#1163: [with patch, needs review] make assume behave more consistently and catch
inconsistent assumptions
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Reporter: zimmerma | Owner: gfurnish
Type: defect | Status: assigned
Priority: major | Milestone: sage-4.1.2
Component: calculus | Keywords:
Reviewer: | Author: Karl-Dieter Crisman
Merged: |
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Changes (by newvalueoldvalue):
* cc: jason (added)
* author: => Karl-Dieter Crisman
Comment:
The non-experimental patch resolves all but one of the issues above (about
which more below). It also fixes a SLEW of subtle problems and provides
better checking in the assumptions code, and fixes a variety of doctests
which are improvements (!) based on making assumptions better. I didn't
end up changing the documentation to tell people to use forget, but
instead gave lots of examples of what Errors you get if you assume
something redundant or inconsistent.
The one "problem" which is still extant is the following:
{{{
sage: assume(x>0)
sage: bool(x!=1)
True
}}}
This is something to which Maxima replies 'unknown', which is a lot better
than True or False. However, Sage doesn't currently have that as an
option (that would be a separate ticket, and not necessarily a desirable
one). Further, since anytime one compares a symbolic variable, e.g.
{{{
def func(x,y):
if x is not y:
do something great
else:
blow a gasket
}}}
Expression.__nonzero__ is called, we are forced to have x!=1 be True,
since x==1 is False, seeing as != and == have opposite truth tables. For
an example of what can go wrong, see the change made in this patch to
desolvers.py, where having x!=1 be False originally meant there was no
dependent variable; this one was fixable, but the fact that symbolic
matrices essentially all go to zero (!) after making that change was much
worse.
So I have left it alone after checking as many cases as I could, and we'll
just have to live with it, unless someone can figure out how to get around
this. I think it's really just a conflict between wanting to say that
x!=1 is True unless you are sure that x==1, and wanting to say that x!=1
is False unless you are sure that x!=1; there is no nonstandard logic
option for Python.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/1163#comment:14>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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