Polynomials.jl
Basic arithmetic, integration, differentiation, evaluation, and root finding over dense univariate polynomials.
Installation
(v1.6) pkg> add Polynomials
Available Types of Polynomials
Polynomial
– standard basis polynomials,a(x) = a₀ + a₁ x + a₂ x² + … + aₙ xⁿ
,n ∈ ℕ
ImmutablePolynomial
– standard basis polynomials backed by a Tuple type for faster evaluation of valuesSparsePolynomial
– standard basis polynomial backed by a dictionary to hold sparse highdegree polynomialsLaurentPolynomial
– Laurent polynomials,a(x) = aₘ xᵐ + … + aₙ xⁿ
m ≤ n
,m,n ∈ ℤ
backed by an offset array; for example, ifm<0
andn>0
,a(x) = aₘ xᵐ + … + a₋₁ x⁻¹ + a₀ + a₁ x + … + aₙ xⁿ
ChebyshevT
– Chebyshev polynomials of the first kind
Usage
julia> using Polynomials
Construction and Evaluation
Construct a polynomial from an array (a vector) of its coefficients, lowest order first.
julia> Polynomial([1,0,3,4])
Polynomial(1 + 3*x^2 + 4*x^3)
Optionally, the variable of the polynomial can be specified.
julia> Polynomial([1,2,3], :s)
Polynomial(1 + 2*s + 3*s^2)
Construct a polynomial from its roots.
julia> fromroots([1,2,3]) # (x1)*(x2)*(x3)
Polynomial(6 + 11*x  6*x^2 + x^3)
Evaluate the polynomial p
at x
.
julia> p = Polynomial([1, 0, 1]);
julia> p(0.1)
0.99
Arithmetic
Methods are added to the usual arithmetic operators so that they work on polynomials, and combinations of polynomials and scalars.
julia> p = Polynomial([1,2])
Polynomial(1 + 2*x)
julia> q = Polynomial([1, 0, 1])
Polynomial(1  x^2)
julia> p  q
Polynomial(2*x + x^2)
julia> p = Polynomial([1,2])
Polynomial(1 + 2*x)
julia> q = Polynomial([1, 0, 1])
Polynomial(1  x^2)
julia> 2p
Polynomial(2 + 4*x)
julia> 2+p
Polynomial(3 + 2*x)
julia> p  q
Polynomial(2*x + x^2)
julia> p * q
Polynomial(1 + 2*x  x^2  2*x^3)
julia> q / 2
Polynomial(0.5  0.5*x^2)
julia> q ÷ p # `div`, also `rem` and `divrem`
Polynomial(0.25  0.5*x)
Operations involving polynomials with different variables will error.
julia> p = Polynomial([1, 2, 3], :x);
julia> q = Polynomial([1, 2, 3], :s);
julia> p + q
ERROR: ArgumentError: Polynomials have different indeterminates
Construction and Evaluation
While polynomials of type Polynomial
are mutable objects, operations such as
+
, 
, *
, always create new polynomials without modifying its arguments.
The time needed for these allocations and copies of the polynomial coefficients
may be noticeable in some use cases. This is amplified when the coefficients
are for instance BigInt
or BigFloat
which are mutable themself.
This can be avoided by modifying existing polynomials to contain the result
of the operation using the MutableArithmetics (MA) API.
Consider for instance the following arrays of polynomials
using Polynomials
d, m, n = 30, 20, 20
p(d) = Polynomial(big.(1:d))
A = [p(d) for i in 1:m, j in 1:n]
b = [p(d) for i in 1:n]
In this case, the arrays are mutable objects for which the elements are mutable
polynomials which have mutable coefficients (BigInt
s).
These three nested levels of mutable objects communicate with the MA
API in order to reduce allocation.
Calling A * b
requires approximately 40 MiB due to 2 M allocations
as it does not exploit any mutability. Using
using MutableArithmetics
const MA = MutableArithmetics
MA.operate(*, A, b)
exploits the mutability and hence only allocate approximately 70 KiB due to 4 k allocations. If the resulting vector is already allocated, e.g.,
z(d) = Polynomial([zero(BigInt) for i in 1:d])
c = [z(2d  1) for i in 1:m]
then we can exploit its mutability with
MA.mutable_operate!(MA.add_mul, c, A, b)
to reduce the allocation down to 48 bytes due to 3 allocations. These remaining
allocations are due to the BigInt
buffer used to store the result of
intermediate multiplications. This buffer can be preallocated with
buffer = MA.buffer_for(MA.add_mul, typeof(c), typeof(A), typeof(b))
MA.mutable_buffered_operate!(buffer, MA.add_mul, c, A, b)
then the second line is allocationfree.
The MA.@rewrite
macro rewrite an expression into an equivalent code that
exploit the mutability of the intermediate results.
For instance
MA.@rewrite(A1 * b1 + A2 * b2)
is rewritten into
c = MA.operate!(MA.add_mul, MA.Zero(), A1, b1)
MA.operate!(MA.add_mul, c, A2, b2)
which is equivalent to
c = MA.operate(*, A1, b1)
MA.mutable_operate!(MA.add_mul, c, A2, b2)
Note that currently, only the Polynomial
implements the API and it only
implements part of it.
Integrals and Derivatives
Integrate the polynomial p
term by term, optionally adding a constant
term k
. The degree of the resulting polynomial is one higher than the
degree of p
(for a nonzero polynomial).
julia> integrate(Polynomial([1, 0, 1]))
Polynomial(1.0*x  0.3333333333333333*x^3)
julia> integrate(Polynomial([1, 0, 1]), 2)
Polynomial(2.0 + 1.0*x  0.3333333333333333*x^3)
Differentiate the polynomial p
term by term. The degree of the
resulting polynomial is one lower than the degree of p
.
julia> derivative(Polynomial([1, 3, 1]))
Polynomial(3  2*x)
Rootfinding
Return the roots (zeros) of p
, with multiplicity. The number of
roots returned is equal to the degree of p
. By design, this is not typestable, the returned roots may be real or complex.
julia> roots(Polynomial([1, 0, 1]))
2element Vector{Float64}:
1.0
1.0
julia> roots(Polynomial([1, 0, 1]))
2element Vector{ComplexF64}:
0.0  1.0im
0.0 + 1.0im
julia> roots(Polynomial([0, 0, 1]))
2element Vector{Float64}:
0.0
0.0
Fitting arbitrary data
Fit a polynomial (of degree deg
or less) to x
and y
using a leastsquares approximation.
julia> xs = 0:4; ys = @. exp(xs) + sin(xs);
julia> fit(xs, ys) > p > round.(coeffs(p), digits=4) > Polynomial
Polynomial(1.0 + 0.0593*x + 0.3959*x^2  0.2846*x^3 + 0.0387*x^4)
julia> fit(ChebyshevT, xs, ys, 2) > p > round.(coeffs(p), digits=4) > ChebyshevT
ChebyshevT(0.5413⋅T_0(x)  0.8991⋅T_1(x)  0.4238⋅T_2(x))
Visual example:
Other methods
Polynomial objects also have other methods:

