Factored Elements

In many applications in number theory related to the multiplicative structure of number fields, interesting elements, e.g. units, are extremely large when written with respect to a fixed basis for the field: for the fundamental unit in $Q[\sqrt{d}]$ it is known that the coefficients with respect to the canonical basis $1,\sqrt{d}$ can have $O(\exp \sqrt{d})$ many digits. All currently known, fast methods construct those elements as power products of smaller elements, allowing the computer to handle them.

Mathematically, one can think of factored elements to formally live in the ring $\mathbb{Z}[K^{\ast }]$ the group ring of the non-zero field elements. Thus elements are of the form $\prod a_i^{e_i}$ where $a_i$ are elements in $K$, typically small and the $e_i\in \mathbb{Z}$ are frequently large exponents. We refer to the $a_i$ as the base and the $e_i$ as the exponents of the factored element.

Presentations of elements of $K^{\ast }$ are non-unique; elements in this form can easily be multiplied, raised to large powers, but in general not compared and not added.

In Hecke, this is captured more generally by the type FacElem, parametrized by the type of the elements in the base and the type of their parent.

Important special cases are

• FacElem{ZZRingElem, ZZRing}, factored integers

• FacElem{AbsSimpleNumFieldElem, AbsSimpleNumField}, factored algebraic numbers

• FacElem{AbsNumFieldOrderIdeal, AbsNumFieldOrderIdealSet}, factored ideals

It should be noted that an object of type FacElem{ZZRingElem, ZZRing} will, in general, not represent an integer as the exponents can be negative.

Construction

In general one can define factored elements by giving 2 arrays, the base and the exponent, or a dictionary containing the pairs:

FacElem Type

FacElem{B, S}

Type for factored elements, that is elements of the form prod a_i^k_i for elements a_i of type B in a ring of type S.

source

FacElem Method

FacElem{B}(R, base::Vector{B}, exp::Vector{ZZRingElem}) -> FacElem{B}

Returns the element $\prod b_i^{e_i}$, un-expanded.

source

FacElem{B}(base::Vector{B}, exp::Vector{ZZRingElem}) -> FacElem{B}

Returns the element $\prod b_i^{e_i}$, un-expanded.

source

FacElem{B}(R, d::Dict{B, ZZRingElem}) -> FacElem{B}
FacElem{B}(R, d::Dict{B, Integer}) -> FacElem{B}

Returns the element $\prod b^{d[p]}$, un-expanded.

source

FacElem{B}(d::Dict{B, ZZRingElem}) -> FacElem{B}
FacElem{B}(d::Dict{B, Integer}) -> FacElem{B}

Returns the element $\prod b^{d[p]}$, un-expanded.

source

ideal Method

 ideal(O::AbsSimpleNumFieldOrder, a::FacElem{AbsSimpleNumFieldElem, AbsSimpleNumField)

The factored fractional ideal $a\ast O$.

source

Conversion

The process of computing the value defined by a factored element is available as evaluate. Depending on the types involved this can be very efficient.

evaluate Method

evaluate{T}(x::FacElem{T}) -> T

Expands or evaluates the factored element, i.e. actually computes the value. Does "square-and-multiply" on the exponent vectors.

source

evaluate Method

evaluate(x::FacElem{QQFieldElem}) -> QQFieldElem
evaluate(x::FacElem{ZZRingElem}) -> ZZRingElem

Expands or evaluates the factored element, i.e. actually computes the the element. Works by first obtaining a simplified version of the power product into coprime base elements.

source

evaluate Method

evaluate{T}(x::FacElem{T}) -> T

Expands or evaluates the factored element, i.e. actually computes the value. Does "square-and-multiply" on the exponent vectors.

source

evaluate_naive Method

evaluate_naive{T}(x::FacElem{T}) -> T

Expands or evaluates the factored element, i.e. actually computes the value. Uses the obvious naive algorithm. Faster for input in finite rings.

source

Special functions

In the case where the parent of the base allows for efficient gcd computation, power products can be made unique:

simplify Method

simplify(x::FacElem{AbsNumFieldOrderIdeal{AbsSimpleNumField, AbsSimpleNumFieldElem}, AbsNumFieldOrderIdealSet{AbsSimpleNumField, AbsSimpleNumFieldElem}}) -> FacElem
simplify(x::FacElem{AbsSimpleNumFieldOrderFractionalIdeal, AbsNumFieldOrderFractionalIdealSet{AbsSimpleNumField, AbsSimpleNumFieldElem}}) -> FacElem

Uses coprime_base to obtain a simplified version of $x$, ie. in the simplified version all base ideals will be pariwise coprime but not necessarily prime!.

source

simplify Method

simplify(x::FacElem{QQFieldElem}) -> FacElem{QQFieldElem}
simplify(x::FacElem{ZZRingElem}) -> FacElem{ZZRingElem}

Simplfies the factored element, i.e. arranges for the base to be coprime.

source

The simplified version can then be used further:

isone Method

isone(x::FacElem{QQFieldElem}) -> Bool
isone(x::FacElem{ZZRingElem}) -> Bool

Tests if $x$ represents $1$ without an evaluation.

source

factor_coprime Method

factor_coprime(x::FacElem{ZZRingElem}) -> Fac{ZZRingElem}

Computed a partial factorisation of $x$, ie. writes $x$ as a product of pariwise coprime integers.

source

factor_coprime Method

factor_coprime(x::FacElem{AbsNumFieldOrderIdeal{AbsSimpleNumField, AbsSimpleNumFieldElem}, AbsNumFieldOrderIdealSet{AbsSimpleNumField, AbsSimpleNumFieldElem}}) -> Dict{AbsNumFieldOrderIdeal{AbsSimpleNumField, AbsSimpleNumFieldElem}, Int}

