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# Matroid construction

## Theory

Matroids are combinatorial structures that capture the abstract properties of (linear/algebraic/...) dependence. Formally, a matroid is a pair \(M = (E, I)\) of a finite set \(E\), the groundset, and a collection of subsets \(I\), the independent sets, subject to the following axioms:

- \(I\) contains the empty set
- If \(X\) is a set in \(I\), then each subset of \(X\) is in \(I\)
- If two subsets \(X\), \(Y\) are in \(I\), and \(|X| > |Y|\), then there exists \(x \in X - Y\) such that \(Y + \{x\}\) is in \(I\).

See the Wikipedia article on matroids for more theory and examples. Matroids can be obtained from many types of mathematical structures, and Sage supports a number of them.

There are two main entry points to Sage’s matroid functionality. The object `matroids.` contains a number of constructors for well-known matroids. The function `Matroid()` allows you to define your own matroids from a variety of sources. We briefly introduce both below; follow the links for more comprehensive documentation.

Each matroid object in Sage comes with a number of built-in operations. An overview can be found in the documentation of `the abstract matroid class`.

## Built-in matroids

For built-in matroids, do the following:

- Within a Sage session, type `matroids.` (Do not press “Enter”, and do not forget the final period ”.”)
- Hit “tab”.

You will see a list of methods which will construct matroids. For example:

    sage: M = matroids.Wheel(4)
    sage: M.is_connected()
    True

or:

    sage: U36 = matroids.Uniform(3, 6)
    sage: U36.equals(U36.dual())
    True

A number of special matroids are collected under a `named_matroids` submenu. To see which, type `matroids.named_matroids.<tab>` as above:

    sage: F7 = matroids.named_matroids.Fano()
    sage: len(F7.nonspanning_circuits())
    7

## Constructing matroids

To define your own matroid, use the function `Matroid()`. This function attempts to interpret its arguments to create an appropriate matroid. The input arguments are documented in detail `below`.

EXAMPLES:

    sage: A = Matrix(GF(2), [[1, 0, 0, 0, 1, 1, 1],
    ....:                    [0, 1, 0, 1, 0, 1, 1],
    ....:                    [0, 0, 1, 1, 1, 0, 1]])
    sage: M = Matroid(A)
    sage: M.is_isomorphic(matroids.named_matroids.Fano())
    True

    sage: M = Matroid(graphs.PetersenGraph())
    sage: M.rank()
    9

AUTHORS:

- Rudi Pendavingh, Michael Welsh, Stefan van Zwam (2013-04-01): initial version

## Methods

`sage.matroids.constructor.``Matroid`(*args, **kwds)

  Construct a matroid.

  Matroids are combinatorial structures that capture the abstract properties of (linear/algebraic/...) dependence. Formally, a matroid is a pair \(M = (E, I)\) of a finite set \(E\), the groundset, and a collection of subsets \(I\), the independent sets, subject to the following axioms:

  - \(I\) contains the empty set
  - If \(X\) is a set in \(I\), then each subset of \(X\) is in \(I\)
  - If two subsets \(X\), \(Y\) are in \(I\), and \(|X| > |Y|\), then there exists \(x \in X - Y\) such that \(Y + \{x\}\) is in \(I\).

  See the Wikipedia article on matroids for more theory and examples. Matroids can be obtained from many types of mathematical structures, and Sage supports a number of them.

  There are two main entry points to Sage’s matroid functionality. For built-in matroids, do the following:

  - Within a Sage session, type “matroids.” (Do not press “Enter”, and do not forget the final period ”.”)
  - Hit “tab”.

