On Tue, Aug 26, 2008 at 9:09 AM, Burcin Erocal <[EMAIL PROTECTED]> wrote:
>
> On Tue, 26 Aug 2008 08:29:33 +0200
> "Ondrej Certik" <[EMAIL PROTECTED]> wrote:
>
>>
>> On Tue, Aug 26, 2008 at 12:49 AM, William Stein <[EMAIL PROTECTED]> wrote:
>> >
>> >> BTW, one important warning: ginac and sympycore are missing
>> >> assumptions and sympy only has very trivial ones, like positive,
>> >> negative, integer, even, odd, etc. This is really important for any
>> >> nontrivial things in a CAS and I changes to the core may be needed. I
>> >> really want to have assumptions in sympy first before saying -- yes,
>> >> this approach to do the core is the best.
>> >>
>> >> Ondrej
>> >
>> > Why are assumptions "really important for any nontrivial things in a CAS"?
>> > In my entire life I've only ever used assumptions to get maxima to do
>> > a symbolic integration. I've never used them in any other context.
>> > Can you please educate me on why they are so important? Thanks.
>>
>> Basically all more nontrivial simplifications. Things like to simplify
>> sqrt(1-sin(x)**2) etc. Or acos(cos(x)). All of those are things that
>> are needed to be automatic, but only work sometimes, e.g. if 0 < x <
>> pi, or -pi/2 < x < pi/2 etc. This is obivously needed in integration
>> and in solvers and also just when the user wants to simplify the
>> expression. So in particular this is done inside ginac, so ginac need
>> some way to handle these. Do you have any ideas how to do it?
>
> Looking briefly at the MMA documentation, I can't find any reference to
> a general assumption context, which can be queried to get information
> on variables/expressions.
>
> It seems that you can give assumptions only for specific functions,
> such as
>
> In[]:= Simplify[ArcCos[Cos[x]],{0<x,x<Pi/2}]
> Out[]= x
>
> same goes for Refine, FullSimplify and FunctionExpand (whatever that
> is).
>
> As already mentioned in another thread, the automatic evaluation (basic
> simplification) in GiNaC is meant to be a cheap operation. MMA doesn't
> do automatic simplification of this statement either,
>
> In[]:= Assuming[0<x, Assuming[x<Pi/2 ,ArcCos[Cos[x]]]]
> Out[]= ArcCos[Cos[x]]
If mathematica really doesn't do this in the core, e.g. you need to
tell the simplifiy -- hey, use these assumptions, then I think we
should do exactly this. We actually converged to this solution too in
sympy, because I really don't want to make the core complicated.
On Tue, Aug 26, 2008 at 9:15 AM, David Philp <[EMAIL PROTECTED]> wrote:
>
>
> On 26/08/2008, at 5:09 PM, Burcin Erocal wrote:
>> In[]:= Assuming[0<x, Assuming[x<Pi/2 ,ArcCos[Cos[x]]]]
>> Out[]= ArcCos[Cos[x]]
>
> In[]:= Simplify[ArcCos[Cos[x]], Assumptions -> 0 < x < Pi/2]
> Out[] = x
Exactly, you pass the assumptions as an object to the simplify
function. That's the point, it's very explicit and what I like.
I really need to learn mathematica and then just do the same, instead
of trying to do something and then burn myself.
On Tue, Aug 26, 2008 at 9:06 AM, Fernando Perez <[EMAIL PROTECTED]> wrote:
>
> On Mon, Aug 25, 2008 at 10:58 AM, William Stein <[EMAIL PROTECTED]> wrote:
>
>>> As to GPL vs BSD, I am sad that some people will not contribute to a
>>> BSD project and some other people will not use a GPL project. But my
>>> intuition says that the license is not the main reason. If sympy was
>>> as fast as ginac (as I hope it will be in not so distant future), I am
>>> sure it'd be ok, even if it's BSD. BTW, I told Burcin and William that
>>> if the license was the only reason, I am willing to consider switching
>>> to GPL (i.e. try to persuade all 44+ contributors), if that would mean
>>> more people would be developing sympy. Currently it seems to be the
>>> opposite, i.e. when we switched from GPL to BSD, people and developers
>>> seemed to like it, but I may well be mistaken. But as I said, I think
>>> the main problem of sympy is not the license, but speed.
>>
>> The GPL/BSD split in the mathematical Python community is
>> unfortunate and a is very real problem. At scipy'08 it was
>> the source of tension for some people...
>
> I don't know if for this particular project it's a
> realistic/valid/interesting solution or not, but how about using LGPL
> as a middle solution? I happen to actually really like the LGPL: I
> find that it protects my code from flat out abuse, while not imposing
> my conditions beyond my own code boundaries. But for many cases I
> find that LGPL strikes an excellent balance and fully protects the
> original code from closed modifications (though not from use as an
> unmodified library), which is what I consider most important.
>
> In fact, I'd originally licensed ipython as LGPL, and switched to BSD
> only to make it easier to share code back and forth with
> numpy/scipy/matplotlib. In this case I'm happy having switched to BSD,
> but I still like the LGPL a lot.
>
> Just a suggestion to consider, though obviously ultimately the
> decision rests with the code authors.
Thanks Fernando for mentioning LPGL. Actually, in fact Kirill, another
sympy developer who works on the core of sympy a lot, he didn't have
time to advocate it yet, but he also very much like LGPL and always
wanted sympy to be LGPL to prevent code abuse. I don't think such code
abuse is actually happening though, so I think it's more of an
irrational fear, but I am in no way trying to make it invaluable
argument -- I may well be completely wrong on this.
But did it ever happen to you Fernando that someone would plainly
abuse ipython/numpy/scipy? Clearly ipython is way more popular than
sympy, so if it doesn't happen for numpy/scipy/ipython, I don't think
we have to fear. (btw. fear of what actually?)
But as I said several times, I am not fundamental in this, if it helps
all the sympy developers to sleep well at night, let's do it. I am not
making fun of it, I mean it. It's simply not worhty if many people
feel that their code will be abused and it's not a question if, but
when. So this clearly has to be resolved in some way. My way is to
question this very fear itself, I suspect it's not happening this way.
The other option is to switch to LGPL or GPL.
Burcin, would LGPL be suitable for you to contribute to sympy, or is
LGPL not protective enough for you?
Ondrej
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