# A question about tensor product

1. Oct 8, 2015

### Harel

Hey it might be a stupid question but I saw that the tensor product of 2 vectors with dim m and n gives another vector with dimension mn and in another context I saw that the tensor product of vector gives a metrix. For example from sean carroll's book: "If T is a (k,l) tensor and S is a (m, n) tensor, we define a (k + m, l + n) tensor T ⊗ S"
so the tensor product of two type 1 tensors,k=1,vectors, is a metrix
and in the context of quantum mechanic I saw
(1,0)⊗(1,0)↦(1,0,0,0) when those our basis vectors.
I'm sure I'm just getting something wrong but I am hopefull that you can explain me what.

2. Oct 8, 2015

### Daeho Ro

Why it would be? If one of them is dual vector, then it might be. I will give you some examples:
$$\begin{pmatrix} 1 \\ 2 \end{pmatrix} \otimes \begin{pmatrix} 1 & 2 \end{pmatrix} = \begin{pmatrix} 1 \times \begin{pmatrix} 1 & 2 \end{pmatrix}\\ 2 \times \begin{pmatrix} 1 & 2 \end{pmatrix} \end{pmatrix} = \begin{pmatrix} 1 & 2 \\ 2 & 4 \end{pmatrix},$$
$$\begin{pmatrix} 1 & 2 \end{pmatrix} \otimes \begin{pmatrix} 1 & 2 \end{pmatrix} = \begin{pmatrix} 1 \times \begin{pmatrix} 1 & 2 \end{pmatrix} & 2 \times \begin{pmatrix} 1 & 2 \end{pmatrix} \end{pmatrix} = \begin{pmatrix} 1 & 2 & 2 & 4 \end{pmatrix}$$

3. Oct 8, 2015

### Harel

Because by the defenition of sean, let's take a type (1,0) tensor which is a vector and another (1,0) tensor which is also a vector and the product will be a (2,0) tensor, which is a metrix.

Last edited: Oct 8, 2015
4. Oct 8, 2015

### WWGD

A vector can be seen as a $1 \times n$ matrix.

5. Oct 8, 2015

### Daeho Ro

That is true. But, the results of mine are matrices with the size $2 \times 2$ and $1 \times 4$.

6. Oct 8, 2015

### WWGD

Actually, if your map is k-linear ( in any " coordinate") for k>2 (where you may have quadratic forms), it is not representable as a matrix anymore. That is the actual point of tensors: to represent k - , or j- ( k,j pos. integers) linear maps in many variables, which is not feasible with matrices alone whenever you have an index >2.
Only linear and bilinear maps may be represented using matrices.

7. Oct 8, 2015

### HallsofIvy

Staff Emeritus
I feel compelled to point out that a tensor can be represented by a matrix in a given coordinate system, but, strictly speaking, a tensor is NOT a matrix.

8. Oct 8, 2015

### WWGD

How do you represent a higher-order tensor as a matrix? e.g., a 3-linear map .

9. Oct 15, 2015

### HallsofIvy

Staff Emeritus
In the same sense that a "vector" is a 1 by 3 matrix, so a higher order tensor can be represented by a "3 by 3 by 3" or higher matrix. I admit that is stretching the concept of "matrix" a bit far. My point was simply that a matrix is NOT a tensor.