  You will see a list of methods which will construct matroids. For example:

      sage: F7 = matroids.named_matroids.Fano()
      sage: len(F7.nonspanning_circuits())
      7

  or:

      sage: U36 = matroids.Uniform(3, 6)
      sage: U36.equals(U36.dual())
      True

  To define your own matroid, use the function `Matroid()`. This function attempts to interpret its arguments to create an appropriate matroid. The following named arguments are supported:

  INPUT:

  - `groundset` – If provided, the groundset of the matroid. If not provided, the function attempts to determine a groundset from the data.
  - `bases` – The list of bases (maximal independent sets) of the matroid.
  - `independent_sets` – The list of independent sets of the matroid.
  - `circuits` – The list of circuits of the matroid.
  - `graph` – A graph, whose edges form the elements of the matroid.
  - `matrix` – A matrix representation of the matroid.
  - `reduced_matrix` – A reduced representation of the matroid: if `reduced_matrix = A` then the matroid is represented by \([I\ \ A]\) where \(I\) is an appropriately sized identity matrix.
  - `rank_function` – A function that computes the rank of each subset. Can only be provided together with a groundset.
  - `circuit_closures` – Either a list of tuples `(k, C)` with `C` the closure of a circuit, and `k` the rank of `C`, or a dictionary `D` with `D[k]` the set of closures of rank-`k` circuits.
  - `matroid` – An object that is already a matroid. Useful only with the `regular` option.

  Up to two unnamed arguments are allowed.

  - One unnamed argument, no named arguments other than `regular` – the input should be either a graph, or a matrix, or a list of independent sets containing all bases, or a matroid.
  - Two unnamed arguments: the first is the groundset, the second a graph, or a matrix, or a list of independent sets containing all bases, or a matroid.
  - One unnamed argument, at least one named argument: the unnamed argument is the groundset, the named argument is as above (but must be different from `groundset`).

  The examples section details how each of the input types deals with explicit or implicit groundset arguments.

  OPTIONS:

  - `regular` – (default: `False`) boolean. If `True`, output a `RegularMatroid` instance such that, if the input defines a valid regular matroid, then the output represents this matroid. Note that this option can be combined with any type of input.
  - `ring` – any ring. If provided, and the input is a `matrix` or `reduced_matrix`, output will be a linear matroid over the ring or field `ring`.
  - `field` – any field. Same as `ring`, but only fields are allowed.
  - `check` – (default: `True`) boolean. If `True` and `regular` is true, the output is checked to make sure it is a valid regular matroid.

  Warning

  Except for regular matroids, the input is not checked for validity. If your data does not correspond to an actual matroid, the behavior of the methods is undefined and may cause strange errors. To ensure you have a matroid, run `M.is_valid()`.

  Note

  The `Matroid()` method will return instances of type `BasisMatroid`, `CircuitClosuresMatroid`, `LinearMatroid`, `BinaryMatroid`, `TernaryMatroid`, `QuaternaryMatroid`, `RegularMatroid`, or `RankMatroid`. To import these classes (and other useful functions) directly into Sage’s main namespace, type:

      sage: from sage.matroids.advanced import *

  See `sage.matroids.advanced`.

  EXAMPLES:

  Note that in these examples we will often use the fact that strings are iterable in these examples. So we type `'abcd'` to denote the list `['a', 'b', 'c', 'd']`.

  1. List of bases:

     > All of the following inputs are allowed, and equivalent:

     >     sage: M1 = Matroid(groundset='abcd', bases=['ab', 'ac', 'ad',
     >     ....:                                       'bc', 'bd', 'cd'])
     >     sage: M2 = Matroid(bases=['ab', 'ac', 'ad', 'bc', 'bd', 'cd'])
     >     sage: M3 = Matroid(['ab', 'ac', 'ad', 'bc', 'bd', 'cd'])
     >     sage: M4 = Matroid('abcd', ['ab', 'ac', 'ad', 'bc', 'bd', 'cd'])
     >     sage: M5 = Matroid('abcd', bases=[['a', 'b'], ['a', 'c'],
     >     ....:                             ['a', 'd'], ['b', 'c'],
     >     ....:                             ['b', 'd'], ['c', 'd']])
     >     sage: M1 == M2
     >     True
     >     sage: M1 == M3
     >     True
     >     sage: M1 == M4
     >     True
     >     sage: M1 == M5
     >     True

