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^s -
a : R -
i : σ, with corresponding monomialX i, often denotedX_iby mathematicians -
p : mv_polynomial σ R
Definitions #
-
mv_polynomial σ R: the type of polynomials with variables of typeσand coefficients in the commutative semiringR -
monomial s a: the monomial which mathematically would be denoteda * X^s -
C a: the constant polynomial with valuea -
X 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 typeR -
map (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
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
Equations
If R is a subsingleton, then mv_polynomial σ R has a unique element
Equations
monomial s a is the monomial with coefficient a and exponents given by s
Equations
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
λ s, monomial s 1 as a homomorphism.
Equations
- mv_polynomial.monomial_one_hom R σ = add_monoid_algebra.of R (σ →₀ ℕ)
Analog of polynomial.induction_on'.
To prove something about mv_polynomials,
it suffices to show the condition is closed under taking sums,
and it holds for monomials.
Similar to mv_polynomial.induction_on but only a weak form of h_add is required.
Similar to mv_polynomial.induction_on but only a yet weaker form of h_add is required.
Analog of polynomial.induction_on.
The finite set of all m : σ →₀ ℕ such that X^m has a non-zero coefficient.
The coefficient of the monomial m in the multi-variable polynomial p.
Equations
- mv_polynomial.coeff m p = ⇑p m
mv_polynomial.coeff m but promoted to an add_monoid_hom.
Equations
- mv_polynomial.coeff_add_monoid_hom m = {to_fun := mv_polynomial.coeff m, map_zero' := _, map_add' := _}
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
- mv_polynomial.eval₂_hom f g = {to_fun := mv_polynomial.eval₂ f g, map_one' := _, map_mul' := _, map_zero' := _, map_add' := _}
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
If f is a left-inverse of g then map f is a left-inverse of map g.
If f is a right-inverse of g then map f is a right-inverse of map g.
If f : S₁ →ₐ[R] S₂ is a morphism of R-algebras, then so is mv_polynomial.map f.
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' := _}
Version of aeval for defining algebra homs out of mv_polynomial σ R over a smaller base ring
than R.