# Generic univariate polynomials

AbstractAlgebra.jl provides a module, implemented in `src/generic/Poly.jl`

for generic polynomials over any commutative ring belonging to the AbstractAlgebra abstract type hierarchy.

As well as implementing the Univariate Polynomial interface, and relevant parts of the Euclidean Ring interface for polynomials over a field, there are many additional generic algorithms implemented for such polynomial rings. We describe this generic functionality below.

All of the generic functionality is part of a submodule of AbstractAlgebra called `Generic`

. This is exported by default so that it is not necessary to qualify the function names with the submodule name.

## Types and parent objects

Polynomials implemented using the AbstractAlgebra generics have type `Generic.Poly{T}`

where `T`

is the type of elements of the coefficient ring. Internally they consist of a Julia array of coefficients and some additional fields for length and a parent object, etc. See the file `src/generic/GenericTypes.jl`

for details.

Parent objects of such polynomials have type `Generic.PolyRing{T}`

.

The string representation of the variable of the polynomial ring, and the base/coefficient ring $R$ is stored in the parent object.

The polynomial element types belong to the abstract type `AbstractAlgebra.PolyElem{T}`

and the polynomial ring types belong to the abstract type `AbstractAlgebra.PolyRing{T}`

. This enables one to write generic functions that can accept any AbstractAlgebra polynomial type.

Note that both the generic polynomial ring type `Generic.PolyRing{T}`

and the abstract type it belongs to, `AbstractAlgebra.PolyRing{T}`

are both called `PolyRing`

. The former is a (parameterised) concrete type for a polynomial ring over a given base ring whose elements have type `T`

. The latter is an abstract type representing all polynomial ring types in AbstractAlgebra.jl, whether generic or very specialised (e.g. supplied by a C library).

## Polynomial ring constructors

In order to construct polynomials in AbstractAlgebra.jl, one must first construct the polynomial ring itself. This is accomplished with the following constructor.

`PolynomialRing(R::AbstractAlgebra.Ring, s::AbstractString; cached::Bool = true)`

Given a base ring `R`

and string `s`

specifying how the generator (variable) should be printed, return a tuple `S, x`

representing the new polynomial ring $S = R[x]$ and the generator $x$ of the ring. By default the parent object `S`

will depend only on `R`

and `x`

and will be cached. Setting the optional argument `cached`

to `false`

will prevent the parent object `S`

from being cached.

A shorthand version of this function is provided: given a base ring `R`

, we abbreviate the constructor as follows.

`R["x"]`

Here are some examples of creating polynomial rings and making use of the resulting parent objects to coerce various elements into the polynomial ring.

**Examples**

```
R, x = PolynomialRing(ZZ, "x")
S, y = PolynomialRing(R, "y")
T, z = QQ["z"]
f = R()
g = R(123)
h = S(BigInt(1234))
k = S(x + 1)
m = T(z + 1)
```

All of the examples here are generic polynomial rings, but specialised implementations of polynomial rings provided by external modules will also usually provide a `PolynomialRing`

constructor to allow creation of their polynomial rings.

## Basic ring functionality

Once a polynomial ring is constructed, there are various ways to construct polynomials in that ring.

The easiest way is simply using the generator returned by the `PolynomialRing`

constructor and build up the polynomial using basic arithmetic, as described in the Ring interface.

The Julia language also has special syntax for the construction of polynomials in terms of a generator, e.g. we can write `2x`

instead of `2*x`

.

The polynomial rings in AbstractAlgebra.jl implement the full Ring interface. Of course the entire Univariate Polynomial Ring interface is also implemented.

We give some examples of such functionality.

**Examples**

```
R, x = PolynomialRing(ZZ, "x")
S, y = PolynomialRing(R, "y")
f = x^3 + 3x + 21
g = (x + 1)*y^2 + 2x + 1
h = zero(S)
k = one(R)
isone(k) == true
iszero(f) == false
n = length(g)
U = base_ring(S)
V = base_ring(y + 1)
v = var(S)
T = parent(y + 1)
g == deepcopy(g)
t = divexact(2g, 2)
```

For polynomials over a field, the Euclidean Ring interface is implemented.

