# Library of Hyperplane Arrangements¶

A collection of useful or interesting hyperplane arrangements. See sage.geometry.hyperplane_arrangement.arrangement for details about how to construct your own hyperplane arrangements.

class sage.geometry.hyperplane_arrangement.library.HyperplaneArrangementLibrary

Bases: object

The library of hyperplane arrangements.

Catalan(n, K=Rational Field, names=None)

Return the Catalan arrangement.

INPUT:

• n – integer
• K – field (default: $$\QQ$$)
• names – tuple of strings or None (default); the variable names for the ambient space

OUTPUT:

The arrangement of $$3n(n-1)/2$$ hyperplanes $$\{ x_i - x_j = -1,0,1 : 1 \leq i \leq j \leq n \}$$.

EXAMPLES:

sage: hyperplane_arrangements.Catalan(5)
Arrangement of 30 hyperplanes of dimension 5 and rank 4

G_Shi(G, K=Rational Field, names=None)

Return the Shi hyperplane arrangement of a graph $$G$$.

INPUT:

• G – graph
• K – field (default: $$\QQ$$)
• names – tuple of strings or None (default); the variable names for the ambient space

OUTPUT:

The Shi hyperplane arrangement of the given graph G.

EXAMPLES:

sage: G = graphs.CompleteGraph(5)
sage: hyperplane_arrangements.G_Shi(G)
Arrangement of 20 hyperplanes of dimension 5 and rank 4
sage: g = graphs.HouseGraph()
sage: hyperplane_arrangements.G_Shi(g)
Arrangement of 12 hyperplanes of dimension 5 and rank 4
sage: a = hyperplane_arrangements.G_Shi(graphs.WheelGraph(4));  a
Arrangement of 12 hyperplanes of dimension 4 and rank 3

G_semiorder(G, K=Rational Field, names=None)

Return the semiorder hyperplane arrangement of a graph.

INPUT:

• G – graph
• K – field (default: $$\QQ$$)
• names – tuple of strings or None (default); the variable names for the ambient space

OUTPUT:

The semiorder hyperplane arrangement of a graph G is the arrangement $$\{ x_i - x_j = -1,1 \}$$ where $$ij$$ is an edge of G.

EXAMPLES:

sage: G = graphs.CompleteGraph(5)
sage: hyperplane_arrangements.G_semiorder(G)
Arrangement of 20 hyperplanes of dimension 5 and rank 4
sage: g = graphs.HouseGraph()
sage: hyperplane_arrangements.G_semiorder(g)
Arrangement of 12 hyperplanes of dimension 5 and rank 4

Ish(n, K=Rational Field, names=None)

Return the Ish arrangement.

INPUT:

• n – integer
• K – field (default:QQ)
• names – tuple of strings or None (default); the variable names for the ambient space

OUTPUT:

The Ish arrangement, which is the set of $$n(n-1)$$ hyperplanes.

$\{ x_i - x_j = 0 : 1 \leq i \leq j \leq n \} \cup \{ x_1 - x_j = i : 1 \leq i \leq j \leq n \}.$

EXAMPLES:

sage: a = hyperplane_arrangements.Ish(3);  a
Arrangement of 6 hyperplanes of dimension 3 and rank 2
sage: a.characteristic_polynomial()
x^3 - 6*x^2 + 9*x
sage: b = hyperplane_arrangements.Shi(3)
sage: b.characteristic_polynomial()
x^3 - 6*x^2 + 9*x


REFERENCES:

Shi(n, K=Rational Field, names=None)

Return the Shi arrangement.

INPUT:

• n – integer
• K – field (default:QQ)
• names – tuple of strings or None (default); the variable names for the ambient space

OUTPUT:

The Shi arrangement is the set of $$n(n-1)$$ hyperplanes: $$\{ x_i - x_j = 0,1 : 1 \leq i \leq j \leq n \}$$.

EXAMPLES:

sage: hyperplane_arrangements.Shi(4)
Arrangement of 12 hyperplanes of dimension 4 and rank 3

bigraphical(G, A=None, K=Rational Field, names=None)

Return a bigraphical hyperplane arrangement.

INPUT:

• G – graph
• A – list, matrix, dictionary (default: None gives semiorder), or the string ‘generic’
• K – field (default: $$\QQ$$)
• names – tuple of strings or None (default); the variable names for the ambient space

OUTPUT:

The hyperplane arrangement with hyperplanes $$x_i - x_j = A[i,j]$$ and $$x_j - x_i = A[j,i]$$ for each edge $$v_i, v_j$$ of G. The indices $$i,j$$ are the indices of elements of G.vertices().

