#15220: Vector_modn_dense used for too large a modulus?
------------------------------+-----------------------------
Reporter: mmezzarobba | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-5.12
Component: linear algebra | Keywords:
Merged in: | Authors:
Reviewers: | Report Upstream: N/A
Work issues: | Branch:
Commit: | Dependencies:
Stopgaps: |
------------------------------+-----------------------------
`sage.rings.finite_rings.integer_mod?` says that `IntegerMod_int64` "is
used if the modulus is less than `2^{31}-1`".
Consider however a vector of elements of `GF(p)` with `p` between 2^31^
and 2^32^:
{{{
sage: K = GF(3037000493)
sage: v = vector(K, [42])
sage: type(v)
<type 'sage.modules.vector_modn_dense.Vector_modn_dense'>
}}}
Then:
{{{
sage: type(v[0])
<type 'sage.rings.finite_rings.integer_mod.IntegerMod_int64'>
}}}
which breaks some operations:
{{{
sage: ~v[0]
---------------------------------------------------------------------------
ZeroDivisionError Traceback (most recent call
last)
<ipython-input-5-43d86000256e> in <module>()
----> 1 ~v[Integer(0)]
/home/marc/co/sage/local/lib/python2.7/site-
packages/sage/rings/finite_rings/integer_mod.so in
sage.rings.finite_rings.integer_mod.IntegerMod_int64.__invert__
(sage/rings/finite_rings/integer_mod.c:29992)()
/home/marc/co/sage/local/lib/python2.7/site-
packages/sage/rings/finite_rings/integer_mod.so in
sage.rings.finite_rings.integer_mod.mod_inverse_int64
(sage/rings/finite_rings/integer_mod.c:30920)()
ZeroDivisionError: Inverse does not exist.
}}}
This does not happens with modular integers themselves:
{{{
sage: type(K(42))
<type 'sage.rings.finite_rings.integer_mod.IntegerMod_gmp'>
sage: ~K(42)
2241595602
}}}
Matrices do not seem affected either.
Things work fine with sage-5.8 on the same machine. This makes me suspect
that the issue might have been introduced in
aa83d6e996d552dfa871f510b1bc9af6f1127076 (#14627)—but I didn't check.
--
Ticket URL: <http://trac.sagemath.org/ticket/15220>
Sage <http://www.sagemath.org>
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