**Examples**

```
R, x = PolynomialRing(QQ, "x")
S = ResidueRing(R, x^3 + 3x + 1)
T, y = PolynomialRing(S, "y")
f = (3*x^2 + x + 2)*y + x^2 + 1
g = (5*x^2 + 2*x + 1)*y^2 + 2x*y + x + 1
h = (3*x^3 + 2*x^2 + x + 7)*y^5 + 2x*y + 1
invmod(f, g)
mulmod(f, g, h)
powmod(f, 3, h)
h = mod(f, g)
q, r = divrem(f, g)
d = gcd(f*h, g*h)
k = gcdinv(f, h)
m = lcm(f, h)
flag, q = divides(g^2, g)
valuation(3g^3, g) == 3
val, q = remove(5g^3, g)
r, s, t = gcdx(g, h)
```

Functions in the Euclidean Ring interface are supported over residue rings that are not fields, except that if an impossible inverse is encountered during the computation an error is thrown.

## Polynomial functionality provided by AbstractAlgebra.jl

The functionality listed below is automatically provided by AbstractAlgebra.jl for any polynomial module that implements the full Univariate Polynomial Ring interface. This includes AbstractAlgebra.jl's own generic polynomial rings.

But if a C library provides all the functionality documented in the Univariate Polynomial Ring interface, then all the functions described here will also be automatically supplied by AbstractAlgebra.jl for that polynomial type.

Of course, modules are free to provide specific implementations of the functions described here, that override the generic implementation.

### Basic functionality

`AbstractAlgebra.Generic.modulus`

— Method.`modulus{T <: ResElem}(a::AbstractAlgebra.PolyElem{T})`

Return the modulus of the coefficients of the given polynomial.

`AbstractAlgebra.Generic.lead`

— Method.`lead(x::AbstractAlgebra.PolyElem)`

Return the leading coefficient of the given polynomial. This will be the nonzero coefficient of the term with highest degree unless the polynomial in the zero polynomial, in which case a zero coefficient is returned.

`AbstractAlgebra.Generic.trail`

— Method.`trail(x::AbstractAlgebra.PolyElem)`

Return the trailing coefficient of the given polynomial. This will be the nonzero coefficient of the term with lowest degree unless the polynomial in the zero polynomial, in which case a zero coefficient is returned.

`AbstractAlgebra.Generic.gen`

— Method.`gen{T <: RingElement}(R::AbsSeriesRing{T})`

Return the generator of the power series ring, i.e. $x + O(x^n)$ where $n$ is the precision of the power series ring $R$.

`AbstractAlgebra.Generic.isgen`

— Method.`isgen(a::AbstractAlgebra.PolyElem)`

Return

`true`

if the given polynomial is the constant generator of its polynomial ring, otherwise return`false`

.

`AbstractAlgebra.Generic.isunit`

— Method.`isunit(a::AbstractAlgebra.PolyElem)`

Return

`true`

if the given polynomial is a unit in its polynomial ring, otherwise return`false`

.

`AbstractAlgebra.Generic.ismonomial`

— Method.`ismonomial(a::AbstractAlgebra.PolyElem)`

Return

`true`

if the given polynomial is a monomial.

`AbstractAlgebra.Generic.isterm`

— Method.`isterm(a::AbstractAlgebra.PolyElem)`

Return

`true`

if the given polynomial is has one term. This function is recursive, with all scalar types returning true.

