerator(g)
x^2 + x + 1

julia> d = denominator(g)
x^3 + 3*x + 1

Note that numerator and denominator are returned as elements of a polynomial ring whose variable is printed the same way as that of the generator of the rational function field.

Rational function field functionality provided by AbstractAlgebra.jl

The following functionality is provided for rational function fields.

Greatest common divisor

Base.gcdMethod
gcd(a::Rat{T}, b::Rat{T}) where {T <: RingElem}

Return a greatest common divisor of $a$ and $b$ if one exists. N.B: we define the GCD of $a/b$ and $c/d$ to be gcd$(ad, bc)/bd$, reduced to lowest terms.

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Examples

julia> R, x = RationalFunctionField(QQ, "x")
(Rational function field over Rationals, x)

julia> f = (x + 1)//(x^3 + 3x + 1)
(x + 1)//(x^3 + 3*x + 1)

julia> g = (x^2 + 2x + 1)//(x^2 + x + 1)
(x^2 + 2*x + 1)//(x^2 + x + 1)

julia> h = gcd(f, g)
(x + 1)//(x^5 + x^4 + 4*x^3 + 4*x^2 + 4*x + 1)

Square root

AbstractAlgebra.is_squareMethod
is_square(f::PolyElem{T}) where T <: RingElement

Return true if $f$ is a perfect square.

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is_square(a::FracElem{T}) where T <: RingElem

Return true if $a$ is a square.

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Base.sqrtMethod
sqrt(a::FieldElem)

Return the square root of the element a. By default the function will throw an exception if the input is not square. If check=false this test is omitted.

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Examples

julia> R, x = RationalFunctionField(QQ, "x")
(Rational function field over Rationals, x)

julia> a = (21//4*x^6 - 15*x^5 + 27//14*x^4 + 9//20*x^3 + 3//7*x + 9//10)//(x + 3)
(21//4*x^6 - 15*x^5 + 27//14*x^4 + 9//20*x^3 + 3//7*x + 9//10)//(x + 3)

julia> sqrt(a^2)
(21//4*x^6 - 15*x^5 + 27//14*x^4 + 9//20*x^3 + 3//7*x + 9//10)//(x + 3)

julia> is_square(a^2)
true

Univariate function fields

Univariate function fields in AbstractAlgebra are algebraic extensions $K/k(x)$ of a rational function field $k(x)$ over a field $k$.

These are implemented in a module implemented in src/generic/FunctionField.jl.

Generic function field types

Function field objects $K/k(x)$ in AbstractAlgebra have type Generic.FunctionField{T} where T is the type of elements of the field k.

Corresponding function field elements have type Generic.FunctionFieldElement{T}. See the file src/generic/GenericTypes.jl for details.

Abstract types

Function field types belong to the abstract type Field and their elements to the abstract type FieldElem.

Function field constructors

In order to construct function fields in AbstractAlgebra.jl, one first constructs the rational function field they are an extension of, then supplies a polynomial over this field to the following constructor:

FunctionField(p::Poly{Rat{T}}, s::AbstractString; cached::Bool=true) where T <: FieldElement

Given an irreducible polynomial p over a rational function field return a tuple (S, z) consisting of the parent object of the function field defined by that polynomial over $k(x)$ and the generator z. By default the parent object S will depend only on p and s and will be cached. Setting the optional argument cached to false will prevent the parent object S from being cached.

Here are some examples of creating function fields and making use of the resulting parent objects to coerce various elements into the function field.

Examples

julia> R1, x1 = RationalFunctionField(QQ, "x1") # characteristic 0
(Rational function field over Rationals, x1)

julia> U1, z1 = R1["z1"]
(Univariate Polynomial Ring in z1 over Rational function field over Rationals, z1)

julia> f = (x1^2 + 1)//(x1 + 1)*z1^3 + 4*z1 + 1//(x1 + 1)
(x1^2 + 1)//(x1 + 1)*z1^3 + 4*z1 + 1//(x1 + 1)

julia> S1, y1 = FunctionField(f, "y1")
(Function Field over Rationals with defining polynomial (x1^2 + 1)*y1^3 + (4*x1 + 4)*y1 + 1, y1)

julia> a = S1()
0

julia> b = S1((x1 + 1)//(x1 + 2))
(x1 + 1)//(x1 + 2)

julia> c = S1(1//3)
1//3

julia> R2, x2 = RationalFunctionField(GF(23), "x1") # characteristic p
(Rational function field over Finite field F_23, x1)

julia> U2, z2 = R2["z2"]
(Univariate Polynomial Ring in z2 over Rational function field over Finite field F_23, z2)

julia> g = z2^2 + 3z2 + 1
z2^2 + 3*z2 + 1

julia> S2, y2 = FunctionField(g, "y2")
(Function Field over Finite field F_23 with defining polynomial y2^2 + 3*y2 + 1, y2)

julia> d = S2(R2(5))
5

julia> e = S2(y2)
y2

Basic function field functionality

Function fields implement the full Ring and Field interfaces. We give some examples of such functionality.