     > We do not check if the provided input forms an actual matroid:

     >     sage: M1 = Matroid(groundset='abcd', bases=['ab', 'cd'])
     >     sage: M1.full_rank()
     >     2
     >     sage: M1.is_valid()
     >     False

     > Bases may be repeated:

     >     sage: M1 = Matroid(['ab', 'ac'])
     >     sage: M2 = Matroid(['ab', 'ac', 'ab'])
     >     sage: M1 == M2
     >     True

  2. List of independent sets:

     >     sage: M1 = Matroid(groundset='abcd',
     >     ....:              independent_sets=['', 'a', 'b', 'c', 'd', 'ab',
     >     ....:                               'ac', 'ad', 'bc', 'bd', 'cd'])

     > We only require that the list of independent sets contains each basis of the matroid; omissions of smaller independent sets and repetitions are allowed:

     >     sage: M1 = Matroid(bases=['ab', 'ac'])
     >     sage: M2 = Matroid(independent_sets=['a', 'ab', 'b', 'ab', 'a',
     >     ....:                                'b', 'ac'])
     >     sage: M1 == M2
     >     True

  3. List of circuits:

     >     sage: M1 = Matroid(groundset='abc', circuits=['bc'])
     >     sage: M2 = Matroid(bases=['ab', 'ac'])
     >     sage: M1 == M2
     >     True

     > A matroid specified by a list of circuits gets converted to a `BasisMatroid` internally:

     >     sage: M = Matroid(groundset='abcd', circuits=['abc', 'abd', 'acd',
     >     ....:                                         'bcd'])
     >     sage: type(M)
     >     <type 'sage.matroids.basis_matroid.BasisMatroid'>

     > Strange things can happen if the input does not satisfy the circuit axioms, and these are not always caught by the `is_valid()` method. So always check whether your input makes sense!

     >     sage: M = Matroid('abcd', circuits=['ab', 'acd'])
     >     sage: M.is_valid()
     >     True
     >     sage: [sorted(C) for C in M.circuits()]
     >     [['a']]

  4. Graph:

     > Sage has great support for graphs, see `sage.graphs.graph`.

     >     sage: G = graphs.PetersenGraph()
     >     sage: Matroid(G)
     >     Regular matroid of rank 9 on 15 elements with 2000 bases

     > Note: if a groundset is specified, we assume it is in the same order as `G.edge_iterator()` provides:

     >     sage: G = Graph([(0, 1), (0, 2), (0, 2), (1, 2)])
     >     sage: M = Matroid('abcd', G)
     >     sage: M.rank(['b', 'c'])
     >     1

     > If no groundset is provided, we attempt to use the edge labels:

     >     sage: G = Graph([(0, 1, 'a'), (0, 2, 'b'), (1, 2, 'c')])
     >     sage: M = Matroid(G)
     >     sage: sorted(M.groundset())
     >     ['a', 'b', 'c']

     > If no edge labels are present and the graph is simple, we use the tuples `(i, j)` of endpoints. If that fails, we simply use a list `[0..m-1]`

     >     sage: G = Graph([(0, 1), (0, 2), (1, 2)])
     >     sage: M = Matroid(G)
     >     sage: sorted(M.groundset())
     >     [(0, 1), (0, 2), (1, 2)]

     >     sage: G = Graph([(0, 1), (0, 2), (0, 2), (1, 2)])
     >     sage: M = Matroid(G)
     >     sage: sorted(M.groundset())
     >     [0, 1, 2, 3]

     > When the `graph` keyword is used, a variety of inputs can be converted to a graph automatically. The following uses a graph6 string (see the `Graph` method’s documentation):

     >     sage: Matroid(graph=':I`AKGsaOs`cI]Gb~')
     >     Regular matroid of rank 9 on 17 elements with 4004 bases

     > However, this method is no more clever than `Graph()`:

     >     sage: Matroid(graph=41/2)
     >     Traceback (most recent call last):
     >     ...
     >     ValueError: input does not seem to represent a graph.

