On Sun, Apr 13, 2008 at 9:37 PM, <[EMAIL PROTECTED]> wrote:
>
> sage: Q.<x,y> = QQ['x,y']
> sage: R.<X,Y> = Q.quo(Q.ideal(x^2 + y^2 -1))
> sage: X/2
> ---------------------------------------------------------------------------
> <type 'exceptions.NotImplementedError'> Traceback (most recent call last)
>
> /home/boothby/.sage/<ipython console> in <module>()
>
> /home/boothby/.sage/element.pyx in
> sage.structure.element.RingElement.__div__ (sage/structure/element.c:9080)()
>
> /home/boothby/.sage/coerce.pyx in
> sage.structure.coerce.CoercionModel_cache_maps.bin_op_c
> (sage/structure/coerce.c:5055)()
>
> /home/boothby/.sage/element.pyx in
> sage.structure.element.RingElement.__div__ (sage/structure/element.c:9063)()
>
> /home/boothby/.sage/coerce.pxi in sage.structure.element._div_c
> (sage/structure/element.c:16174)()
>
>
> /home/boothby/sage/local/lib/python2.5/site-packages/sage/rings/quotient_ring_element.py
> in _div_(self, right)
> 114 if not right.is_unit():
> 115 raise ZeroDivisionError
> --> 116 raise NotImplementedError
> 117
> 118 def __int__(self):
>
> <type 'exceptions.NotImplementedError'>:
>
>
> I'd understand if I were trying to divide by a non-unit, or even a unit
> polynomial. But I'm trying to divide by a unit in the base ring -- shouldn't
> that work? Is this a problem in the coercion model, or is it validly not
> implemented?
>
It's simply not implemented. Implement it and post a patch.... :-)
The source code for the function _div_ that is called is
sage: X._div_??
Source:
def _div_(self, right):
if not right.is_unit():
raise ZeroDivisionError
raise NotImplementedError
which does *not* presently try to invert right if it does happen to be a unit
in the base ring. Why? Because it's not implemented yet.
Your best bet is to either implement better functionality
for computing in quotient rings or to type
sage: X * (1/2)
1/2*X
By the way, looking at the file quotient_ring_element.py, I would
bring the doctest coverage to 100% before adding or subtracting
any code from that file. Right now coverage is a terrible 24%.
-- william
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