Examples

julia> R, x = RationalFunctionField(GF(23), "x") # characteristic p
(Rational function field over Finite field F_23, x)

julia> U, z = R["z"]
(Univariate Polynomial Ring in z over Rational function field over Finite field F_23, z)

julia> g = z^2 + 3z + 1
z^2 + 3*z + 1

julia> S, y = FunctionField(g, "y")
(Function Field over Finite field F_23 with defining polynomial y^2 + 3*y + 1, y)

julia> f = (x + 1)*y + 1
(x + 1)*y + 1

julia> base_ring(f)
Rational function field over Finite field F_23

julia> f^2
(20*x^2 + 19*x + 22)*y + 22*x^2 + 21*x

julia> f*inv(f)
1

Function field functionality provided by AbstractAlgebra.jl

The following functionality is provided for function fields.

Basic manipulation

AbstractAlgebra.varMethod
var(R::FunctionField)

Return the variable name of the generator of the function field R as a symbol.

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Base.numeratorMethod
Base.numerator(R::FunctionField{T}, canonicalise::Bool=true) where T <: FieldElement
Base.denominator(R::FunctionField{T}, canonicalise::Bool=true) where T <: FieldElement

Thinking of elements of the rational function field as fractions, put the defining polynomial of the function field over a common denominator and return the numerator/denominator respectively. Note that the resulting polynomials belong to a different ring than the original defining polynomial. The canonicalise is ignored, but exists for compatibility with the Generic interface.

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Base.numeratorMethod
Base.numerator(a::FunctionFieldElem{T}, canonicalise::Bool=true) where T <: FieldElement
Base.denominator(a::FunctionFieldElem{T}, canonicalise::Bool=true) where T <: FieldElement

Return the numerator and denominator of the function field element a. Note that elements are stored in fraction free form so that the denominator is a common denominator for the coefficients of the element a. If canonicalise is set to true the fraction is first canonicalised.

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AbstractAlgebra.degreeMethod
degree(S::FunctionField)

Return the degree of the defining polynomial of the function field, i.e. the degree of the extension that the function field makes of the underlying rational function field.

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AbstractAlgebra.genMethod
gen(S::FunctionField{T}) where T <: FieldElement

Return the generator of the function field returned by the function field constructor.

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AbstractAlgebra.is_genMethod
is_gen(a::FunctionFieldElem)

Return true if a is the generator of the function field returned by the function field constructor.

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AbstractAlgebra.coeffMethod
coeff(a::FunctionFieldElem, n::Int)

Return the degree n coefficient of the element a in its polynomial representation in terms of the generator of the function field. The coefficient is returned as an element of the underlying rational function field.

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AbstractAlgebra.Generic.num_coeffMethod
num_coeff(a::FunctionFieldElem, n::Int)

Return the degree n coefficient of the numerator of the element a (in its polynomial representation in terms of the generator of the function field, rationalised as per numerator/denominator described above). The coefficient will be an polynomial over the base_ring of the underlying rational function field.

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Examples

julia> R, x = RationalFunctionField(QQ, "x")
(Rational function field over Rationals, x)

julia> U, z = R["z"]
(Univariate Polynomial Ring in z over Rational function field over Rationals, z)

julia> g = z^2 + 3*(x + 1)//(x + 2)*z + 1
z^2 + (3*x + 3)//(x + 2)*z + 1

julia> S, y = FunctionField(g, "y")
(Function Field over Rationals with defining polynomial (x + 2)*y^2 + (3*x + 3)*y + x + 2, y)

julia> base_field(S)
Rational function field over Rationals

julia> var(S)
:y

julia> characteristic(S)
0

julia> defining_polynomial(S)
z^2 + (3*x + 3)//(x + 2)*z + 1

julia> numerator(S)
(x + 2)*y^2 + (3*x + 3)*y + x + 2

julia> denominator(S)
x + 2

julia> a = (x + 1)//(x^2 + 1)*y + 3x + 2
((x + 1)*y + 3*x^3 + 2*x^2 + 3*x + 2)//(x^2 + 1)

julia> numerator(a, false)
(x + 1)*y + 3*x^3 + 2*x^2 + 3*x + 2

julia> denominator(a, false)
x^2 + 1

julia> degree(S)
2

julia> gen(S)
y

julia> is_gen(y)
true

julia> coeff(a, 1)
(x + 1)//(x^2 + 1)

julia> num_coeff(a, 1)
x + 1

Trace and norm

LinearAlgebra.normMethod
norm(a::FunctionFieldElem)

Return the absolute norm of a as an element of the underlying rational function field.

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julia> R, x = RationalFunctionField(QQ, "x")
(Rational function field over Rationals, x)

julia> U, z = R["z"]
(Univariate Polynomial Ring in z over Rational function field over Rationals, z)

julia> g = z^2 + 3*(x + 1)//(x + 2)*z + 1
z^2 + (3*x + 3)//(x + 2)*z + 1

julia> S, y = FunctionField(g, "y")
(Function Field over Rationals with defining polynomial (x + 2)*y^2 + (3*x + 3)*y + x + 2, y)

julia> f = (-3*x - 5//3)//(x - 2)*y + (x^3 + 1//9*x^2 + 5)//(x - 2)
((-3*x - 5//3)*y + x^3 + 1//9*x^2 + 5)//(x - 2)

julia> norm(f)
(x^7 + 20//9*x^6 + 766//81*x^5 + 2027//81*x^4 + 110//3*x^3 + 682//9*x^2 + 1060//9*x + 725//9)//(x^3 - 2*x^2 - 4*x + 8)

julia> tr(f)
(2*x^4 + 38//9*x^3 + 85//9*x^2 + 24*x + 25)//(x^2 - 4)