EXAMPLES:

sage: G = graphs.CycleGraph(4)
sage: G.edges()
[(0, 1, None), (0, 3, None), (1, 2, None), (2, 3, None)]
sage: G.edges(labels=False)
[(0, 1), (0, 3), (1, 2), (2, 3)]
sage: A = {0:{1:1, 3:2}, 1:{0:3, 2:0}, 2:{1:2, 3:1}, 3:{2:0, 0:2}}
sage: HA = hyperplane_arrangements.bigraphical(G, A)
sage: HA.n_regions()
63
sage: hyperplane_arrangements.bigraphical(G, 'generic').n_regions()
65
sage: hyperplane_arrangements.bigraphical(G).n_regions()
59


REFERENCES:

braid(n, K=Rational Field, names=None)

The braid arrangement.

INPUT:

• n – integer
• K – field (default: QQ)
• names – tuple of strings or None (default); the variable names for the ambient space

OUTPUT:

The hyperplane arrangement consisting of the $$n(n-1)/2$$ hyperplanes $$\{ x_i - x_j = 0 : 1 \leq i \leq j \leq n \}$$.

EXAMPLES:

sage: hyperplane_arrangements.braid(4)
Arrangement of 6 hyperplanes of dimension 4 and rank 3

coordinate(n, K=Rational Field, names=None)

Return the coordinate hyperplane arrangement.

INPUT:

• n – integer
• K – field (default: $$\QQ$$)
• names – tuple of strings or None (default); the variable names for the ambient space

OUTPUT:

The coordinate hyperplane arrangement, which is the central hyperplane arrangement consisting of the coordinate hyperplanes $$x_i = 0$$.

EXAMPLES:

sage: hyperplane_arrangements.coordinate(5)
Arrangement of 5 hyperplanes of dimension 5 and rank 5

graphical(G, K=Rational Field, names=None)

Return the graphical hyperplane arrangement of a graph G.

INPUT:

• G – graph
• K – field (default: $$\QQ$$)
• names – tuple of strings or None (default); the variable names for the ambient space

OUTPUT:

The graphical hyperplane arrangement of a graph G, which is the arrangement $$\{ x_i - x_j = 0 \}$$ for all edges $$ij$$ of the graph G.

EXAMPLES:

sage: G = graphs.CompleteGraph(5)
sage: hyperplane_arrangements.graphical(G)
Arrangement of 10 hyperplanes of dimension 5 and rank 4
sage: g = graphs.HouseGraph()
sage: hyperplane_arrangements.graphical(g)
Arrangement of 6 hyperplanes of dimension 5 and rank 4

linial(n, K=Rational Field, names=None)

Return the linial hyperplane arrangement.

INPUT:

• n – integer
• K – field (default: $$\QQ$$)
• names – tuple of strings or None (default); the variable names for the ambient space

OUTPUT:

The linial hyperplane arrangement is the set of hyperplanes $$\{x_i - x_j = 1 : 1\leq i < j \leq n\}$$.

EXAMPLES:

sage: a = hyperplane_arrangements.linial(4);  a
Arrangement of 6 hyperplanes of dimension 4 and rank 3
sage: a.characteristic_polynomial()
x^4 - 6*x^3 + 15*x^2 - 14*x

semiorder(n, K=Rational Field, names=None)

Return the semiorder arrangement.

INPUT:

• n – integer
• K – field (default: $$\QQ$$)
• names – tuple of strings or None (default); the variable names for the ambient space

OUTPUT:

The semiorder arrangement, which is the set of $$n(n-1)$$ hyperplanes $$\{ x_i - x_j = -1,1 : 1 \leq i \leq j \leq n\}$$.

EXAMPLES:

sage: hyperplane_arrangements.semiorder(4)
Arrangement of 12 hyperplanes of dimension 4 and rank 3

sage.geometry.hyperplane_arrangement.library.make_parent(base_ring, dimension, names=None)

Construct the parent for the hyperplane arrangements.

For internal use only.

INPUT:

• base_ring – a ring
• dimension – integer
• namesNone (default) or a list/tuple/iterable of strings

OUTPUT:

A new HyperplaneArrangements instance.

EXAMPLES:

sage: from sage.geometry.hyperplane_arrangement.library import make_parent
sage: make_parent(QQ, 3)
Hyperplane arrangements in 3-dimensional linear space over
Rational Field with coordinates t0, t1, t2