Some useful functions for the matroid class.¶
For direct access to the methods newlabel()
, setprint()
and
get_nonisomorphic_matroids()
, type:
sage: from sage.matroids.advanced import *
See also sage.matroids.advanced
.
AUTHORS:
 Stefan van Zwam (20110624): initial version

sage.matroids.utilities.
get_nonisomorphic_matroids
(MSet)¶ Return nonisomorphic members of the matroids in set
MSet
.For direct access to
get_nonisomorphic_matroids
, run:sage: from sage.matroids.advanced import *
INPUT:
MSet
– an iterable whose members are matroids.
OUTPUT:
A list containing one representative of each isomorphism class of members of
MSet
.EXAMPLES:
sage: from sage.matroids.advanced import * sage: L = matroids.Uniform(3, 5).extensions() sage: len(list(L)) 32 sage: len(get_nonisomorphic_matroids(L)) 5

sage.matroids.utilities.
lift_cross_ratios
(A, lift_map=None)¶ Return a matrix which arises from the given matrix by lifting cross ratios.
INPUT:
A
– a matrix over a ringsource_ring
.lift_map
– a python dictionary, mapping each cross ratio ofA
to some element of a target ring, and such thatlift_map[source_ring(1)] = target_ring(1)
.
OUTPUT:
Z
– a matrix over the ringtarget_ring
.
The intended use of this method is to create a (reduced) matrix representation of a matroid
M
over a ringtarget_ring
, given a (reduced) matrix representation ofA
ofM
over a ringsource_ring
and a maplift_map
fromsource_ring
totarget_ring
.This method will create a unique candidate representation
Z
, but will not verify ifZ
is indeed a representation ofM
. However, this is guaranteed if the conditions of the lift theorem (see [PvZ2010]) hold for the lift map in combination with the matrixA
.For a lift map \(f\) and a matrix \(A\) these conditions are as follows. First of all \(f: S \rightarrow T\), where \(S\) is a set of invertible elements of the source ring and \(T\) is a set of invertible elements of the target ring. The matrix \(A\) has entries from the source ring, and each cross ratio of \(A\) is contained in \(S\). Moreover:
 \(1 \in S\), \(1 \in T\);
 for all \(x \in S\): \(f(x) = 1\) if and only if \(x = 1\);
 for all \(x, y \in S\): if \(x + y = 0\) then \(f(x) + f(y) = 0\);
 for all \(x, y \in S\): if \(x + y = 1\) then \(f(x) + f(y) = 1\);
 for all \(x, y, z \in S\): if \(xy = z\) then \(f(x)f(y) = f(z)\).
Any ring homomorphism \(h: P \rightarrow R\) induces a lift map from the set of units \(S\) of \(P\) to the set of units \(T\) of \(R\). There exist lift maps which do not arise in this manner. Several such maps can be created by the function
lift_map()
.See also
EXAMPLES:
sage: from sage.matroids.advanced import lift_cross_ratios, lift_map, LinearMatroid sage: R = GF(7) sage: to_sixth_root_of_unity = lift_map('sru') sage: A = Matrix(R, [[1, 0, 6, 1, 2],[6, 1, 0, 0, 1],[0, 6, 3, 6, 0]]) sage: A [1 0 6 1 2] [6 1 0 0 1] [0 6 3 6 0] sage: Z = lift_cross_ratios(A, to_sixth_root_of_unity) sage: Z [ 1 0 1 1 1] [ 1 1 0 0 z] [ 0 z  1 1 z + 1 0] sage: M = LinearMatroid(reduced_matrix = A) sage: sorted(M.cross_ratios()) [3, 5] sage: N = LinearMatroid(reduced_matrix = Z) sage: sorted(N.cross_ratios()) [z + 1, z] sage: M.is_isomorphism(N, {e:e for e in M.groundset()}) True

