Weakly chordal graphs¶
This module deals with everything related to weakly chordal graphs. It currently contains the following functions:
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Author:
Birk Eisermann (initial implementation)
Nathann Cohen (some doc and optimization)
David Coudert (remove recursion)
Methods¶
- sage.graphs.weakly_chordal.is_long_antihole_free(g, certificate=False)[source]¶
Test whether the given graph contains an induced subgraph that is isomorphic to the complement of a cycle of length at least 5.
INPUT:
certificate– boolean (default:False)Whether to return a certificate. When
certificate = True, then the function returns(False, Antihole)ifgcontains an induced complement of a cycle of length at least 5 returned asAntihole.(True, [])ifgdoes not contain an induced complement of a cycle of length at least 5. For this case it is not known how to provide a certificate.
When
certificate = False, the function returns justTrueorFalseaccordingly.
ALGORITHM:
This algorithm tries to find a cycle in the graph of all induced \(\overline{P_4}\) of \(g\), where two copies \(\overline{P}\) and \(\overline{P'}\) of \(\overline{P_4}\) are adjacent if there exists a (not necessarily induced) copy of \(\overline{P_5}=u_1u_2u_3u_4u_5\) such that \(\overline{P}=u_1u_2u_3u_4\) and \(\overline{P'}=u_2u_3u_4u_5\).
This is done through a depth-first-search. For efficiency, the auxiliary graph is constructed on-the-fly and never stored in memory.
The run time of this algorithm is \(O(n+m^2)\) for
SparseGraphand \(O(n^2\log{m} + m^2)\) forDenseGraph[NP2007] (where \(n\) is the number of vertices and \(m\) is the number of edges of the graph).EXAMPLES:
The Petersen Graph contains an antihole:
sage: g = graphs.PetersenGraph() sage: g.is_long_antihole_free() False
>>> from sage.all import * >>> g = graphs.PetersenGraph() >>> g.is_long_antihole_free() False
The complement of a cycle is an antihole:
sage: g = graphs.CycleGraph(6).complement() sage: r,a = g.is_long_antihole_free(certificate=True) sage: r False sage: a.complement().is_isomorphic(graphs.CycleGraph(6)) True
>>> from sage.all import * >>> g = graphs.CycleGraph(Integer(6)).complement() >>> r,a = g.is_long_antihole_free(certificate=True) >>> r False >>> a.complement().is_isomorphic(graphs.CycleGraph(Integer(6))) True
- sage.graphs.weakly_chordal.is_long_hole_free(g, certificate=False)[source]¶
Test whether
gcontains an induced cycle of length at least 5.INPUT:
certificate– boolean (default:False)Whether to return a certificate. When
certificate = True, then the function returns(True, [])ifgdoes not contain such a cycle. For this case, it is not known how to provide a certificate.(False, Hole)ifgcontains an induced cycle of length at least 5.Holereturns this cycle.
If
certificate = False, the function returns justTrueorFalseaccordingly.
ALGORITHM:
This algorithm tries to find a cycle in the graph of all induced \(P_4\) of \(g\), where two copies \(P\) and \(P'\) of \(P_4\) are adjacent if there exists a (not necessarily induced) copy of \(P_5=u_1u_2u_3u_4u_5\) such that \(P=u_1u_2u_3u_4\) and \(P'=u_2u_3u_4u_5\).
This is done through a depth-first-search. For efficiency, the auxiliary graph is constructed on-the-fly and never stored in memory.
The run time of this algorithm is \(O(n+m^2)\) for
SparseGraphand \(O(n^2 + m^2)\) forDenseGraph[NP2007] (where \(n\) is the number of vertices and \(m\) is the number of edges of the graph).EXAMPLES:
The Petersen Graph contains a hole:
sage: g = graphs.PetersenGraph() sage: g.is_long_hole_free() False
>>> from sage.all import * >>> g = graphs.PetersenGraph() >>> g.is_long_hole_free() False
The following graph contains a hole, which we want to display:
sage: g = graphs.FlowerSnark() sage: r,h = g.is_long_hole_free(certificate=True) sage: r False sage: Graph(h).is_isomorphic(graphs.CycleGraph(h.order())) True
>>> from sage.all import * >>> g = graphs.FlowerSnark() >>> r,h = g.is_long_hole_free(certificate=True) >>> r False >>> Graph(h).is_isomorphic(graphs.CycleGraph(h.order())) True
- sage.graphs.weakly_chordal.is_weakly_chordal(g, certificate=False)[source]¶
Test whether the given graph is weakly chordal, i.e., the graph and its complement have no induced cycle of length at least 5.
INPUT:
certificate– boolean (default:False); whether to return a certificate. Ifcertificate = False, returnTrueorFalseaccording to the graph. Ifcertificate = True, return(False, forbidden_subgraph)when the graph contains a forbidden subgraph H, this graph is returned.(True, [])when the graph is weakly chordal.For this case, it is not known how to provide a certificate.
ALGORITHM:
This algorithm checks whether the graph
gor its complement contain an induced cycle of length at least 5.Using is_long_hole_free() and is_long_antihole_free() yields a run time of \(O(n+m^2)\) for
SparseGraphand \(O(n^2 + m^2)\) forDenseGraph(where \(n\) is the number of vertices and \(m\) is the number of edges of the graph).EXAMPLES:
The Petersen Graph is not weakly chordal and contains a hole:
sage: g = graphs.PetersenGraph() sage: r,s = g.is_weakly_chordal(certificate=True) sage: r False sage: l = s.order() sage: s.is_isomorphic(graphs.CycleGraph(l)) True
>>> from sage.all import * >>> g = graphs.PetersenGraph() >>> r,s = g.is_weakly_chordal(certificate=True) >>> r False >>> l = s.order() >>> s.is_isomorphic(graphs.CycleGraph(l)) True