Re: [sage-support] Is this a bug?

[kcrisman]

> Users mistake FAR too often symbolics with polynomials

Well, of course, and I'm sure rjf would say that Sage developers far too 
often mistake expressions for functions.  But what percentage of users 
actually know the difference in any case? (Rhetorical.)

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Re: [sage-support] Is this a bug?

[kcrisman]

> I am mildly in favor of dropping this predefinition (FWIW). 
>
>
>
Far be it for me to acquire the usual "hobgoblin of little minds" trope, 
but in this case consistency is probably good.  This argument was had ~10 
years ago in great detail, with several hilarious variants implemented one 
after another.  This has been pretty stable as a reasonable compromise 
between usability for bored students and the technical needs of the 
development crew, and I think one of the very nice things about Sage is 
that for all the learning curve, at least you can "just do math" with it. 
 Sage cell would not be nearly as useful as Wolfram Alpha without 
predefined x, for instance.

(Now let the attacks begin ...)

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[sage-support] Fwd: Typo in Sage documentation

[William Stein]

-- Forwarded message --
From: Philip Turecek 
Date: Wed, Feb 28, 2018 at 1:47 PM
Subject: Typo in Sage documentation
To: wst...@sagemath.com


Dear Mr Stein,

I found a typo in the Sage documentation, at the function period():

http://doc.sagemath.org/html/en/reference/rings_standard/sage/rings/rational.html#sage.rings.rational.Rational.period

It should be
"In general if d=2^a5^bm where m is coprime to 10"
instead of
"In general if d=2^a3^bm where m is coprime to 10".

Best regards,

Philip Turecek


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Best Regards,
William Stein

CEO, SageMath, Inc.

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Re: [sage-support] Is this a bug?

[Justin C. Walker]

> On Feb 28, 2018, at 00:09, Dima Pasechnik  wrote:
> 
> I would be for dropping 'x' as the only "default" variable (defined at start 
> time).
> Sage is unique in this sense AFAIK; there are CASes which don't require 
> declaration of any variable, 
> there are ones that don't have any special variables like Sage's 'x’.

I am mildly in favor of dropping this predefinition (FWIW).

Also, for this discussion, one can always access the variable by something like 
this:
sage: x=P.variables()[0]

Justin

--
Justin C. Walker, Curmudgeon-At-Large, Director
Institute for the Enhancement of the Director's Income

The path of least resistance:
it's not just for electricity any more.




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Re: [sage-support] Re: Is this a bug?

[Ralf Stephan]

Sorry for the noise, it already works fine, just not with characteristic 
polynomials of symbolic matrices. They messed up my Sage session, so the 
minpoly example seemed to fail too.

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Re: [sage-support] Re: Is this a bug?

[Ralf Stephan]

It might not be necessary to (re)assign the global variable. Only if the 
user wants to do operations with the polynomial he wants an x to be that 
poly variable, and certainly not another x. So, the parser can do the part 
of figuring out what x is meant, maybe by checking all generator names. 
Example:

sage: charpoly()
x + 1
sage: _ - x
(parser sees underscore and operation, and an unassigned global, so it 
checks the object referred by the underscore for variable names)

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Re: [sage-support] Re: Is this a bug?

[John Cremona]

On 28 February 2018 at 13:49, Ralf Stephan  wrote:

> Why should I define x when Sage gives me a polynomial with x, doesn't it
> already know it?
>
> That's what a user would ask and, frankly, s/he would be right.
>

Here is one reason.  In this example:

sage: M=Matrix([[1,2,3],[4,5,6],[7,8,9]])
sage: p = M.charpoly()
sage: p.parent()
Univariate Polynomial Ring in x over Integer Ring
sage: p
x^3 - 15*x^2 - 18*x
sage: x.parent()
Symbolic Ring

when we see the polynomial displayed we see the variable displayed as x,
but there has been no assignment to the python variable 'x' behind the
scenes.  You are suggesting (it seems) that after computing p here the
python variable 'x' should be bound to p.parent().gen().  But it is surely
a very Bad Idea for a function to (re)-assign global variables?  This would
be a source of many bugs.

It would probably be less confusing if there was no globally defined 'x' at
all as has been suggested earlier in this thread.  Then the last line of my
example would raise an error, and the documentation could tell users to
enter something like 'x = p.parent().gen()' or 'x = p.variables()[0]'
 (the latter would be better if a method variable() existed for univariate
polynomials!).

John



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[sage-support] Re: Is this a bug?