0based indexing is used to extract the coefficients of
[a0, a1, a2, ...]
, coefficients may be changed using indexing notation. 
coeffs
: returns the entire coefficient vector 
degree
: returns the polynomial degree,length
is number of stored coefficients 
variable
: returns the polynomial symbol as polynomial in the underlying type 
norm
: find thep
norm of a polynomial 
conj
: finds the conjugate of a polynomial over a complex field 
truncate
: set to 0 all small terms in a polynomial; 
chop
chops off any small leading values that may arise due to floating point operations. 
gcd
: greatest common divisor of two polynomials. 
Pade
: Return the Pade approximant of orderm/n
for a polynomial as aPade
object.
Related Packages

StaticUnivariatePolynomials.jl Fixedsize univariate polynomials backed by a Tuple

MultiPoly.jl for sparse multivariate polynomials

DynamicPolynomals.jl Multivariate polynomials implementation of commutative and noncommutative variables

MultivariatePolynomials.jl for multivariate polynomials and moments of commutative or noncommutative variables

PolynomialRings.jl A library for arithmetic and algebra with multivariable polynomials.

AbstractAlgebra.jl, Nemo.jl for generic polynomial rings, matrix spaces, fraction fields, residue rings, power series, Hecke.jl for algebraic number theory.

CommutativeAlgebra.jl the start of a computer algebra system specialized to discrete calculations with support for polynomials.

PolynomialRoots.jl for a fast complex polynomial root finder. For larger degree problems, also FastPolynomialRoots and AMRVW.

SpecialPolynomials.jl A package providing various polynomial types beyond the standard basis polynomials in
Polynomials.jl
. Includes interpolating polynomials, Bernstein polynomials, and classical orthogonal polynomials. 
ClassicalOrthogonalPolynomials.jl A Julia package for classical orthogonal polynomials and expansions. Includes
chebyshevt
,chebyshevu
,legendrep
,jacobip
,ultrasphericalc
,hermiteh
, andlaguerrel
. The same repository includesFastGaussQuadrature.jl
,FastTransforms.jl
, and theApproxFun
packages.
Legacy code
As of v0.7, the internals of this package were greatly generalized and new types and method names were introduced. For compatability purposes, legacy code can be run after issuing using Polynomials.PolyCompat
.
Contributing
If you are interested in contributing, feel free to open an issue or pull request to get started.