Can Tensor Products Define (M,N) Tensors?

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Discussion Overview

The discussion revolves around the definition and representation of (M,N) tensors using tensor products of basis vectors and basis one-forms. Participants explore whether such a product is defined and how it relates to the representation of (0,N) and (M,0) tensors.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant proposes that an (M,N) tensor can be expressed in terms of its components and tensor products of M basis vectors and N basis one-forms, suggesting that this is a logical extension of the definitions for (0,N) and (M,0) tensors.
  • Another participant provides a mathematical representation of an (M,N) tensor using a summation of components and tensor products of basis vectors and one-forms.
  • A later reply expresses appreciation for the clarification and seeks confirmation of the reasonableness of the initial proposal.
  • An example is presented to illustrate how an (M,N) tensor can be applied to vectors and one-forms, demonstrating the relationship between the components and the tensor product structure.

Areas of Agreement / Disagreement

Participants generally agree that the concept of expressing (M,N) tensors in terms of tensor products is valid, but there is no explicit consensus on the formal definition or implications of this product.

Contextual Notes

The discussion does not resolve the formal definition of the tensor product between basis vectors and one-forms, nor does it clarify any assumptions regarding the commutativity of tensor products in this context.

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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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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Ah cool thank you! It seemed reasonable but I just wanted to make sure because I'd never seen it stated explicitly.
 
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}
 
Thanks for the example :)
 

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