# Mixed product identity between tensors in Mathematica 9

How can we simplify tensor expressions in Mathematica 9 using the mixed-product identity $(A\otimes B)(C \otimes D) \equiv AC \otimes BD$ ?

Is it possible to implement this kind of evaluations using the new Mathematica 9 tensor capabilities?

The following expression is false:

TensorProduct[a, b].TensorProduct[c, d] === TensorProduct[ a.c, b.d]


I don't really need to prove this identity, but rather to use it for simplifying some expressions. In my case, $a$ and $c$ are some (unknown) symbolic matrices, while $b$ and $d$ are explicit integer $2\times 2$ matrices. I'd like Mathematica to evaluate the matrix product between $b$ and $d$ explicitly.

For instance, when

b=PauliMatrix; d=PauliMatrix;


as the product of two Pauli matrices gives another Pauli matrix, I'd like to obtain the simplified result

TensorProduct[a.c, -I PauliMatrix]


## 2 Answers

You could also do

$Assumptions = (a | b | c | d) \[Element] Matrices[{k, k}] KroneckerProduct[a, b] . KroneckerProduct[c, d] // TensorExpand  which returns KroneckerProduct[a.c, b.d]  • That is exactly what I was looking for. Thanks – benkj Feb 3 '14 at 16:13 1. The mixed-product identity $$(A\otimes B)(C \otimes D) \equiv AC \otimes BD$$ is written in terms of the Kronecker product (see here) not TensorProduct. They have different dimensions A = RandomReal[1.0, {10, 10}]; KroneckerProduct[A, A] // Dimensions TensorProduct[A, A] // Dimensions  {100, 100} {10, 10, 10, 10}  2. lhs === rhs gives exact comparison of lhs and rhs. You need to use ==. 3. Unfortunately tensors operations with KroneckerProduct and Dot is not fully implemented in Mathematica 9. However, for example, Dot[a,b] is equivalent to TensorContract[a\[TensorProduct]b, {{2, 3}}]. Finally, you can rewrite your identity as $Assumptions = (a | b | c | d) \[Element] Matrices[{k, k}];

TensorTranspose[
TensorContract[a\[TensorProduct]b\[TensorProduct]c\[TensorProduct]d, {{2, 5}, {4, 7}}],
{1, 3, 2, 4}] ==
TensorContract[a\[TensorProduct]c, {{2, 3}}]\[TensorProduct]TensorContract[
b\[TensorProduct]d, {{2, 3}}] // TensorReduce


True

Unfortunately, I can't explain derivation of this formula because I don't understand it completely.

• Thanks, your answer makes me understand a little bit better the new Tensor capabilities of Mathematica 9. However, I'm not able to use your commands for simplifying expression. This was indeed, my main problem. I edited the question to better clarify this point. – benkj Oct 16 '13 at 7:44
• @benkj I think it is much simpler to implement it with pattern replacements or do it manually. – ybeltukov Oct 16 '13 at 9:55