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fix issue with immutable graphs in src/sage/graphs/chrompoly.pyx #39896

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fix issue with immutable graphs in src/sage/graphs/chrompoly.pyx #39896

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17 changes: 14 additions & 3 deletions src/sage/graphs/chrompoly.pyx
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Original file line number Diff line number Diff line change
Expand Up @@ -141,6 +141,12 @@ def chromatic_polynomial(G, return_tree_basis=False, algorithm='C', cache=None):
Traceback (most recent call last):
...
ValueError: algorithm must be "C" or "Python"

Check the behavior with immutable graphs::

sage: G = Graph([(0, 1), (0, 1)], multiedges=True, immutable=True)
sage: G.chromatic_polynomial()
x^2 - x
"""
algorithm = algorithm.lower()
if algorithm not in ['c', 'python']:
Expand Down Expand Up @@ -168,9 +174,8 @@ def chromatic_polynomial(G, return_tree_basis=False, algorithm='C', cache=None):
cdef mpz_t m, coeff
cdef mpz_t *tot
cdef mpz_t *coeffs
G = G.relabel(inplace=False)
G = G.relabel(inplace=False, immutable=False)
G.remove_multiple_edges()
G.remove_loops()
nverts = G.num_verts()
nedges = G.num_edges()

Expand Down Expand Up @@ -436,6 +441,12 @@ def chromatic_polynomial_with_cache(G, cache=None):
Traceback (most recent call last):
...
TypeError: parameter cache must be a dictionary or None

Check the behavior with immutable graphs::

sage: G = Graph([(0, 1), (0, 1)], multiedges=True, immutable=True)
sage: chromatic_polynomial_with_cache(G)
x^2 - x
"""
cdef CommutativeRing R = PolynomialRing(ZZ, "x", implementation="FLINT")
cdef Polynomial_integer_dense_flint one = R.one()
Expand All @@ -448,7 +459,7 @@ def chromatic_polynomial_with_cache(G, cache=None):
return zero

# Make a copy of the input graph and ensure that it's labeled [0..n-1]
G = G.relabel(inplace=False)
G = G.relabel(inplace=False, immutable=False)
G.remove_multiple_edges()

# We use a cache to avoid computing twice the chromatic polynomial of
Expand Down
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