I have two matrices, $A$ and $B$. The elements of these matrices are some abstract non-commuting objects, which I'm just representing as variables. For now, I don't care about having Mathematica know about the specifics of these commutation relations - all I care about is that it doesn't assume that they commute, so that, for example, when I calculate something like $\text{Tr}(A B)$, the expression that it returns keeps the elements of $A$ in front of the elements of $B$. Of course for something as simple as $\text{Tr}(AB)$ I can just rearrange the elements by hand, but I would like to have the ability to do this for more complex expressions, with potentially more than two matrices. It seems Mathematica has some built-in functionality for non-commutative multiplication, but this is only for scalars - is there something that generalizes this to matrices of non-commuting objects?
2 Answers
As Dot is a special case of Inner with Dot[##] == Inner[Times,##, Plus], we can just tell Mathematica to use NonCommutativeMultiply in the first slot to take the place of regular multiplication.
ncdot[ten1_, ten2_] := Inner[NonCommutativeMultiply, ten1, ten2, Plus]
ncdot[{x, y}, {u, v}]
ncdot[{{x, y}, {z, w}}, {{a, b}, {c, d}}]
Tr@%
x ** u + y ** v
{{x ** a + y ** c, x ** b + y ** d}, {w ** c + z ** a, w ** d + z ** b}}
w ** d + x ** a + y ** c + z ** b
This contrasts the automatic canonical sorting MMA does under the hood of the individual symbols as in
{x, y}.{u, v}
(*and*)
{{x, y}, {z, w}}.{{a, b}, {c, d}} // Tr
u x + v y
d w + a x + c y + b z
To use this for several items (e.g. matrices $A,B,C$), we incorporate/redefine Fold into the definition as
ncdot[tens__] := Fold[Inner[NonCommutativeMultiply, ##] &, {tens}]
(*Without the inclusion of the fourth argument of Inner, the default is Plus*)
Which we can use as
ncdot @@ Table[i[j, k], {i, {a, b, c}}, {j, 2}, {k, 2}];
% /. (i : (a | b | c))[j_, k_] :> Subscript[i, j\[InvisibleComma]k] // MatrixForm
$\left( \begin{array}{cc} \left(a_{1,1}\text{**}b_{1,1}+a_{1,2}\text{**}b_{2,1}\right)\text{**}c_{1,1}+\left(a_{1,1}\text{**}b_{ 1,2}+a_{1,2}\text{**}b_{2,2}\right)\text{**}c_{2,1} & \left(a_{1,1}\text{**}b_{1,1}+a_{1,2}\text{**}b_{2,1}\right)\text{**}c_{1,2}+\left(a_{1,1}\text{**}b_{ 1,2}+a_{1,2}\text{**}b_{2,2}\right)\text{**}c_{2,2} \\ \left(a_{2,1}\text{**}b_{1,1}+a_{2,2}\text{**}b_{2,1}\right)\text{**}c_{1,1}+\left(a_{2,1}\text{**}b_{ 1,2}+a_{2,2}\text{**}b_{2,2}\right)\text{**}c_{2,1} & \left(a_{2,1}\text{**}b_{1,1}+a_{2,2}\text{**}b_{2,1}\right)\text{**}c_{1,2}+\left(a_{2,1}\text{**}b_{ 1,2}+a_{2,2}\text{**}b_{2,2}\right)\text{**}c_{2,2} \\ \end{array} \right)$
Maybe this?
Tr[{{a, b}, {c, d}}.{{x, y}, {z, w}}] /. Times -> NonCommutativeMultiply
Tr[{{a, b}, {c, d}}.{{x, y}, {z, w}} /. Times -> NonCommutativeMultiply]
a ** x + b ** z + c ** y + d ** w