sage.matroids.utilities.
lift_map
(target)¶ Create a lift map, to be used for lifting the cross ratios of a matroid representation.
See also
INPUT:
target
– a string describing the target (partial) field.
OUTPUT:
 a dictionary
Depending on the value of
target
, the following lift maps will be created: “reg”: a lift map from \(\GF3\) to the regular partial field \((\ZZ, <1>)\).
 “sru”: a lift map from \(\GF7\) to the sixthrootofunity partial field \((\QQ(z), <z>)\), where \(z\) is a sixth root of unity. The map sends 3 to \(z\).
 “dyadic”: a lift map from \(\GF{11}\) to the dyadic partial field \((\QQ, <1, 2>)\).
 “gm”: a lift map from \(\GF{19}\) to the golden mean partial field \((\QQ(t), <1,t>)\), where \(t\) is a root of \(t^2t1\). The map sends \(5\) to \(t\).
The example below shows that the latter map satisfies three necessary conditions stated in
lift_cross_ratios()
EXAMPLES:
sage: from sage.matroids.utilities import lift_map sage: lm = lift_map('gm') sage: for x in lm: ....: if (x == 1) is not (lm[x] == 1): ....: print('not a proper lift map') ....: for y in lm: ....: if (x+y == 0) and not (lm[x]+lm[y] == 0): ....: print('not a proper lift map') ....: if (x+y == 1) and not (lm[x]+lm[y] == 1): ....: print('not a proper lift map') ....: for z in lm: ....: if (x*y==z) and not (lm[x]*lm[y]==lm[z]): ....: print('not a proper lift map')

sage.matroids.utilities.
make_regular_matroid_from_matroid
(matroid)¶ Attempt to construct a regular representation of a matroid.
INPUT:
matroid
– a matroid.
OUTPUT:
Return a \((0, 1, 1)\)matrix over the integers such that, if the input is a regular matroid, then the output is a totally unimodular matrix representing that matroid.
EXAMPLES:
sage: from sage.matroids.utilities import make_regular_matroid_from_matroid sage: make_regular_matroid_from_matroid( ....: matroids.CompleteGraphic(6)).is_isomorphic( ....: matroids.CompleteGraphic(6)) True

sage.matroids.utilities.
newlabel
(groundset)¶ Create a new element label different from the labels in
groundset
.INPUT:
groundset
– A set of objects.
OUTPUT:
A string not in the set
groundset
.For direct access to
newlabel
, run:sage: from sage.matroids.advanced import *
ALGORITHM:
 Create a set of all onecharacter alphanumeric strings.
 Remove the string representation of each existing element from this list.
 If the list is nonempty, return any element.
 Otherwise, return the concatenation of the strings of each existing element, preceded by ‘e’.
EXAMPLES:
sage: from sage.matroids.advanced import newlabel sage: S = set(['a', 42, 'b']) sage: newlabel(S) in S False sage: S = set('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789') sage: t = newlabel(S) sage: len(t) 63 sage: t[0] 'e'

sage.matroids.utilities.
sanitize_contractions_deletions
(matroid, contractions, deletions)¶ Return a fixed version of sets
contractions
anddeletions
.INPUT:
matroid
– aMatroid
instance.contractions
– a subset of the groundset.deletions
– a subset of the groundset.
OUTPUT:
An independent set
C
and a coindependent setD
such thatmatroid / contractions \ deletions == matroid / C \ D
Raise an error if either is not a subset of the groundset of
matroid
or if they are not disjoint.This function is used by the
Matroid.minor()
method.EXAMPLES:
sage: from sage.matroids.utilities import setprint sage: from sage.matroids.utilities import sanitize_contractions_deletions sage: M = matroids.named_matroids.Fano() sage: setprint(sanitize_contractions_deletions(M, 'abc', 'defg')) [{'a', 'b', 'c'}, {'d', 'e', 'f', 'g'}] sage: setprint(sanitize_contractions_deletions(M, 'defg', 'abc')) [{'d', 'e', 'g'}, {'a', 'b', 'c', 'f'}] sage: setprint(sanitize_contractions_deletions(M, [1, 2, 3], 'efg')) Traceback (most recent call last): ... ValueError: input contractions is not a subset of the groundset. sage: setprint(sanitize_contractions_deletions(M, 'efg', [1, 2, 3])) Traceback (most recent call last): ... ValueError: input deletions is not a subset of the groundset. sage: setprint(sanitize_contractions_deletions(M, 'ade', 'efg')) Traceback (most recent call last): ... ValueError: contraction and deletion sets are not disjoint.

