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# Polynomials

Basic arithmetic, integration, differentiation, evaluation, and root finding over dense univariate polynomials. #### Poly(a::Vector) where {T<:Number}

Construct a polynomial from its coefficients, lowest order first.

``````julia> Poly([1,0,3,4])
Poly(1 + 3x^2 + 4x^3)
``````

An optional variable parameter can be added.

``````julia> Poly([1,2,3], :s)
Poly(1 + 2s + 3s^2)
``````

#### poly(r::AbstractVector)

Construct a polynomial from its roots. This is in contrast to the `Poly` constructor, which constructs a polynomial from its coefficients.

``````// Represents (x-1)*(x-2)*(x-3)
julia> poly([1,2,3])
Poly(-6 + 11x - 6x^2 + x^3)
``````

#### +, -, *, /, div, ==

The usual arithmetic operators are overloaded to work on polynomials, and combinations of polynomials and scalars.

``````julia> p = Poly([1,2])
Poly(1 + 2x)

julia> q = Poly([1, 0, -1])
Poly(1 - x^2)

julia> 2p
Poly(2 + 4x)

julia> 2+p
Poly(3 + 2x)

julia> p - q
Poly(2x + x^2)

julia> p * q
Poly(1 + 2x - x^2 - 2x^3)

julia> q / 2
Poly(0.5 - 0.5x^2)

julia> q ÷ p      # `div`, also `rem` and `divrem`
Poly(0.25 - 0.5x)
``````

Note that operations involving polynomials with different variables will error.

``````julia> p = Poly([1, 2, 3], :x)
julia> q = Poly([1, 2, 3], :s)
julia> p + q
ERROR: Polynomials must have same variable.
``````

To get the degree of the polynomial use the `degree` method

``````julia> degree(p)
2

julia> degree(p^2)
4

julia> degree(p-p)
-1
``````

#### polyval(p::Poly, x::Number)

Evaluate the polynomial `p` at `x`.

``````julia> p = Poly([1, 0, -1])
julia> polyval(p, 0.1)
0.99
``````

A call method is also available:

``````julia> p(0.1)
0.99
``````

#### polyint(p::Poly, k::Number=0)

Integrate the polynomial `p` term by term, optionally adding constant term `k`. The order of the resulting polynomial is one higher than the order of `p`.

``````julia> polyint(Poly([1, 0, -1]))
Poly(x - 0.3333333333333333x^3)

julia> polyint(Poly([1, 0, -1]), 2)
Poly(2.0 + x - 0.3333333333333333x^3)
``````

#### polyder(p::Poly)

Differentiate the polynomial `p` term by term. The order of the resulting polynomial is one lower than the order of `p`.

``````julia> polyder(Poly([1, 3, -1]))
Poly(3 - 2x)
``````

#### roots(p::Poly)

Return the roots (zeros) of `p`, with multiplicity. The number of roots returned is equal to the order of `p`. By design, this is not type-stable, the returned roots may be real or complex.

``````julia> roots(Poly([1, 0, -1]))
2-element Array{Float64,1}:
-1.0
1.0

julia> roots(Poly([1, 0, 1]))
2-element Array{Complex{Float64},1}:
0.0+1.0im
0.0-1.0im

julia> roots(Poly([0, 0, 1]))
2-element Array{Float64,1}:
0.0
0.0
``````

#### polyfit(x, y, n=length(x)-1)

• `polyfit`: fits a polynomial (of order `n`) to `x` and `y` using a least-squares approximation. ``` julia> xs = 1:4; ys = exp.(xs); polyfit(xs, ys) Poly(-7.717211620141281 + 17.9146616149694x - 9.77757245502143x^2 + 2.298404288652356x^3) ```

Visual example: #### Other methods

Polynomial objects also have other methods:

• 0-based indexing is used to extract the coefficients of \$a_0 + a_1 x + a_2 x^2 + ...\$, coefficients may be changed using indexing notation.

• `coeffs`: returns the entire coefficient vector

• `degree`: returns the polynomial degree, `length` is 1 plus the degree

• `variable`: returns the polynomial symbol as a degree 1 polynomial

• `norm`: find the `p`-norm of a polynomial

• `conj`: finds the conjugate of a polynomial over a complex fiel

• `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 order `m/n` for a polynomial as a `Pade` object.

• MultiPoly.jl for sparse multivariate polynomials

• MultivariatePolynomials.jl for multivariate polynomials and moments of commutative or non-commutative variables

• Nemo.jl for generic polynomial rings, matrix spaces, fraction fields, residue rings, power series

• PolynomialRoots.jl for a fast complex polynomial root finder

02/24/2014

5 days ago

249 commits