[Ralf Stephan]

Why should I define x when Sage gives me a polynomial with x, doesn't it 
already know it?

That's what a user would ask and, frankly, s/he would be right.

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[sage-support] Re: Is this a bug?

[Simon King]

On 2018-02-28, Ralf Stephan  wrote:
> On Wednesday, February 28, 2018 at 9:09:04 AM UTC+1, Dima Pasechnik wrote:
>>
>> I would be for dropping 'x' as the only "default" variable (defined at 
>> start time).
>>
>
> I agree but does it solve the problem I demonstrated. Can you then add x to 
> the minpoly?

If you define x as the variable of the minpoly, then certainly you can.

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[sage-support] Re: Is this a bug?

[Ralf Stephan]

On Wednesday, February 28, 2018 at 9:09:04 AM UTC+1, Dima Pasechnik wrote:
>
> I would be for dropping 'x' as the only "default" variable (defined at 
> start time).
>

I agree but does it solve the problem I demonstrated. Can you then add x to 
the minpoly?

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[sage-support] Re: Is this a bug?

[Dima Pasechnik]

I would be for dropping 'x' as the only "default" variable (defined at 
start time).
Sage is unique in this sense AFAIK; there are CASes which don't require 
declaration of any variable, 
there are ones that don't have any special variables like Sage's 'x'.


On Wednesday, February 28, 2018 at 8:00:34 AM UTC, Simon King wrote:
>
> Hi Ralf, 
>
> On 2018-02-28, Ralf Stephan  wrote: 
> > The reason is apparently that the polynomial ring was not created by the 
> > user on the command line but by the charpoly() code. 
>
> The parent of the pre-defined variable x wasn't created by the user 
> either, so, please don't blame polynomial rings or the 
> minpoly()/charpoly() code for it. 
>
>  sage: QQbar(sqrt(2)).minpoly().parent() 
>  Univariate Polynomial Ring in x over Rational Field 
>  sage: x.parent() 
>  Symbolic Ring 
>
>
> > i would consider this a high priority bug. Is there a ticket? 
>
> Not in the way you think, IMHO. 
>
> One problem that I see is the fact that it is possible to create rings 
> in which one variable name occurs twice, such as here: 
>   sage: RR['x']['x'] 
>   Univariate Polynomial Ring in x over Univariate Polynomial Ring in x 
>   over Real Field with 53 bits of precision 
>   sage: RR['x'].gen()-RR['x']['x'].gen() 
>   -x + x 
>   sage: _==0 
>   False 
>
> So, we have two x with different roles. It would make sense to disallow 
> the creation of such rings. 
>
> Also, I belive it is a bug that x is pre-defined as a symbolic variable; 
> we should finally drop that pseudo-courtesy. Users mistake FAR too often 
> symbolics with polynomials; by pre-defining x, we let the users believe 
> that x is good for *all* computations. So, I believe it would be better 
> to let the user create x him/herself, for the specific application that 
> he/she has in mind. However, I am aware that I belong to a minority with 
> that opinion. 
>
> Best regards, 
> Simon 
>
>

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[sage-support] Re: Is this a bug?

[Simon King]

Hi Ralf,

On 2018-02-28, Ralf Stephan  wrote:
> The reason is apparently that the polynomial ring was not created by the 
> user on the command line but by the charpoly() code.

The parent of the pre-defined variable x wasn't created by the user
either, so, please don't blame polynomial rings or the
minpoly()/charpoly() code for it.

 sage: QQbar(sqrt(2)).minpoly().parent()
 Univariate Polynomial Ring in x over Rational Field
 sage: x.parent()
 Symbolic Ring


> i would consider this a high priority bug. Is there a ticket?

Not in the way you think, IMHO.

One problem that I see is the fact that it is possible to create rings
in which one variable name occurs twice, such as here:
  sage: RR['x']['x']
  Univariate Polynomial Ring in x over Univariate Polynomial Ring in x
  over Real Field with 53 bits of precision
  sage: RR['x'].gen()-RR['x']['x'].gen()
  -x + x
  sage: _==0
  False

So, we have two x with different roles. It would make sense to disallow
the creation of such rings.

Also, I belive it is a bug that x is pre-defined as a symbolic variable;
we should finally drop that pseudo-courtesy. Users mistake FAR too often
symbolics with polynomials; by pre-defining x, we let the users believe
that x is good for *all* computations. So, I believe it would be better
to let the user create x him/herself, for the specific application that
he/she has in mind. However, I am aware that I belong to a minority with
that opinion.

Best regards,
Simon

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