From: Michael Orlitzky Date: Tue, 2 Mar 2021 19:15:27 +0000 (-0500) Subject: eja: update the design document. X-Git-Url: https://gitweb.michael.orlitzky.com/?a=commitdiff_plain;h=91e2cb17af6ef788820dd6975335457411ecc8ae;p=sage.d.git eja: update the design document. --- diff --git a/mjo/eja/DESIGN b/mjo/eja/DESIGN index 35f3858..7607b1c 100644 --- a/mjo/eja/DESIGN +++ b/mjo/eja/DESIGN @@ -48,14 +48,38 @@ the matrix algebras? 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. -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... +Fielding questions +------------------ + +All Euclidean Jordan algebras are over the real scalar field. This +presents a problem: in SageMath, the matrix and vector classes don't +support scalar fields that are different than their entries. And three +of the simple families of Euclidean Jordan algebras are matrices with +non-real entries: the Hermitian comples, quaternion, and octonion +algebras. + +At least in the complex and quaternion case, we can "embed" the +complex numbers and quaternions into the space of 2-by-2 or 4-by-4 +matrices. But the octonions are not associative, so they can't be +embedded (via a homomorphism) into any real matrix space. So what +do we do? Write it ourselves, obviously. + +The octonion matrix algebra is implemented separately, as a subclass +of CombinatorialFreeModule, to work around that issue. The custom +class supports a scalar field that is different than the entries of +the matrices. However, this means that we actually have FOUR +different types of "matrices" to support: + + (1) Sage vectors + (2) Sage matrices + (3) Our custom matrices + (4) Cartesian products of the (1) through (3) + +All other Euclidean Jordan algebras could of course be implemented in +the same way as the octonion algebra, but for the sake of the user +interface, we must also support at least the usual SageMath vectors +and matrices. Basis normalization -------------------