Finite field embeddings

Introduction

Nemo allows the construction of finite field embeddings making use of the algorithm of Bosma, Cannon and Steel behind the scenes to ensure compatibility. Critical routines (e.g. polynomial factorization, matrix computations) are provided by the C library Flint, whereas high level tasks are written directly in Nemo.

Embedding functionality

It is possible to explicitly call the embedding embed function to create an embedding, but it is also possible to directly ask for the conversion of a finite field element x in some other finite field k via calling k(x). The resulting embedding is of type FinFieldMorphism. It is also possible to compute the preimage map of an embedding via the preimage_map function, applied to an embedding or directly to the finite fields (this actually first computes the embedding), or via conversion. An error is thrown if the element you want to compute the preimage of is not in the image of the embedding.

Computing an embedding

embed — Method

embed(k::T, K::T) where T <: FinField

Embed $k$ in $K$, with some additional computations in order to satisfy compatibility conditions with previous and future embeddings.

Examples

julia> k2, x2 = FiniteField(19, 2, "x2")
(Finite field of degree 2 over GF(19), x2)

julia> k4, x4 = FiniteField(19, 4, "x4")
(Finite field of degree 4 over GF(19), x4)

julia> f = embed(k2, k4)
Morphism of finite fields from
  from finite field of degree 2 over gF(19)
  to finite field of degree 4 over gF(19)

julia> y = f(x2)
6*x4^3 + 5*x4^2 + 9*x4 + 17

julia> z = k4(x2)
6*x4^3 + 5*x4^2 + 9*x4 + 17

Computing the preimage of an embedding

preimage_map — Method

preimage_map(k::T, k::T) where T <: FinField

Computes the preimage map corresponding to the embedding of $k$ into $K$.

preimage_map — Method

preimage_map(f::FinFieldMorphism)

Compute the preimage map corresponding to the embedding $f$.

Examples

julia> k7, x7 = FiniteField(13, 7, "x7")
(Finite field of degree 7 over GF(13), x7)

julia> k21, x21 = FiniteField(13, 21, "x21")
(Finite field of degree 21 over GF(13), x21)

julia> s = preimage_map(k7, k21)
Preimage of the morphism from Finite field of degree 7 over GF(13) to Finite field of degree 21 over GF(13)

julia> y = k21(x7);

julia> z = s(y)
x7

julia> t = k7(y)
x7