# Series Ring Interface

Univariate power series rings are supported in AbstractAlgebra in a variety of different forms, including absolute and relative precision models and Laurent series.

In addition to the standard Ring interface, numerous additional functions are required to be present for power series rings.

## Types and parents

AbstractAlgebra provides two abstract types for power series rings and their elements:

`SeriesRing{T}`

is the abstract type for all power series ring parent types`SeriesElem{T}`

is the abstract type for all power series types

We have that `SeriesRing{T} <: AbstractAlgebra.Ring`

and `SeriesElem{T} <: AbstractAlgebra.RingElem`

.

Note that both abstract types are parameterised. The type `T`

should usually be the type of elements of the coefficient ring of the power series ring. For example, in the case of $\mathbb{Z}[[x]]$ the type `T`

would be the type of an integer, e.g. `BigInt`

.

Within the `SeriesElem{T}`

abstract type is the abstract type `RelSeriesElem{T}`

for relative power series, and `AbsSeriesElem{T}`

for absolute power series.

Relative series are typically stored with a valuation and a series that is either zero or that has nonzero constant term. Absolute series are stored starting from the constant term, even if it is zero.

If the parent object for a relative series ring over the bignum integers has type `MySeriesRing`

and series in that ring have type `MySeries`

then one would have:

`MySeriesRing <: SeriesRing{BigInt}`

`MySeries <: RelSeriesElem{BigInt}`

Series rings should be made unique on the system by caching parent objects (unless an optional `cache`

parameter is set to `false`

). Series rings should at least be distinguished based on their base (coefficient) ring. But if they have the same base ring and symbol (for their variable/generator) and same default precision, they should certainly have the same parent object.

See `src/generic/GenericTypes.jl`

for an example of how to implement such a cache (which usually makes use of a dictionary).

## Required functionality for series

In addition to the required functionality for the Ring interface the Series Ring interface has the following required functions.

We suppose that `R`

is a fictitious base ring (coefficient ring) and that `S`

is a series ring over `R`

(e.g. $S = R[[x]]$) with parent object `S`

of type `MySeriesRing{T}`

. We also assume the series in the ring have type `MySeries{T}`

, where `T`

is the type of elements of the base (coefficient) ring.

Of course, in practice these types may not be parameterised, but we use parameterised types here to make the interface clearer.

Note that the type `T`

must (transitively) belong to the abstract type `RingElem`

.

### Constructors

In addition to the standard constructors, the following constructors, taking an array of coefficients, must be available.

For relative power series and Laurent series we have:

`(S::MySeriesRing{T})(A::Array{T, 1}, len::Int, prec::Int, val::Int) where T <: AbstractAlgebra.RingElem`

Create the series in the given ring whose valuation is `val`

, whose absolute precision is given by `prec`

and the coefficients of which are given by `A`

, starting from the first nonzero term. Only `len`

terms of the array are used, the remaining terms being ignored. The value `len`

cannot exceed the length of the supplied array.

It is permitted to have trailing zeros in the array, but it is not needed, even if the precision minus the valuation is bigger than the length of the array.

**Examples**

```
julia> S, x = PowerSeriesRing(QQ, 10, "x"; model=:capped_relative)
(Univariate power series ring in x over Rationals, x+O(x^11))
julia> T, y = LaurentSeriesRing(ZZ, 10, "y")
(Laurent series ring in y over Integers, y+O(y^11))
julia> U, z = LaurentSeriesField(QQ, 10, "z")
(Laurent series field in z over Rationals, z+O(z^11))
```

For absolute power series we have:

`(S::MySeriesRing{T})(A::Array{T, 1}, len::Int, prec::Int) where T <: AbstractAlgebra.RingElem`

Create the series in the given ring whose absolute precision is given by `prec`

and the coefficients of which are given by `A`

, starting from the constant term. Only `len`

terms of the array are used, the remaining terms being ignored.

Note that `len`

is usually maintained separately of any polynomial that is underlying the power series. This allows for easy trucation of a power series without actually modifying the polynomial underlying it.

It is permitted to have trailing zeros in the array, but it is not needed, even if the precision is bigger than the length of the array.

**Examples**

```
julia> S, x = PowerSeriesRing(QQ, 10, "x"; model=:capped_absolute)
(Univariate power series ring in x over Rationals, x+O(x^10))
julia> f = S(Rational{BigInt}[0, 2, 3, 1], 4, 6)
(2//1)*x+(3//1)*x^2+x^3+O(x^6)
```

### Data type and parent object methods

`var(S::MySeriesRing{T}) where T <: AbstractAlgebra.RingElem`

Return a `Symbol`

representing the variable (generator) of the series ring. Note that this is a `Symbol`

not a `String`

, though its string value will usually be used when printing series.