**Examples**

```
R, x = PolynomialRing(ZZ, "x")
S, y = PolynomialRing(R, "y")
T, z = PolynomialRing(QQ, "z")
U = ResidueRing(ZZ, 17)
V, w = PolynomialRing(U, "w")
a = zero(S)
b = one(S)
c = BigInt(1)//2*z^2 + BigInt(1)//3
d = x*y^2 + (x + 1)*y + 3
f = lead(d)
y = gen(S)
g = isgen(w)
m = isunit(b)
n = degree(d)
r = modulus(w)
isterm(2y^2) == true
ismonomial(y^2) == true
```

### Truncation

`Base.truncate`

— Method.`truncate(a::AbstractAlgebra.PolyElem, n::Int)`

Return $a$ truncated to $n$ terms.

`AbstractAlgebra.Generic.mullow`

— Method.`mullow{T <: RingElement}(a::AbstractAlgebra.PolyElem{T}, b::AbstractAlgebra.PolyElem{T}, n::Int)`

Return $a\times b$ truncated to $n$ terms.

**Examples**

```
R, x = PolynomialRing(ZZ, "x")
S, y = PolynomialRing(R, "y")
f = x*y^2 + (x + 1)*y + 3
g = (x + 1)*y + (x^3 + 2x + 2)
h = truncate(f, 1)
k = mullow(f, g, 4)
```

### Reversal

`Base.reverse`

— Method.`reverse(x::AbstractAlgebra.PolyElem, len::Int)`

Return the reverse of the polynomial $x$, thought of as a polynomial of the given length (the polynomial will be notionally truncated or padded with zeroes before the leading term if necessary to match the specified length). The resulting polynomial is normalised. If

`len`

is negative we throw a`DomainError()`

.

`Base.reverse`

— Method.`reverse(x::AbstractAlgebra.PolyElem)`

Return the reverse of the polynomial $x$, i.e. the leading coefficient of $x$ becomes the constant coefficient of the result, etc. The resulting polynomial is normalised.

**Examples**

```
R, x = PolynomialRing(ZZ, "x")
S, y = PolynomialRing(R, "y")
f = x*y^2 + (x + 1)*y + 3
g = reverse(f, 7)
h = reverse(f)
```

### Shifting

`AbstractAlgebra.Generic.shift_left`

— Method.`shift_left(x::AbstractAlgebra.PolyElem, n::Int)`

Return the polynomial $f$ shifted left by $n$ terms, i.e. multiplied by $x^n$.

`AbstractAlgebra.Generic.shift_right`

— Method.`shift_right(f::AbstractAlgebra.PolyElem, n::Int)`

Return the polynomial $f$ shifted right by $n$ terms, i.e. divided by $x^n$.

**Examples**

```
R, x = PolynomialRing(ZZ, "x")
S, y = PolynomialRing(R, "y")
f = x*y^2 + (x + 1)*y + 3
g = shift_left(f, 7)
h = shift_right(f, 2)
```

### Pseudodivision

Given two polynomials $a, b$, pseudodivision computes polynomials $q$ and $r$ with length$(r) <$ length$(b)$ such that $L^d a = bq + r,$ where $d =$ length$(a) -$ length$(b) + 1$ and $L$ is the leading coefficient of $b$.

We call $q$ the pseudoquotient and $r$ the pseudoremainder.

`AbstractAlgebra.Generic.pseudorem`

— Method.`pseudorem{T <: RingElement}(f::AbstractAlgebra.PolyElem{T}, g::AbstractAlgebra.PolyElem{T})`

Return the pseudoremainder of $a$ divided by $b$. If $b = 0$ we throw a

`DivideError()`

.

`AbstractAlgebra.Generic.pseudodivrem`

— Method.`pseudodivrem{T <: RingElement}(f::AbstractAlgebra.PolyElem{T}, g::AbstractAlgebra.PolyElem{T})`

Return a tuple $(q, r)$ consisting of the pseudoquotient and pseudoremainder of $a$ divided by $b$. If $b = 0$ we throw a

`DivideError()`

.

