[sage-devel] Re: no conversion between number fields (extensions) over base rings which have conversion

2018-10-18 Thread Nils Bruin
On Thursday, October 18, 2018 at 11:55:14 AM UTC-7, Simon Brandhorst wrote:

> I would say because there is no canonically defined embedding as one can 
> send i to -i as well.
>
> That's in principle true, but in sage names of generators carry essential 
information. For instance:

sage: parent(QQ['x'].0+ZZ['x,y'].1)
Multivariate Polynomial Ring in x, y over Rational Field

so there's an argument to be made that the "natural" map in this case would 
be to map i to i. I think that choice could be justifiable in sage, but I 
also expect it will be too expensive to support properly in this case: One 
would also need to check that the minimal polynomials of the different "i"s 
are compatible. They are in this case, but that requires quite a bit of 
work to figure out.
 

> On Thursday, October 18, 2018 at 1:38:24 PM UTC+2, Daniel Krenn wrote:
>>
>> sage: CyclotomicField(3).extension(x^2+1, 'i')(QQ.extension(x^2+1, 
>> 'i').gen()) 
>>
>> returns 
>>
>> TypeError: Cannot coerce element into this number field 
>>
>> Does anyone have some idea why this is not working? 
>>
>> (This is https://trac.sagemath.org/ticket/26443) 
>>
>> Best, Daniel 
>>
>

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[sage-devel] Re: no conversion between number fields (extensions) over base rings which have conversion

2018-10-18 Thread Simon Brandhorst
I would say because there is no canonically defined embedding as one can 
send i to -i as well.

On Thursday, October 18, 2018 at 1:38:24 PM UTC+2, Daniel Krenn wrote:
>
> sage: CyclotomicField(3).extension(x^2+1, 'i')(QQ.extension(x^2+1, 
> 'i').gen()) 
>
> returns 
>
> TypeError: Cannot coerce element into this number field 
>
> Does anyone have some idea why this is not working? 
>
> (This is https://trac.sagemath.org/ticket/26443) 
>
> Best, Daniel 
>

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