Multivariate polynomials
This file defines polynomial rings over a base ring (or even semiring),
with variables from a general type σ (which could be infinite).
Important definitions
Let R be a commutative ring (or a semiring) and let σ be an arbitrary
type. This file creates the type mv_polynomial σ R, which mathematicians
might denote $R[X_i : i \in σ]$. It is the type of multivariate
(a.k.a. multivariable) polynomials, with variables
corresponding to the terms in σ, and coefficients in R.
Notation
In the definitions below, we use the following notation:
σ : Type*(indexing the variables)R : Type*[comm_semiring R](the coefficients)s : σ →₀ ℕ, a function fromσtoℕwhich is zero away from a finite set. This will give rise to a monomial inmv_polynomial σ Rwhich mathematicians might callX^sa : Ri : σ, with corresponding monomialX i, often denotedX_iby mathematiciansp : mv_polynomial σ R
Definitions
mv_polynomial σ R: the type of polynomials with variables of typeσand coefficients in the commutative semiringRmonomial s a: the monomial which mathematically would be denoteda * X^sC a: the constant polynomial with valueaX i: the degree one monomial corresponding to i; mathematically this might be denotedXᵢ.coeff s p: the coefficient ofsinp.eval₂ (f : R → S₁) (g : σ → S₁) p: given a semiring homomorphism fromRto another semiringS₁, and a mapσ → S₁, evaluatespat this valuation, returning a term of typeS₁. Note thateval₂can be made usingevalandmap(see below), and it has been suggested that sticking toevalandmapmight make the code less brittle.eval (g : σ → R) p: given a mapσ → R, evaluatespat this valuation, returning a term of typeRmap (f : R → S₁) p: returns the multivariate polynomial obtained frompby the change of coefficient semiring corresponding tof
Implementation notes
Recall that if Y has a zero, then X →₀ Y is the type of functions from X to Y with finite
support, i.e. such that only finitely many elements of X get sent to non-zero terms in Y.
The definition of mv_polynomial σ R is (σ →₀ ℕ) →₀ R ; here σ →₀ ℕ denotes the space of all
monomials in the variables, and the function to R sends a monomial to its coefficient in
the polynomial being represented.
Tags
polynomial, multivariate polynomial, multivariable polynomial
S₁ S₂ S₃
Multivariate polynomial, where σ is the index set of the variables and
R is the coefficient ring
Equations
- mv_polynomial σ R = add_monoid_algebra R (σ →₀ ℕ)
Equations
- mv_polynomial.inhabited = {default := 0}
Equations
the coercion turning an mv_polynomial into the function which reports the coefficient of a given monomial
monomial s a is the monomial a * X^s
Equations
- mv_polynomial.monomial s a = finsupp.single s a
C a is the constant polynomial with value a
Equations
- mv_polynomial.C = {to_fun := mv_polynomial.monomial 0, map_one' := _, map_mul' := _, map_zero' := _, map_add' := _}
X n is the degree 1 monomial $X_n$.
Equations
- mv_polynomial.X n = mv_polynomial.monomial (finsupp.single n 1) 1
The coefficient of the monomial m in the multi-variable polynomial p.
Equations
- mv_polynomial.coeff m p = ⇑p m
constant_coeff p returns the constant term of the polynomial p, defined as coeff 0 p.
This is a ring homomorphism.
Equations
- mv_polynomial.constant_coeff = {to_fun := mv_polynomial.coeff 0, map_one' := _, map_mul' := _, map_zero' := _, map_add' := _}
Evaluate a polynomial p given a valuation g of all the variables
and a ring hom f from the scalar ring to the target
Equations
- mv_polynomial.eval₂ f g p = finsupp.sum p (λ (s : σ →₀ ℕ) (a : R), (⇑f a) * s.prod (λ (n : σ) (e : ℕ), g n ^ e))
mv_polynomial.eval₂ as a ring_hom.
Equations
Evaluate a polynomial p given a valuation f of all the variables
Equations
map f p maps a polynomial p across a ring hom f
Equations
The algebra of multivariate polynomials
A map σ → S₁ where S₁ is an algebra over R generates an R-algebra homomorphism
from multivariate polynomials over σ to S₁.
Equations
- mv_polynomial.aeval f = {to_fun := (mv_polynomial.eval₂_hom (algebra_map R S₁) f).to_fun, map_one' := _, map_mul' := _, map_zero' := _, map_add' := _, commutes' := _}