  5. Matrix:

     > The basic input is a `Sage matrix`:

     >     sage: A = Matrix(GF(2), [[1, 0, 0, 1, 1, 0],
     >     ....:                    [0, 1, 0, 1, 0, 1],
     >     ....:                    [0, 0, 1, 0, 1, 1]])
     >     sage: M = Matroid(matrix=A)
     >     sage: M.is_isomorphic(matroids.CompleteGraphic(4))
     >     True

     > Various shortcuts are possible:

     >     sage: M1 = Matroid(matrix=[[1, 0, 0, 1, 1, 0],
     >     ....:                      [0, 1, 0, 1, 0, 1],
     >     ....:                      [0, 0, 1, 0, 1, 1]], ring=GF(2))
     >     sage: M2 = Matroid(reduced_matrix=[[1, 1, 0],
     >     ....:                              [1, 0, 1],
     >     ....:                              [0, 1, 1]], ring=GF(2))
     >     sage: M3 = Matroid(groundset=[0, 1, 2, 3, 4, 5],
     >     ....:              matrix=[[1, 1, 0], [1, 0, 1], [0, 1, 1]],
     >     ....:              ring=GF(2))
     >     sage: A = Matrix(GF(2), [[1, 1, 0], [1, 0, 1], [0, 1, 1]])
     >     sage: M4 = Matroid([0, 1, 2, 3, 4, 5], A)
     >     sage: M1 == M2
     >     True
     >     sage: M1 == M3
     >     True
     >     sage: M1 == M4
     >     True

     > However, with unnamed arguments the input has to be a `Matrix` instance, or the function will try to interpret it as a set of bases:

     >     sage: Matroid([0, 1, 2], [[1, 0, 1], [0, 1, 1]])
     >     Traceback (most recent call last):
     >     ...
     >     ValueError: basis has wrong cardinality.

     > If the groundset size equals number of rows plus number of columns, an identity matrix is prepended. Otherwise the groundset size must equal the number of columns:

     >     sage: A = Matrix(GF(2), [[1, 1, 0], [1, 0, 1], [0, 1, 1]])
     >     sage: M = Matroid([0, 1, 2], A)
     >     sage: N = Matroid([0, 1, 2, 3, 4, 5], A)
     >     sage: M.rank()
     >     2
     >     sage: N.rank()
     >     3

     > We automatically create an optimized subclass, if available:

     >     sage: Matroid([0, 1, 2, 3, 4, 5],
     >     ....:         matrix=[[1, 1, 0], [1, 0, 1], [0, 1, 1]],
     >     ....:         field=GF(2))
     >     Binary matroid of rank 3 on 6 elements, type (2, 7)
     >     sage: Matroid([0, 1, 2, 3, 4, 5],
     >     ....:         matrix=[[1, 1, 0], [1, 0, 1], [0, 1, 1]],
     >     ....:         field=GF(3))
     >     Ternary matroid of rank 3 on 6 elements, type 0-
     >     sage: Matroid([0, 1, 2, 3, 4, 5],
     >     ....:         matrix=[[1, 1, 0], [1, 0, 1], [0, 1, 1]],
     >     ....:         field=GF(4, 'x'))
     >     Quaternary matroid of rank 3 on 6 elements
     >     sage: Matroid([0, 1, 2, 3, 4, 5],
     >     ....:         matrix=[[1, 1, 0], [1, 0, 1], [0, 1, 1]],
     >     ....:         field=GF(2), regular=True)
     >     Regular matroid of rank 3 on 6 elements with 16 bases

     > Otherwise the generic LinearMatroid class is used:

     >     sage: Matroid([0, 1, 2, 3, 4, 5],
     >     ....:         matrix=[[1, 1, 0], [1, 0, 1], [0, 1, 1]],
     >     ....:         field=GF(83))
     >     Linear matroid of rank 3 on 6 elements represented over the Finite
     >     Field of size 83