sage.matroids.utilities.
setprint
(X)¶ Print nested data structures nicely.
Python’s data structures
set
andfrozenset
do not print nicely. This function can be used as replacement forprint
to overcome this. For direct access tosetprint
, run:sage: from sage.matroids.advanced import *
Note
This function will be redundant when Sage moves to Python 3, since the default
print
will suffice then.INPUT:
X
– Any Python object
OUTPUT:
None
. However, the function prints a nice representation ofX
.EXAMPLES:
Output looks much better:
sage: from sage.matroids.advanced import setprint sage: L = [{1, 2, 3}, {1, 2, 4}, {2, 3, 4}, {4, 1, 3}] sage: print(L) [set([1, 2, 3]), set([1, 2, 4]), set([2, 3, 4]), set([1, 3, 4])] sage: setprint(L) [{1, 2, 3}, {1, 2, 4}, {2, 3, 4}, {1, 3, 4}]
Note that for iterables, the effect can be undesirable:
sage: from sage.matroids.advanced import setprint sage: M = matroids.named_matroids.Fano().delete('efg') sage: M.bases() Iterator over a system of subsets sage: setprint(M.bases()) [{'a', 'b', 'c'}, {'a', 'c', 'd'}, {'a', 'b', 'd'}]
An exception was made for subclasses of SageObject:
sage: from sage.matroids.advanced import setprint sage: G = graphs.PetersenGraph() sage: list(G) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] sage: setprint(G) Petersen graph: Graph on 10 vertices

sage.matroids.utilities.
setprint_s
(X, toplevel=False)¶ Create the string for use by
setprint()
.INPUT:
X
– any Python objecttoplevel
– (default:False
) indicates whether this is a recursion or not.
OUTPUT:
A string representation of the object, with nice notation for sets and frozensets.
EXAMPLES:
sage: from sage.matroids.utilities import setprint_s sage: L = [{1, 2, 3}, {1, 2, 4}, {2, 3, 4}, {4, 1, 3}] sage: setprint_s(L) '[{1, 2, 3}, {1, 2, 4}, {2, 3, 4}, {1, 3, 4}]'
The
toplevel
argument only affects strings, to mimicprint
‘s behavior:sage: X = 'abcd' sage: setprint_s(X) "'abcd'" sage: setprint_s(X, toplevel=True) 'abcd'

sage.matroids.utilities.
spanning_forest
(M)¶ Return a list of edges of a spanning forest of the bipartite graph defined by \(M\)
INPUT:
M
– a matrix defining a bipartite graph G. The vertices are the rows and columns, if \(M[i,j]\) is nonzero, then there is an edge between row \(i\) and column \(j\).
OUTPUT:
A list of tuples \((r_i,c_i)\) representing edges between row \(r_i\) and column \(c_i\).
EXAMPLES:
sage: len(sage.matroids.utilities.spanning_forest(matrix([[1,1,1],[1,1,1],[1,1,1]]))) 5 sage: len(sage.matroids.utilities.spanning_forest(matrix([[0,0,1],[0,1,0],[0,1,0]]))) 3

sage.matroids.utilities.
spanning_stars
(M)¶ Returns the edges of a connected subgraph that is a union of all edges incident some subset of vertices.
INPUT:
M
– a matrix defining a bipartite graph G. The vertices are the rows and columns, if \(M[i,j]\) is nonzero, then there is an edge between row i and column 0.
OUTPUT:
A list of tuples \((row,column)\) in a spanning forest of the bipartite graph defined by
M
EXAMPLES:
sage: edges = sage.matroids.utilities.spanning_stars(matrix([[1,1,1],[1,1,1],[1,1,1]])) sage: Graph([(x+3, y) for x,y in edges]).is_connected() True