Computed a partial factorisation of $x$, ie. writes $x$ as a product of pariwise coprime integral ideals.

source

factor_coprime Method

factor_coprime(Q::FacElem{AbsSimpleNumFieldOrderFractionalIdeal, AbsNumFieldOrderFractionalIdealSet{AbsSimpleNumField, AbsSimpleNumFieldElem}}) -> Dict{AbsNumFieldOrderIdeal{AbsSimpleNumField, AbsSimpleNumFieldElem}, Int}

A coprime factorisation of $Q$: each ideal in $Q$ is split using \code{integral_split} and then a coprime basis is computed. This does {\bf not} use any factorisation.

source

factor_coprime Method

factor_coprime(I::AbsNumFieldOrderIdealSet{AbsSimpleNumField, AbsSimpleNumFieldElem}, a::FacElem{AbsSimpleNumFieldElem, AbsSimpleNumField}) -> Dict{AbsNumFieldOrderIdeal{AbsSimpleNumField, AbsSimpleNumFieldElem}, ZZRingElem}

Factors the rincipal ideal generated by $a$ into coprimes by computing a coprime basis from the principal ideals in the factorisation of $a$.

source

factor Method

 factor(Q::FacElem{AbsSimpleNumFieldOrderFractionalIdeal, AbsNumFieldOrderFractionalIdealSet{AbsSimpleNumField, AbsSimpleNumFieldElem}}) -> Dict{AbsNumFieldOrderIdeal{AbsSimpleNumField, AbsSimpleNumFieldElem}, Int}

The factorisation of $Q$, by refining a coprime factorisation.

source

factor Method

factor(I::AbsNumFieldOrderIdealSet{AbsSimpleNumField, AbsSimpleNumFieldElem}, a::FacElem{AbsSimpleNumFieldElem, AbsSimpleNumField}) -> Dict{AbsNumFieldOrderIdeal{AbsSimpleNumField, AbsSimpleNumFieldElem}, ZZRingElem}

Factors the principal ideal generated by $a$ by refining a coprime factorisation.

source

For factorised algebraic numbers a unique simplification is not possible, however, this allows still do obtain partial results:

compact_presentation Function

compact_presentation(a::FacElem{AbsSimpleNumFieldElem, AbsSimpleNumField}, n::Int = 2; decom, arb_prec = 100, short_prec = 1000) -> FacElem

Computes a presentation $a=\prod a_i^{n_i}$ where all the exponents $n_i$ are powers of $n$ and, the elements $a_i$ are "small", generically, they have a norm bounded by $d^{n/2}$ where $d$ is the discriminant of the maximal order. As the algorithm needs the factorisation of the principal ideal generated by $a$, it can be passed in in \code{decom}.

source

valuation Method

valuation(a::FacElem{AbsSimpleNumFieldElem, AbsSimpleNumField}, P::AbsNumFieldOrderIdeal{AbsSimpleNumField, AbsSimpleNumFieldElem}) -> ZZRingElem

The valuation of $a$ at $P$.

source

valuation Method

valuation(A::FacElem{AbsSimpleNumFieldOrderFractionalIdeal, AbsNumFieldOrderFractionalIdealSet{AbsSimpleNumField, AbsSimpleNumFieldElem}}, p::AbsNumFieldOrderIdeal{AbsSimpleNumField, AbsSimpleNumFieldElem})
valuation(A::FacElem{AbsNumFieldOrderIdeal{AbsSimpleNumField, AbsSimpleNumFieldElem}, AbsNumFieldOrderIdealSet{AbsSimpleNumField, AbsSimpleNumFieldElem}}, p::AbsNumFieldOrderIdeal{AbsSimpleNumField, AbsSimpleNumFieldElem})

The valuation of $A$ at $P$.

source

evaluate_mod Method

evaluate_mod(a::FacElem{AbsSimpleNumFieldElem, AbsSimpleNumField}, B::AbsSimpleNumFieldOrderFractionalIdeal) -> AbsSimpleNumFieldElem

Evaluates $a$ using CRT and small primes. Assumes that the ideal generated by $a$ is in fact $B$. Useful in cases where $a$ has huge exponents, but the evaluated element is actually "small".

source

reduce_ideal Method

reduce_ideal(A::FacElem{AbsNumFieldOrderIdeal{AbsSimpleNumField, AbsSimpleNumFieldElem}}) -> AbsNumFieldOrderIdeal{AbsSimpleNumField, AbsSimpleNumFieldElem}, FacElem{AbsSimpleNumFieldElem}

Computes $B$ and $\alpha$ in factored form, such that $\alpha B=A$.

source

modular_proj Method

modular_proj(a::FacElem{AbsSimpleNumFieldElem, AbsSimpleNumField}, me::modular_env) -> Vector{fqPolyRepFieldElem}

Given an algebraic number $a$ in factored form and data \code{me} as computed by \code{modular_init}, project $a$ onto the residue class fields.

source

Positivity & Signs

Factored elements can be used instead of number field elements for the functions sign, signs, is_positive, is_negative and is_totally_positive, see Positivity & Signs

Miscellaneous

max_exp Method

max_exp(a::FacElem)

Finds the largest exponent in the factored element $a$.

source

min_exp Method

min_exp(a::FacElem)

Finds the smallest exponent in the factored element $a$.

source

maxabs_exp Method

maxabs_exp(a::FacElem)

Finds the largest exponent by absolute value in the factored element $a$.

source