From: Michael Orlitzky Date: Thu, 3 Dec 2020 16:44:15 +0000 (-0500) Subject: eja: update the DESIGN and TODO documents. X-Git-Url: https://gitweb.michael.orlitzky.com/?a=commitdiff_plain;h=c89792b4772d4012799a6e7fcd63c1cdba6fce12;p=sage.d.git eja: update the DESIGN and TODO documents. --- diff --git a/mjo/eja/DESIGN b/mjo/eja/DESIGN index 248af84..467e2ae 100644 --- a/mjo/eja/DESIGN +++ b/mjo/eja/DESIGN @@ -6,28 +6,64 @@ the documentation. I'm just not sure where they go yet. Matrix representations ---------------------- -Why allow matrix representations for all algebras? +All algebras have a "matrix representation" of their elements. This is +the original, ambient representation of the elements as either column +(n-by-1) or square (n-by-n) matrices. For example, the elements of the +Jordan spin algebra are column vectors, and the elements of real +symmetric matrix algebra are... real symmetric matrices. + +The CombinatorialFreeModule class actually supports such an +alternative representation of its generators since it subclasses +IndexedGenerators. However, using matrices as the index set turns out +to be ugly: printing the generators, and especially printing an +algebra element as a sum of said generators comes out wonky, since +the matrices require more than one line. For example, + + sage: A = matrix(QQ,[[1,2],[3,4]]) + sage: B = matrix(QQ,[[5,6],[7,8]]) + sage: A.set_immutable() + sage: B.set_immutable() + sage: M = CombinatorialFreeModule(QQ,[A,B],bracket=False,prefix="") + sage: 2*M(A) + 3*M(B) + 2*[1 2] + [3 4] + 3*[5 6] + [7 8] + +And things only get worse if you leave the prefix in there to +distinguish between e.g. the super- and sub-algebra elements +corresponding to the same matrix. Thus, we store out own copy +of the matrix generators, and have our own set of methods for +accessing them. + +Why allow matrix representations for all algebras, rather than just +the matrix algebras? 1. We already have a to_vector() operation that turns an algebra -element into a vector whose coordinates live in the algebra's -base_ring(). Adding a to_matrix() operation is a natural -generalization of that. + element into a vector whose coordinates live in the algebra's + base_ring(). Adding a to_matrix() operation is a natural + generalization of that. + +2. Having access to the ambient coordinates in a general way is useful + when converting between other coordinate systems. If we have two + subalgebras B and C of A, we can use to_matrix() to go from, say, + C -> A -> B rather than having to convert from C to B directly. -2. When constructing a Cartesian product algebra, we don't know a -priori whether or not the result will have matrix-algebra factors. We -can figure it out at runtime, but it would be nice if DirectSumEJA -always returned the same class. Maybe more importantly, if a Cartesian -product has one matrix and one non-matrix factor, then what would its -own matrix representation look like? We want to delegate to the -factors... +3. When constructing a Cartesian product algebra, we don't know a + priori whether or not the result will have matrix-algebra + factors. We can figure it out at runtime, but it would be nice if + DirectSumEJA always returned the same class. Maybe more + importantly, if a Cartesian product has one matrix and one + non-matrix factor, then what would its own matrix representation + look like? We want to delegate to the factors... Basis normalization ------------------- -For performance reasons, we need a class (RationalBasis...) that -orthonormalizes its own basis. We *could* insist that the user do -this, of course, but the reason we don't want him to is because we -need to know how to undo the process. If we run Gram-Schmidt on the -basis matrix ourselves, then we can save the matrix that undoes the -process. And by undoing the process, we can get to a basis where -computations are fast again. +For performance reasons, we prefer the algebra constructors to +orthonormalize their own bases. We _could_ ask the user to do that, +but there's a good reason to do it ourselves: if _we_ run +Gram-Schmidt, then we can compute/store the matrix that undoes the +process. Undoing the change-of-coordinates allows us to perform some +computations in the original basis (like the "characteristic +polynomial of"), which can be much faster when the original basis +contains only rational entries. diff --git a/mjo/eja/TODO b/mjo/eja/TODO index 08b8e81..b298e69 100644 --- a/mjo/eja/TODO +++ b/mjo/eja/TODO @@ -21,8 +21,5 @@ sage: a0 = (1/4)*X[4]**2*X[6]**2 - (1/2)*X[2]*X[5]*X[6]**2 - (1/2)*X[3]*X[4]*X[6 6. The main EJA element constructor is happy to convert between e.g. HadamardEJA(3) and JordanSpinEJA(3). -7. Figure out if CombinatorialFreeModule's use of IndexedGenerators - can be used to replace the matrix_basis(). - -9. Add back the check_field=False and check_axioms=False parameters +8. Add back the check_field=False and check_axioms=False parameters for the EJAs we've constructed ourselves.