On Thursday, May 10, 2012 7:14:01 PM UTC-4, John H Palmieri wrote:
>
>
>
> On Thursday, May 10, 2012 2:41:07 PM UTC-7, Duc Trung Ha wrote:
>>
>> Hola,
>>
>> would anybody be so kind and explain to me the mechanism behind
>> "assume(sth)"?
>>
>
> The mechanism seems to be broken. Actually, the mechanism that compares
> boolean expressions seems to be broken, which means that assumptions don't
> work right, either.
>
> sage: a = (x - 2 <= 0)
> sage: b = (x <= 2)
> sage: c = (x - 1 <= 0)
> sage: a == b
> False
> sage: a == c
> True
> sage: b == c
> False
>
> The first two claimed equalities are a bit troubling...
>
>
Remember, False = we didn't/couldn't prove it was True, so the first one is
annoying but technically ok.
As to the original post, we must be preprocessing things not quite right.
Maxima:
(%i1) assume(x-2<=0);
(%o1) [x <= 2]
(%i2) assume(x-1<=0);
(%o2) [x <= 1]
(%i3) assumptions;
(%o3) assumptions
(%i4) facts();
(%o4) [2 >= x, 1 >= x]
And indeed
sage: c = (x-2<=0)
sage: maxima.assume(c._maxima_init_assume_())
[x<=2]
sage: c = (x-2<=0)
sage: maxima.assume(c._maxima_init_assume_())
[redundant]
But we don't use "facts()" per se, we instead append the relation x-2<=0
directly to the assumption list.
sage: a = (x-1<=0)
sage: sage.symbolic.assumptions._assumptions
[x - 2 <= 0]
sage: if a in sage.symbolic.assumptions._assumptions:
print 'yes'
....:
yes
Aack! Something horrible is happening here. Okay, John, now I understand
your somewhat cryptic post; since
sage: a == c
True
assumptions thinks this is already in the list. And this is already true
in a Sage from two years ago (4.4.4). Why on earth is a == c True?
Apparently Ginac thinks so?
cdef int _cmp_c_impl(left, Element right) except -2:
return left._gobj.compare((<Expression>right)._gobj)
I must be missing something here. Burcin, any ideas - maybe this is
already on some ticket that should have been merged but got hung up on a
technicality, there was some stuff about symbolic expression comparisons a
while back.
Sorry I had to go through all this - hopefully it clarifies things a little
for the original poster.
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