`max_precision(S::MySeriesRing{T}) where T <: AbstractAlgebra.RingElem`

Return the (default) maximum precision of the power series ring. This is the precision that the output of an operation will be if it cannot be represented to full precision (e.g. because it mathematically has infinite precision).

This value is usually supplied upon creation of the series ring and stored in the ring. It is independent of the precision which each series in the ring actually has. Those are stored on a per element basis in the actual series elements.

**Examples**

```
julia> S, x = PowerSeriesRing(QQ, 10, "x")
(Univariate power series ring in x over Rationals, x+O(x^11))
julia> vsym = var(S)
:x
julia> max_precision(S) == 10
true
```

### Basic manipulation of rings and elements

`pol_length(f::MySeries{T}) where T <: AbstractAlgebra.RingElem`

Return the length of the polynomial underlying the given power series. This is not generally useful to the user, but is used internally.

`set_length!(f::MySeries{T}, n::Int) where T <: AbstractAlgebra.RingElem`

This function sets the effective length of the polynomial underlying the given series. The function doesn't modify the actual polynomial, but simply changes the number of terms of the polynomial which are considered to belong to the power series. The remaining terms are ignored.

This function cannot set the length to a value greater than the length of any underlying polynomial.

The function mutates the series in-place but does not return the mutated series.

`precision(f::MySeries{T})`

Return the absolute precision of $f$.

`set_prec!(f::MySeries{T}, prec::Int)`

Set the absolute precision of the given series to the given value.

This function mutates the series in-place but does not return the mutated series.

`valuation(f::MySeries{T})`

Return the valuation of the given series.

`set_val!(f::MySeries{T}, val::Int)`

For relative series and Laurent series only, this function alters the valuation of the given series to the given value.

The series is mutated in-place but does not return the mutated series.

`polcoeff(f::MySeries{T}, n::Int)`

Return the coefficient of degree `n`

of the polynomial underlying the series. If `n`

is larger than the degree of this polynomial, zero is returned. This function is not generally of use to the user but is used internally.

`setcoeff!(f::MySeries{T}, n::Int, a::T) where T <: AbstractAlgebra.RingElem`

Set the degree $n$ coefficient of the polynomial underlying $f$ to $a$. This mutates the polynomial in-place if possible and returns the mutated series (so that immutable types can also be supported). The function must not assume that the polynomial already has space for $n + 1$ coefficients. The polynomial must be resized if this is not the case.

Note that this function is not required to normalise the polynomial and is not necessarily useful to the user, but is used extensively by the generic functionality in AbstractAlgebra.jl. It is for setting raw coefficients in the representation.

`normalise(f::MySeries{T}, n::Int)`

Given a series $f$ represented by a polynomial of at least the given length, return the normalised length of the underlying polynomial assuming it has length at most $n$. This function does not actually normalise the polynomial and is not particularly useful to the user. It is used internally.

`renormalize!(f::MySeries{T}) where T <: AbstractAlgebra.RingElem`

Given a relative series or Laurent series whose underlying polynomial has zero constant term, say as the result of some internal computation, renormalise the series so that the polynomial has nonzero constant term. The precision and valuation of the series are adjusted to compensate. This function is not intended to be useful to the user, but is used internally.

`fit!(f::MySeries{T}, n::Int) where T <: AbstractAlgebra.RingElem`

Ensure that the polynomial underlying $f$ internally has space for $n$ coefficients. This function must mutate the series in-place if it is mutable. It does not return the mutated series. Immutable types can still be supported by defining this function to do nothing.

Some interfaces for C polynomial types automatically manage the internal allocation of polynomials in every function that can be called on them. Explicit adjustment by the generic code in AbstractAlgebra.jl is not required. In such cases, this function can also be defined to do nothing.

`gen(R::MySeriesRing{T}) where T <: AbstractAlgebra.RingElem`

Return the generator `x`

of the series ring.

**Examples**

```
julia> S, x = PowerSeriesRing(ZZ, 10, "x")
(Univariate power series ring in x over Integers, x+O(x^11))
julia> f = 1 + 3x + x^3 + O(x^5)
1+3*x+x^3+O(x^5)
julia> g = S(BigInt[1, 2, 0, 1, 0, 0, 0], 4, 10, 3);
julia> n = pol_length(f)
4
julia> c = polcoeff(f, 1)
3
julia> set_length!(g, 3)
3
julia> g = setcoeff!(g, 2, BigInt(11))
x^3+2*x^4+11*x^5+O(x^10)
julia> fit!(g, 8)
julia> g = setcoeff!(g, 7, BigInt(4))
x^3+2*x^4+11*x^5+O(x^10)
julia> w = gen(S)
x+O(x^11)
julia> isgen(w)
true
```