Multivariate Puiseux polynomials

A Puiseux polynomial over a field $K$ is a finite sum of terms $c x_1^{e_1} \cdots x_n^{e_n}$ with $c \in K$ and rational exponents $e_i$, which may be negative. Puiseux polynomials form a ring containing the Laurent polynomial ring $K[x_1^{\pm 1}, \dots, x_n^{\pm 1}]$.

Generic Puiseux polynomial types

AbstractAlgebra.jl provides a generic implementation of multivariate Puiseux polynomials in the file src/generic/PuiseuxMPoly.jl, built on top of the generic multivariate Laurent polynomials.

The element type is PuiseuxMPolyRingElem{T} <: RingElem and the parent type is PuiseuxMPolyRing{T} <: Ring, parameterized on the coefficient type T.

A Puiseux polynomial $f$ is stored as a pair $(g, d)$ of a Laurent polynomial $g$ and a positive integer $d$, its scale, with

\[f(x_1, \dots, x_n) = g(x_1^{1/d}, \dots, x_n^{1/d}).\]

The representation is normalized: $d$ is coprime to the greatest common divisor of all exponents of $g$, so $d$ is the least common denominator of the exponents of $f$. Every Puiseux polynomial has a unique normalized representation, which makes comparing elements cheap. Arithmetic rescales both operands to a common scale, operates on the underlying Laurent polynomials and normalizes the result.

Constructor

puiseux_polynomial_ring(K::Field, variableName::Vector{String})

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

julia> R
Puiseux polynomial ring in 2 variables x, y
  over rationals

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

julia> f^2
x + 4*x^(1//2)*y^(-1//3) + 4*y^(-2//3)

Elements can be created from the generators by the usual arithmetic operations, and constants can be coerced into the ring by calling it:

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

julia> R(3)
3

julia> 3*x^(1//2)*y^(-2//3) - 1
3*x^(1//2)*y^(-2//3) - 1

Basic functionality

Puiseux polynomial rings implement the Ring interface. In addition, the following functions are available.

base_ring(R::PuiseuxMPolyRing)
coefficient_ring(R::PuiseuxMPolyRing)
symbols(R::PuiseuxMPolyRing)
number_of_variables(R::PuiseuxMPolyRing)
gens(R::PuiseuxMPolyRing)
is_univariate(R::PuiseuxMPolyRing)

Here base_ring returns the underlying Laurent polynomial ring, and coefficient_ring returns the field $K$.

length(f::PuiseuxMPolyRingElem)
coefficients(f::PuiseuxMPolyRingElem)
monomials(f::PuiseuxMPolyRingElem)
exponent_vectors(f::PuiseuxMPolyRingElem)
leading_coefficient(f::PuiseuxMPolyRingElem)
is_gen(f::PuiseuxMPolyRingElem)
is_monomial(f::PuiseuxMPolyRingElem)
is_term(f::PuiseuxMPolyRingElem)
is_unit(f::PuiseuxMPolyRingElem)
canonical_unit(f::PuiseuxMPolyRingElem)

The exponent vectors have entries of type Rational{Int}. The units are the nonzero terms, i.e. the nonzero constant multiples of monomials. As for Laurent polynomials, the canonical unit of f is the canonical unit of its leading coefficient times $x_1^{m_1} \cdots x_n^{m_n}$, where $m_i$ is the smallest exponent of $x_i$ in f.

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

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

julia> collect(coefficients(f))
2-element Vector{Rational{BigInt}}:
 1
 2

julia> exponent_vectors(f)
2-element Vector{Vector{Rational{Int64}}}:
 [1//2, 0]
 [0, 1//3]

julia> is_unit(f), is_unit(2*x^(1//2))
(false, true)

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

The normalized representation can be inspected with the following functions.

poly(f::PuiseuxMPolyRingElem)

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

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

scale(f::PuiseuxMPolyRingElem)

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

julia> scale(x^(1//2) + 2*y^(1//3))
6

Powering

^(f::PuiseuxMPolyRingElem, n::Integer)
^(f::PuiseuxMPolyRingElem, q::Rational)

Any Puiseux polynomial can be raised to a nonnegative integer power. A negative integer power requires f to be a monomial times a constant, and a non-integer rational power requires f to be a monomial with coefficient one.

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

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

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

julia> (x + y)^(1//2)
ERROR: ArgumentError: only monomials can be exponentiated to non-integer powers
[...]

Divisibility

divexact(f::PuiseuxMPolyRingElem, g::PuiseuxMPolyRingElem; check::Bool = true)
divexact(f::PuiseuxMPolyRingElem, a::RingElement; check::Bool = true)
divides(f::PuiseuxMPolyRingElem, g::PuiseuxMPolyRingElem)

divexact returns the quotient of f by a nonzero Puiseux polynomial or constant g, provided the division is exact. divides(f, g) returns a tuple (flag, q) where flag is true if $f = qg$ for some Puiseux polynomial $q$; otherwise q is zero.

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

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

julia> divexact(f^2, f)
x^(1//2) + 2*y^(1//3)

julia> divexact(f, x^(1//6))
x^(1//3) + 2*x^(-1//6)*y^(1//3)

julia> divides(f^2, f)
(true, x^(1//2) + 2*y^(1//3))

julia> divides(x, f)
(false, 0)

Changing the coefficient ring

change_base_ring(K::Ring, f::PuiseuxMPolyRingElem; cached::Bool = true, parent::PuiseuxMPolyRing)
map_coefficients(g, f::PuiseuxMPolyRingElem; cached::Bool = true, parent::PuiseuxMPolyRing)

Return the Puiseux polynomial obtained from f by coercing each coefficient into K, respectively by applying g to each coefficient. Unless parent is given, the result lies in the Puiseux polynomial ring over K, respectively over the parent of the images of g, in the same variables as parent(f).

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

julia> map_coefficients(c -> 2*c, x^(1//2) + 3*y^(1//3))
2*x^(1//2) + 6*y^(1//3)

Valuation

valuation(f::PuiseuxMPolyRingElem)

julia> R, (t,) = puiseux_polynomial_ring(QQ, ["t"]);

julia> valuation(t^(-2//3) + t^(1//2))
-2//3