Reference for cospectral_graphs function?

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The cospectral_graphs function was added in #9141. Credits:

• Jason Grout (2010-06-04): cospectral_graphs

The implementation is quite straightforward. First the optional matrix_function argument is read:

if matrix_function is None:
    matrix_function = lambda g: g.adjacency_matrix()

Then the optional graphs argument is read:

from sage.graphs.graph_generators import graphs as graph_gen
if graphs is None:
    graph_list = graph_gen(vertices, property=lambda _: True)
elif callable(graphs):
    graph_list = iter(g for g in graph_gen(vertices, property=lambda _: True) if graphs(g))
else:
    graph_list = iter(graphs)

Finally the cospectral graphs are generated, by putting each graph in a bucket labeled by the graph's characteristic polynomial:

from collections import defaultdict
charpolys = defaultdict(list)
for g in graph_list:
    cp = matrix_function(g).charpoly()
    charpolys[cp].append(g)

... and then taking the contents of those buckets containing more than one graph:

cospectral_graphs = []
for cp, g_list in charpolys.items():
    if len(g_list) > 1:
        cospectral_graphs.append(g_list)

return cospectral_graphs