Can Tensor Products Define (M,N) Tensors?

In summary, an arbitrary (0,N) tensor can be expressed in terms of its components and the tensor products of N basis one-forms. Similarly, an arbitrary (M,0) tensor can be expressed in terms of its components and tensor products of its M basis vectors. However, an (M,N) tensor cannot be expressed in terms of its components and the M basis vectors and N basis one-forms because the tensor product is not commutative.
  • #1
guitarphysics
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From my understanding, an arbitrary (0,N) tensor can be expressed in terms of its components and the tensor products of N basis one-forms. Similarly, an arbitrary (M,0) tensor can be expressed in terms of its components and tensor products of its M basis vectors. What about an (M,N) tensor, though? I'm reading from Schutz right now and he doesn't mention anything like that, but it seems pretty logical to me that you should be able to have a tensor product between basis vectors and basis one-forms. If you did that then every (M,N) tensor could be expressed just in terms of its components and the M basis vectors and N basis one-forms. The only thing you'd need to keep track of is the order in which you 'supply' the (M,N) tensor with vectors and one-forms (since obviously you can't support a basis one-form with a one-form, and likewise for basis vectors), but you have to do that anyway for (0,N) and (M,0) tensors as well since the tensor product isn't commutative.

Is this type of product defined? Is it the same as the one between one-forms and one-forms, and between vectors and vectors?

Thanks!
 
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  • #2
Certainly:

$$T=\sum_{ijk...lmn...}T_{ijk...}^{lmn...}\tilde{q}^i\otimes\tilde{q}^j\otimes\tilde{q}^k\otimes...\otimes \vec{e}_l\otimes\vec{e}_m\otimes\vec{e}_n\otimes...$$
 
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  • #3
Ah cool thank you! It seemed reasonable but I just wanted to make sure because I'd never seen it stated explicitly.
 
  • #4
I see that the question has been answered, but I think an example of what you can do may be useful:
\begin{align}
&T(\omega,u,v)=T(\omega_i e^i,u^j e_j,v^k e_k)=\omega_i u^j v^k T(e^i,e_j,e_k) = e_i(\omega)e^j(u)e^k(v) T^i{}_{jk} = (T^i{}_{jk} e_i\otimes e^j\otimes e^k)(\omega,u,v)\\
&T=T^i{}_{jk} e_i\otimes e^j\otimes e^k
\end{align}
 
  • #5
Thanks for the example :)
 

1. What is a tensor product?

A tensor product is a mathematical operation that combines two tensors to create a new tensor. It is a way of multiplying tensors in a specific way, similar to how matrices are multiplied.

2. How are tensors defined using tensor products?

Tensors can be defined using tensor products by specifying the number of dimensions and the type of tensor (contravariant or covariant) for each factor tensor. The resulting tensor will have a total of (M+N) dimensions, with M contravariant and N covariant indices.

3. What is the significance of using tensor products to define tensors?

Tensor products provide a systematic and concise way of defining tensors. It allows for the creation of higher-dimensional tensors from lower-dimensional ones, making it a powerful tool in many mathematical and scientific applications.

4. Can tensor products define tensors of any rank?

Yes, tensor products can define tensors of any rank. The resulting tensor will have a total of (M+N) dimensions, where M and N represent the number of dimensions for each factor tensor.

5. Are there any limitations to defining tensors using tensor products?

While tensor products are a powerful tool for defining tensors, they can become computationally complex for higher-dimensional tensors. In addition, the resulting tensor may not always have desirable properties, such as symmetry or antisymmetry, depending on the properties of the factor tensors.

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