**Examples**

```
R, x = PolynomialRing(ZZ, "x")
S, y = PolynomialRing(R, "y")
f = x*y^2 + (x + 1)*y + 3
g = (x + 1)*y + (x^3 + 2x + 2)
h = pseudorem(f, g)
q, r = pseudodivrem(f, g)
```

### Content and primitive part

`AbstractAlgebra.Generic.content`

— Method.`content(a::AbstractAlgebra.PolyElem)`

Return the content of $a$, i.e. the greatest common divisor of its coefficients.

`AbstractAlgebra.Generic.primpart`

— Method.`primpart(a::AbstractAlgebra.PolyElem)`

Return the primitive part of $a$, i.e. the polynomial divided by its content.

**Examples**

```
R, x = PolynomialRing(ZZ, "x")
S, y = PolynomialRing(R, "y")
k = x*y^2 + (x + 1)*y + 3
n = content(k)
p = primpart(k*(x^2 + 1))
```

### Evaluation, composition and substitution

`AbstractAlgebra.Generic.evaluate`

— Method.`evaluate{T <: RingElement}(a::AbstractAlgebra.PolyElem{T}, b::T)`

Evaluate the polynomial $a$ at the value $b$ and return the result.

`AbstractAlgebra.Generic.evaluate`

— Method.`evaluate{T <: RingElement}(a::AbstractAlgebra.PolyElem{T}, b::T)`

Evaluate the polynomial $a$ at the value $b$ and return the result.

`AbstractAlgebra.Generic.compose`

— Method.`compose(a::AbstractAlgebra.PolyElem, b::AbstractAlgebra.PolyElem)`

Compose the polynomial $a$ with the polynomial $b$ and return the result, i.e. return $a\circ b$.

`AbstractAlgebra.Generic.subst`

— Method.`subst{T <: RingElement}(f::AbstractAlgebra.PolyElem{T}, a::Any)`

Evaluate the polynomial $f$ at $a$. Note that $a$ can be anything, whether a ring element or not.

We also overload the functional notation so that the polynomial $f$ can be evaluated at $a$ by writing $f(a)$.

**Examples**

```
R, x = PolynomialRing(ZZ, "x")
S, y = PolynomialRing(R, "y")
f = x*y^2 + (x + 1)*y + 3
g = (x + 1)*y + (x^3 + 2x + 2)
M = R[x + 1 2x; x - 3 2x - 1]
k = evaluate(f, 3)
m = evaluate(f, x^2 + 2x + 1)
n = compose(f, g)
p = subst(f, M)
q = f(M)
r = f(23)
```

### Derivative and integral

`AbstractAlgebra.Generic.derivative`

— Method.`derivative(a::AbstractAlgebra.PolyElem)`

Return the derivative of the polynomial $a$.

`AbstractAlgebra.Generic.integral`

— Method.`integral{T <: Union{AbstractAlgebra.ResElem, FieldElement}}(x::AbstractAlgebra.PolyElem{T})`

Return the integral of the polynomial $a$.

**Examples**

```
R, x = PolynomialRing(ZZ, "x")
S, y = PolynomialRing(R, "y")
T, z = PolynomialRing(QQ, "z")
U = ResidueRing(T, z^3 + 3z + 1)
V, w = PolynomialRing(U, "w")
f = x*y^2 + (x + 1)*y + 3
g = (z^2 + 2z + 1)*w^2 + (z + 1)*w - 2z + 4
h = derivative(f)
k = integral(g)
```

### Resultant and discriminant

`AbstractAlgebra.Generic.resultant`

— Method.`resultant{T <: RingElem}(a::AbstractAlgebra.PolyElem{T}, b::AbstractAlgebra.PolyElem{T})`

Return the resultant of the given polynomials.

`AbstractAlgebra.Generic.resx`

— Method.`resx{T <: RingElement}(a::AbstractAlgebra.PolyElem{T}, b::AbstractAlgebra.PolyElem{T})`

Return a tuple $(r, s, t)$ such that $r$ is the resultant of $a$ and $b$ and such that $r = a\times s + b\times t$.

