# Matroid Realization Spaces

Let $M$ be a matroid of rank $d$ on a ground set $E$ of size $n$. Its *realization space* $\mathcal{R}(M)$ is an affine scheme that parameterizes all hyperplane arrangements that realize the matroid $M$ (up to the action of $PGL(r)$). We provide functions that determine the affine coordinate ring of $\mathcal{R}(M)$.

`is_realizable`

— Method`is_realizable(M; char::Union{Int,Nothing}=nothing, q::Union{Int,Nothing}=nothing)`

If char = nothing, then this function determines whether the matroid is realizable over some field.

If

`char == 0`

, then this function determines whether the matroid is realizable over some field of characteristic 0.If char = p is prime, this function determines whether the matroid is realizable over the finite field $GF(p)$.

If

`char == p`

and`q`

is a power of`p`

, this function determines whether the matroid is realizable over the finite field $GF(q)$.

`defining_ideal`

— Method`defining_ideal(RS::MatroidRealizationSpace)`

The ideal of the matroid realization space `RS`

.

`inequations`

— Method`inequations(RS::MatroidRealizationSpace)`

Generators of the localizing semigroup of `RS`

. These are the polynomials that need to be nonzero in any realization.

`ambient_ring`

— Method`ambient_ring(RS::MatroidRealizationSpace)`

The polynomial ring containing the ideal `defining_ideal(RS)`

and the polynomials in `inequations(RS)`

.

`realization_space`

— Method```
realization_space(
M::Matroid;
B::Union{GroundsetType,Nothing}=nothing,
saturate::Bool=false,
simplify::Bool=true,
char::Union{Int,Nothing}=nothing,
q::Union{Int,Nothing}=nothing,
ground_ring::Ring=ZZ
)::MatroidRealizationSpace
```

This function returns the data for the coordinate ring of the matroid realization space of the matroid `M`

as a `MatroidRealizationSpace`

. This function has several optional parameters.

`B`

is a basis of M that specifies which columns of`realization_matrix(M)`

form an identity matrix. The default is`nothing`

, in which case the basis is chosen for you.`saturate`

determines whether`defining_ideal(M)`

should be saturated with respect to the semigroup generated by`inequations(M)`

. The default is`false`

. The saturation can be rather slow for large instances.`simplify`

determines whether a reduced realization space is returned which means that the equations are used to eliminate variables as far as possible. The default is`true`

.`char`

specifies the characteristic of the coefficient ring. The returned realization space is then the space of all realizations over fields of characteristic`char`

. The default is`nothing`

.`q`

is an integer and assumed to be a prime power`q=p^k`

. The returned realization space is then the space of all realizations over the field $GF(p^k)$. The default is`nothing`

.`ground_ring`

is a ring and specifies the ground*ring over which one wants to consider the realization space, e.g.*ring`QQ`

or`GF(p)`

. The groud`ZZ`

means that we compute the space of realizations over all fields. The default is`ZZ`

.

**Examples**

```
julia> M = fano_matroid();
julia> RS = realization_space(M)
The realization space is
[0 1 1 1 1 0 0]
[1 0 1 1 0 1 0]
[1 0 1 0 1 0 1]
in the integer ring
within the vanishing set of the ideal
2ZZ
julia> realization_space(non_fano_matroid())
The realization space is
[1 1 0 0 1 1 0]
[0 1 1 1 1 0 0]
[0 1 1 0 0 1 1]
in the integer ring
avoiding the zero loci of the polynomials
RingElem[2]
julia> realization_space(pappus_matroid(), char=0)
The realization space is
[1 0 1 0 x2 x2 x2^2 1 0]
[0 1 1 0 1 1 -x1*x2 + x1 + x2^2 1 1]
[0 0 0 1 x2 x1 x1*x2 x1 x2]
in the multivariate polynomial ring in 2 variables over QQ
avoiding the zero loci of the polynomials
RingElem[x1 - x2, x2, x1, x2 - 1, x1 + x2^2 - x2, x1 - 1, x1*x2 - x1 - x2^2]
julia> realization_space(uniform_matroid(3,6))
The realization space is
[1 0 0 1 1 1]
[0 1 0 1 x1 x3]
[0 0 1 1 x2 x4]
in the multivariate polynomial ring in 4 variables over ZZ
avoiding the zero loci of the polynomials
RingElem[x1*x4 - x2*x3, x2 - x4, x1 - x3, x1*x4 - x1 - x2*x3 + x2 + x3 - x4, x3 - x4, x4 - 1, x3 - 1, x3, x4, x1 - x2, x2 - 1, x1 - 1, x1, x2]
```

`realization`

— Method```
realization(M::Matroid; B::Union{GroundsetType,Nothing} = nothing,
saturate::Bool=false,
char::Union{Int,Nothing}=nothing, q::Union{Int,Nothing}=nothing
)::MatroidRealizationSpace
```

This function tries to find one realization in the matroid realization space of the matroid `M`

. The output is again a `MatroidRealizationSpace`

.

If the matroid is only realizable over an extension of the prime field the extension field is specified as a splitting field of an irreducible polynomial. Every root of this polynomial gives an equivalent realization of the matroid.

This function has several optional parameters. Note that one must input either the characteristic or a specific field of definition for the realization.

`B`

is a basis of M that specifies which columns of`realization_matrix(M)`

form the identity matrix. The default is`nothing`

, in which case the basis is chosen for you.`char`

specifies the characteristic of the coefficient ring, and is used to determine if the matroid is realizable over a field of this characteristic. The default is`nothing`

.`q`

is an integer, and when char = p, this input is used to determine whether the matroid is realizable over the finite field $GF(p^{q})$. The default is`nothing`

.`reduce`

determines whether a reduced realization space is returned which means that the equations are used to eliminate variables as far as possible. The default is`true`

.`saturate`

determines whether`defining_ideal(M)`

should be saturated with respect to the semigroup generated by`inequations(M)`

. The default is`false`

. This can be rather slow for large instances.

**Examples**

```
julia> realization(pappus_matroid(), char=0)
One realization is given by
[1 0 1 0 2 2 4 1 0]
[0 1 1 0 1 1 1 1 1]
[0 0 0 1 2 3 6 3 2]
in the rational field
julia> realization(pappus_matroid(), q=4)
One realization is given by
[1 0 1 0 x1 + 1 x1 + 1 x1 1 0]
[0 1 1 0 1 1 1 1 1]
[0 0 0 1 x1 + 1 x1 1 x1 x1 + 1]
in the multivariate polynomial ring in 1 variable over GF(2)
within the vanishing set of the ideal
Ideal (x1^2 + x1 + 1)
julia> realization(uniform_matroid(3,6), char=5)
One realization is given by
[1 0 0 1 1 1]
[0 1 0 1 4 3]
[0 0 1 1 3 2]
in the prime field of characteristic 5
```

`realization`

— Method`realization(RS::MatroidRealizationSpace)`

This function tries to find one realization in the matroid realization `RS`

. The output is again a `MatroidRealizationSpace`

.

If $B$ is the polynomial ring `ambient_ring(RS)`

, $I$ the ideal `defining_ideal(RS)`

, and $U$ the multiplicative semigroup generated by `inequations(RS)`

, then the coordinate ring of the realization space $\mathcal{R}(M)$ is isomorphic to $U^{-1}B/I$.