# factorization

© 2012,2014 John Abbott, Anna Bigatti
GNU Free Documentation License, Version 1.2

- Examples
- User documentation
  - Constructor
  - Accessors
  - Operations
- Maintainer documentation
- Bugs, shortcomings and other ideas
- Main changes

CoCoALib Documentation Index

## Examples

- ex-factor1.C

## User documentation

In CoCoALib `factorization` is a template class for representing (partial) factorizations. Conceptually it comprises a list of factors and their multiplicities, and an extra remaining factor (which may be, for instance, an unfactorized part, or an invertible element).

The class itself imposes few conditions: the factors in the list cannot be invertible or zero-divisors, and their multiplicities are all positive. The remaining factor is a non-zero-divisor. The exact characteristics of the factors depend on the function which generated the `factorization`. Naturally the vectors returned by `myFactors` and `myMultiplicities` will be of the same length.

See also: `factor`, `SqFreeFactor`, `ContentFreeFactor` (see section `factor`), `SmoothFactor` (see section `NumTheory`)

### Constructor

- `factorization(remfactor)` specifies an initial remaining factor, the factor/multiplicity lists are empty
- `factorization(facs, mults, remfactor)` specifies initial values for the 3 components

### Accessors

Let `FactorInfo` be of type `factorization<T>`. These are the accessor functions:

- `FactorInfo.myFactors()` all the factors as a read-only `std::vector`
- `FactorInfo.myMultiplicities()` all the multiplicities as a `std::vector<long>` (read-only)
- `FactorInfo.myRemainingFactor()` the remaining factor (read-only)

For better readability of code using `factorization` we recommend using const ref aliases for the lists of factors and multiplicities; for instance

        const factorization<RingElem> FactorInfo = factor(f);
        const vector<RingElem>& facs = FactorInfo.myFactors();
        const vector<long>& mults = FactorInfo.myMultiplicities();
        ...
        // code using the arrays "facs" and "mults"

### Operations

Let `facs` be of type `factorization<T>`. These are the operations available:

- `facs.myAppend(fac, mult)` appends a new factor with its multiplicity
- `facs.myNewRemainingFactor(RemFac)` sets `RemFac` as the remaining factor

## Maintainer documentation

Being template code it's all in the header file. It's mostly fairly straightfoward.

The main point to note is that `ourCheckNotZeroDiv` and `ourCheckNotUnit` need to be written by hand for each instantiation -- this is enforced by the absence of a default template impl. Note that the impls for `DUPFp` are defined in the file `DUPFp.H`.

The fn `ourCheckCompatibility` is needed for `RingElem` but not for other types (so the default impl is empty). It simply checks that all the factors belong to the same ring (equiv. that they belong to ring of `myRemainingFactorValue`).

In CoCoALib there are just 4 instantiations of this template:

- `factorization<BigInt>` for the fns `factor` and `SmoothFactor` in `NumTheory`
- `factorization<RingElem>` for the fns `factor` and `SqFreeFactor` and `ContentFreeFactor` in `PolyRing` (actually `TmpFactor`)
- `factorization<long>`
- `factorization<DUPFp>`

## Bugs, shortcomings and other ideas

It would be safer to have pairs of factor-and-multiplicity rather than two separate vectors whose length must be the same. However it may be less convenient for the user.

Maybe add fn to get length of a `factorization`? (same as length of `myFactors()`)

Maybe add fn to get ring of a `factorization<RingElem>`?

Maybe add fn to change the multiplicity of a factor?

Bruns questioned the necessity of the restriction that factors be non-zero-divisiors and non-units. JAA prefers to apply these restrictions for the time being, because it will be easier to relax the restrictions later than it would be to tighten them (might break some existing code).

Bruns/Soeger asked whether requiring all factors to be in the same ring is necessary (esp. once CoCoA allows arithmetic between different rings). They cite the example of factors in ZZ[x] and remaining factor in QQ.

## Main changes

2014

- March: fields are now private: new accessor fns; new append fn, and new fn to update rem factor

2012

- October: chose "myMultiplicities" rather than "myExponents" as the field name.
- April: first version of doc (v0.9950)