     > An integer matrix is automatically converted to a matrix over \(\QQ\). If you really want integers, you can specify the ring explicitly:

     >     sage: A = Matrix([[1, 1, 0], [1, 0, 1], [0, 1, -1]])
     >     sage: A.base_ring()
     >     Integer Ring
     >     sage: M = Matroid([0, 1, 2, 3, 4, 5], A)
     >     sage: M.base_ring()
     >     Rational Field
     >     sage: M = Matroid([0, 1, 2, 3, 4, 5], A, ring=ZZ)
     >     sage: M.base_ring()
     >     Integer Ring

  6. Rank function:

     > Any function mapping subsets to integers can be used as input:

     >     sage: def f(X):
     >     ....:     return min(len(X), 2)
     >     ....:
     >     sage: M = Matroid('abcd', rank_function=f)
     >     sage: M
     >     Matroid of rank 2 on 4 elements
     >     sage: M.is_isomorphic(matroids.Uniform(2, 4))
     >     True

  7. Circuit closures:

     > This is often a really concise way to specify a matroid. The usual way is a dictionary of lists:

     >     sage: M = Matroid(circuit_closures={3: ['edfg', 'acdg', 'bcfg',
     >     ....:     'cefh', 'afgh', 'abce', 'abdf', 'begh', 'bcdh', 'adeh'],
     >     ....:     4: ['abcdefgh']})
     >     sage: M.equals(matroids.named_matroids.P8())
     >     True

     > You can also input tuples \((k, X)\) where \(X\) is the closure of a circuit, and \(k\) the rank of \(X\):

     >     sage: M = Matroid(circuit_closures=[(2, 'abd'), (3, 'abcdef'),
     >     ....:                               (2, 'bce')])
     >     sage: M.equals(matroids.named_matroids.Q6())
     >     True

  8. Matroid:

     > Most of the time, the matroid itself is returned:

     >     sage: M = matroids.named_matroids.Fano()
     >     sage: N = Matroid(M)
     >     sage: N is M
     >     True

     > But it can be useful with the `regular` option:

     >     sage: M = Matroid(circuit_closures={2:['adb', 'bec', 'cfa',
     >     ....:                                  'def'], 3:['abcdef']})
     >     sage: N = Matroid(M, regular=True)
     >     sage: N
     >     Regular matroid of rank 3 on 6 elements with 16 bases
     >     sage: Matrix(N)
     >     [1 0 0 1 1 0]
     >     [0 1 0 1 1 1]
     >     [0 0 1 0 1 1]

  The `regular` option:

  >     sage: M = Matroid(reduced_matrix=[[1, 1, 0],
  >     ....:                             [1, 0, 1],
  >     ....:                             [0, 1, 1]], regular=True)
  >     sage: M
  >     Regular matroid of rank 3 on 6 elements with 16 bases

  >     sage: M.is_isomorphic(matroids.CompleteGraphic(4))
  >     True

  > By default we check if the resulting matroid is actually regular. To increase speed, this check can be skipped:

  >     sage: M = matroids.named_matroids.Fano()
  >     sage: N = Matroid(M, regular=True)
  >     Traceback (most recent call last):
  >     ...
  >     ValueError: input does not correspond to a valid regular matroid.
  >     sage: N = Matroid(M, regular=True, check=False)
  >     sage: N
  >     Regular matroid of rank 3 on 7 elements with 32 bases

  >     sage: N.is_valid()
  >     False

  > Sometimes the output is regular, but represents a different matroid from the one you intended:

  >     sage: M = Matroid(Matrix(GF(3), [[1, 0, 1, 1], [0, 1, 1, 2]]))
  >     sage: N = Matroid(Matrix(GF(3), [[1, 0, 1, 1], [0, 1, 1, 2]]),
  >     ....:             regular=True)
  >     sage: N.is_valid()
  >     True
  >     sage: N.is_isomorphic(M)
  >     False

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