# [sage-devel] Re: BUGS in tensor products of algebras

Hey Vit,

Some of these issues are probably related to bad input. Let us start
>> with *.algebras_generators(). It is useful to look at the output:
>>
>> sage: PBW.algebra_generators()
>> Finite family {-alpha[1]: PBW[-alpha[1]], alpha[1]: PBW[alpha[1]],
>> alphacheck[1]: PBW[alphacheck[1]]}
>>
>> So it is expecting a simple root as input. Similarly, for your tensor
>> product:
>>
>
>
>>
>> sage: UC = PBW.tensor(C)
>> Lazy family (Term map from Image of Cartesian product of Free abelian
>> monoid indexed by {alpha[1], alphacheck[1], -alpha[1]}, Subsets of {0, 1}
>> by <type 'tuple'> to Universal enveloping algebra of Lie algebra of ['A',
>> 1] in the Chevalley basis in the Poincare-Birkhoff-Witt basis # The
>> Clifford algebra of the Quadratic form in 2 variables over Rational Field
>> with coefficients:
>> [ 1 0 ]
>> [ * 1 ](i))_{i in Image of Cartesian product of Free abelian monoid
>> indexed by {alpha[1], alphacheck[1], -alpha[1]}, Subsets of {0, 1} by <type
>> 'tuple'>}
>>
>> The tensor product as QQ-modules knows that it is a QQ-algebra, but it
>> plays it safe and uses its basis as the generating set, which in turn, is a
>> Cartesian product of the bases of its factors. Subsequently, the keys for
>> the basis is the Cartesian product of the keys of the factors. So in this
>> case, the (1,1) corresponds to the keys for the Clifford algebra basis
>> (more of a by-product of the implementation, but the subsets are natural):
>>
>>
> That sounds fine. But for a newcomer it's really hard to figure out what
> should he input. I am still not sure myself. Consider this:
>
> alpha = PBW.algebra_generators().keys()
> PBW.basis()[alpha[0]] == PBW.algebra_generators()[alpha[0]]
>
>
>
> I think this is a bug.
>

No, it is not. The keys for the basis of the PBW are different than those
for the algebra generators. See the output:

sage: PBW.basis()
Lazy family (Term map from Free abelian monoid indexed by {alpha[1],
alphacheck[1], -alpha[1]} to Universal enveloping algebra of Lie algebra of
['A', 1] in the Chevalley basis in the Poincare-Birkhoff-Witt basis(i))_{i
in Free abelian monoid indexed by {alpha[1], alphacheck[1], -alpha[1]}}
sage: PBW.algebra_generators()
Finite family {-alpha[1]: PBW[-alpha[1]], alpha[1]: PBW[alpha[1]],
alphacheck[1]: PBW[alphacheck[1]]}

It clearly indicates that the basis keys should be an element of a free
abelian monoid. LBYL. Now in this case, if we did try to convert the input
into the keys, it should work. See https://trac.sagemath.org/ticket/18750.
Actually, it is not as bad as I remembered in terms of outright timings,
but there are some other technical issues.

>
>
>> sage: C.basis()
>> Lazy family (Term map from Subsets of {0, 1} to The Clifford algebra of
>> the Quadratic form in 2 variables over Rational Field with coefficients:
>> [ 1 0 ]
>> [ * 1 ](i))_{i in Subsets of {0, 1}}
>>
>> So it is expecting subsets and that the user will not input bad data.
>> There is no reason for it to simplify and not a bug. Granted, we could put
>> a check on the user input here, but there is a speed penalty as this can be
>> a well-used code path internally. Moreover, ducktyping can also be useful.
>> So we are fairly permissive here, but not without due cause IMO.
>>
>>
> Pardon my ignorance, but If there is no simplification then what is all
> this good for? It does simplify for universal enveloping algebra and so it
> should do it for Clifford algebras as well. Also I have a big issue with
> naming convention here. The method basis() does not produce a basis!
>

it is *bad input*. It does produce a basis, one indexed by *subsets*, not
words/multisets. In math terms, if you have a sequence (x_i)_{i \in I},
then want x_j, where j \notin I, then you have an error. With #18750, this
input might instead raise an error. If you want to do that, then you can do
this:

sage: Q = QuadraticForm(QQ, 2, [1,0,1])
sage: C = CliffordAlgebra(Q)
sage: g = list(C.algebra_generators()); g
[e0, e1]
sage: prod(g[i] for i in [0,0,1,0,1,1,0,0])
-e0*e1

If you took your input and wrapped it with set:

sage: set((1,1,0,1))
{0, 1}

which would be the other possible outcome with #18750.

