On Dec 19, 2008, at 10:40 AM, Jason Grout wrote: > Marshall Hampton wrote: >> Something odd is happening here. I just noticed that if we define v >> as: >> >> v = vector(QQ,[1,1]) >> >> then there are no problems, even though the type(v) is the same as in >> your code. I don't understand how the same type of object, with the >> same values, would have different coercion behavior.
Coercion, or wether arithmetic even makes sense, is determined by the *parent* of an object, not its type. Consider the following session. sage: a = mod(2, 5) sage: b = mod(2, 7) sage: type(a) <type 'sage.rings.integer_mod.IntegerMod_int'> sage: type(b) <type 'sage.rings.integer_mod.IntegerMod_int'> sage: a*b # This doesn't makes sense, and SHOULD fail Traceback (most recent call last): File "<ipython console>", line 1, in <module> File "element.pyx", line 1102, in sage.structure.element.RingElement.__mul__ (sage/structure/element.c: 8632) File "coerce.pyx", line 697, in sage.structure.coerce.CoercionModel_cache_maps.bin_op (sage/structure/ coerce.c:5833) TypeError: unsupported operand parent(s) for '*': 'Ring of integers modulo 5' and 'Ring of integers modulo 7' sage: parent(a) Ring of integers modulo 5 sage: parent(b) Ring of integers modulo 7 > I think that the issue might be the issue in #3058: > http://trac.sagemath.org/sage_trac/ticket/3058 > > The problem comes up when the parent of v has a user-defined basis, > instead of the standard basis: > > sage: v.parent() > > Vector space of degree 2 and dimension 1 over Rational Field > User basis matrix: > [1 1] Yes, you hit the nail on the head. Note that (for better or for worse) multiplication by the identity matrix makes it forget the user- defined basis. sage: A=matrix([[1,2],[2,1]]) sage: eig=A.eigenvectors_right() sage: v=eig[0][1][0] sage: t = var('t') sage: I = A.parent()(1); I [1 0] [0 1] sage: parent(v*t) Vector space of degree 2 and dimension 1 over Symbolic Ring User basis matrix: [1 1] sage: parent(I*v*t) Vector space of dimension 2 over Symbolic Ring sage: A*v - I*v*t (3 - t, 3 - t) - Robert --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to sage-support@googlegroups.com To unsubscribe from this group, send email to sage-support-unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/sage-support URLs: http://www.sagemath.org -~----------~----~----~----~------~----~------~--~---
