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fix issue 40953 with line_graph #40963

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fix issue 40953 with line_graph #40963

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15 changes: 13 additions & 2 deletions src/sage/graphs/line_graph.pyx
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Original file line number Diff line number Diff line change
Expand Up @@ -399,6 +399,14 @@ def line_graph(g, labels=True, return_labels=False, immutable=None):
sage: C.line_graph().is_isomorphic(g.line_graph())
True

:issue:`40953`::

sage: G = Graph([(0,0)],loops=True)
sage: G.line_graph()
Graph on 1 vertex
sage: G.to_directed().line_graph()
Looped digraph on 1 vertex

Check the behavior of parameter ``immutable``::

sage: G = Graph([(0, 1), (1, 2)])
Expand Down Expand Up @@ -431,6 +439,7 @@ def line_graph(g, labels=True, return_labels=False, immutable=None):
cdef dict origlabels_dic = {} # stores original labels of edges in case of multiple edges

multiple = g.has_multiple_edges()
loops = g.has_loops()

if immutable is None:
immutable = g.is_immutable()
Expand All @@ -442,7 +451,8 @@ def line_graph(g, labels=True, return_labels=False, immutable=None):
labels = True
origlabels_dic = {(u, v, id): (u, v, label)
for id, (u, v, label) in enumerate(g.edge_iterator())}
g = parent(g)([g, origlabels_dic.keys()], format='vertices_and_edges', multiedges=True)
g = parent(g)([g, origlabels_dic.keys()], format='vertices_and_edges',
multiedges=True, loops=loops)

if g._directed:
from sage.graphs.digraph import DiGraph
Expand All @@ -451,7 +461,8 @@ def line_graph(g, labels=True, return_labels=False, immutable=None):
for e in g.incoming_edge_iterator(v, labels=labels)
for f in g.outgoing_edge_iterator(v, labels=labels))
G = DiGraph([g.edge_iterator(labels=labels), arcs],
format='vertices_and_edges', immutable=immutable)
format='vertices_and_edges', immutable=immutable,
multiedges=multiple, loops=loops)
if return_labels and multiple:
return [G, origlabels_dic]
return G
Expand Down
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