>  I understand that it might be convenient for implementation details to
> use tuples of numbers (not subsets!!!) to index elements, but why can I
> index with E, F and H in the universal enveloping algebra case and not with
> e and f in the Clifford algebra case?
>
> It seems that accessing Clifford algebra elements through basis just
> messes up with the simplification.
>
> x = cb[(1,1,0,1)]
> print(simplify(x))
> print(type(x))
> print(f*f*e*f)
> print(type(f*f*e*f))
> print(f*f*e*f == x)
>
>
> Do not confuse a bug with a not-yet-implemented feature: (noncommutative)
>> polynomial rings are not in AlgebrasWithBasis, so it is not expected that
>> they work with something like tensor(). However, it would be a good feature
>>
>
> I guess are referring to my complaints about not being able to take tensor
> product with noncommutative ring. I see the naming convention as a BUG. If
> the method has "algebra" in it's name, it should produce algebra and not a
> ring. If a method has "basis" in its name, it should produce basis not a
> generating set as C.basis() does or a basis and not an algebra as
> pbw_basis() does.
>

Remember that we are bound by the facilities of computer science and
programming within Sage as well, which are usually much stronger
constraints than mathematics (e.g., consider the set of real numbers). For
the current implementation of tensor products, we require the algebra to be
in AlgebrasWithBasis. I would not say limitations of the current framework
to be a bug. Also, NC poly rings are in the category of algebras, so it
does produce an algebra.

C.basis() does produce a basis.

sage: list(C.basis())
[1, e0, e1, e0*e1]

Now pbw_basis() does return an algebra in a particular basis, so in some
ways, it is returning a basis, just not for the Lie algebra. So now I see
why you feel it is not completely natural. However, do you have a proposal
for how to have convenient access to the PBW basis of U(*g*)? IMO, it
cannot be connected with universal_enveloping_algebra() because that only
has to return a class modeling the UEA, which should not be required to
have a PBW basis method. Also, I feel that a PBW basis is really more
closely connected to the Lie algebra and it is clear to someone working in
this field what pbw_basis would return.

>
>
>>
>> if you want to access the basis, use the .basis() method.
>>
>> Also, how is pbw_basis not an intuitive name for obtaining the PBW basis
>> (there is also the fully spelled out version too)? If you want to change
>> the ordering, RTM of "PBW?" to pass in an ordering function to
>> L.pbw_basis().
>>
>>
> For naming conventions see above. I checked out helpstring for PBW and I
> am still not sure how to produce e.g. E, F, H ordering. I suggest to move
> this helpstring to L.pbw_basis().
>

-1 It is connected with the construction of the PBW class and not handled
in the pbw_basis() method. However, I do agree that it would be good to
have an (different) example in the pbw_basis() and a reference to the PBW
class in the docstring.

> Now the fact that you sometimes obtaining random attribute errors when
>> running that code block is a definite bug. Please submit a trac ticket.
>>
>>
> Submitted as https://trac.sagemath.org/ticket/24822
>

Great, thank you.

> In general, I think it is difficult to determine the tensor product of two
>> (non commutative, non PBW) algebras over an arbitrary subalgebra. There is
>> some code for doing smash products somewhere on #15874, but it has
>> bitrotted and IDK if that will give you what you want. I did write some
>> code to compute Verma modules via the PBW basis. I had to work somewhat
>> hard to make it work, and it wasn't apparent to me how to extend that to
>> more general framework.
>>
>>
> Verma modules (for finite-dimensional semisimple Lie algebras) were
> exactly what I had in mind. Are they already in Sage somewhere? I haven't
> seen them.
>

See https://trac.sagemath.org/ticket/23517. I try to track the overall
progress of Lie algebras in Sage in essentially metaticket
https://trac.sagemath.org/ticket/14901.

Best,
Travis

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