`AbstractAlgebra.Generic.discriminant`

— Method.`discriminant(a::AbstractAlgebra.PolyElem)`

Return the discrimnant of the given polynomial.

**Examples**

```
R, x = PolynomialRing(ZZ, "x")
S, y = PolynomialRing(R, "y")
f = 3x*y^2 + (x + 1)*y + 3
g = 6(x + 1)*y + (x^3 + 2x + 2)
h = resultant(f, g)
k = discriminant(f)
```

### Newton representation

`monomial_to_newton!{T <: RingElement}(P::Array{T, 1}, roots::Array{T, 1})`

Converts a polynomial $p$, given as an array of coefficients, in-place from its coefficients given in the standard monomial basis to the Newton basis for the roots $r_0, r_1, \ldots, r_{n-2}$. In other words, this determines output coefficients $c_i$ such that $c_0 + c_1(x-r_0) + c_2(x-r_0)(x-r_1) + \ldots + c_{n-1}(x-r_0)(x-r_1)\cdots(x-r_{n-2})$ is equal to the input polynomial.

`newton_to_monomial!{T <: RingElement}(P::Array{T, 1}, roots::Array{T, 1})`

Converts a polynomial $p$, given as an array of coefficients, in-place from its coefficients given in the Newton basis for the roots $r_0, r_1, \ldots, r_{n-2}$ to the standard monomial basis. In other words, this evaluates $c_0 + c_1(x-r_0) + c_2(x-r_0)(x-r_1) + \ldots + c_{n-1}(x-r_0)(x-r_1)\cdots(x-r_{n-2})$ where $c_i$ are the input coefficients given by $p$.

**Examples**

```
R, x = PolynomialRing(ZZ, "x")
S, y = PolynomialRing(R, "y")
f = 3x*y^2 + (x + 1)*y + 3
g = deepcopy(f)
roots = [R(1), R(2), R(3)]
monomial_to_newton!(g.coeffs, roots)
newton_to_monomial!(g.coeffs, roots)
```

### Interpolation

`AbstractAlgebra.Generic.interpolate`

— Method.`interpolate{T <: RingElement}(S::AbstractAlgebra.PolyRing, x::Array{T, 1}, y::Array{T, 1})`

Given two arrays of values $xs$ and $ys$ of the same length $n$, find the polynomial $f$ in the polynomial ring $R$ of length at most $n$ such that $f$ has the value $ys$ at the points $xs$. The values in the arrays $xs$ and $ys$ must belong to the base ring of the polynomial ring $R$. If no such polynomial exists, an exception is raised.

**Examples**

```
R, x = PolynomialRing(ZZ, "x")
S, y = PolynomialRing(R, "y")
xs = [R(1), R(2), R(3), R(4)]
ys = [R(1), R(4), R(9), R(16)]
f = interpolate(S, xs, ys)
```

### Special functions

The following special functions can be computed for any polynomial ring. Typically one uses the generator $x$ of a polynomial ring to get the respective special polynomials expressed in terms of that generator.

`AbstractAlgebra.Generic.chebyshev_t`

— Method.`chebyshev_t(n::Int, x::AbstractAlgebra.PolyElem)`

Return the Chebyshev polynomial of the first kind $T_n(x)$, defined by $T_n(x) = \cos(n \cos^{-1}(x))$.

`AbstractAlgebra.Generic.chebyshev_u`

— Method.`chebyshev_u(n::Int, x::AbstractAlgebra.PolyElem)`

Return the Chebyshev polynomial of the first kind $U_n(x)$, defined by $(n+1) U_n(x) = T'_{n+1}(x)$.

**Examples**

```
R, x = PolynomialRing(ZZ, "x")
S, y = PolynomialRing(R, "y")
f = chebyshev_t(20, y)
g = chebyshev_u(15, y)
```