What is the dot product of tensors?

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Discussion Overview

The discussion centers around the concept of the dot product of rank 2 tensors, specifically how to calculate the dot product of two 3x3 tensors, A and B. Participants explore interpretations and definitions related to this operation, including its application in various contexts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about the calculation of the dot product of two rank 2 tensors, A and B, both represented as 3x3 matrices.
  • Another participant expresses unfamiliarity with the term "dot product" in the context of tensors and requests further details or references.
  • A participant references a document that suggests the dot product of two rank-2 tensors U and V in 3-space can be expressed as UikVkj.
  • A different participant questions the terminology of "dot product," suggesting that it resembles matrix multiplication and proposing that the operation should be referred to as a "contraction" of indices from a rank 4 tensor derived from the dyadic product of U and V.
  • Another participant mentions the existence of a single dot tensor product and a double dot scalar product of tensors, indicating its relevance in computational fluid dynamics.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the terminology or the definition of the dot product for tensors. Multiple interpretations and definitions are presented, indicating ongoing debate and uncertainty regarding the concept.

Contextual Notes

Some participants highlight the ambiguity in the use of the term "dot product" for tensors and suggest that the operation may be more accurately described as a contraction. The discussion reflects varying levels of familiarity with tensor operations and their applications.

Who May Find This Useful

This discussion may be of interest to individuals studying tensor calculus, computational fluid dynamics, or those seeking clarification on tensor operations in mathematical physics.

sugarmolecule
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Hello,

I was trying to follow a proof that uses the dot
product of two rank 2 tensors, as in A dot B.

How is this dot product calculated?

A is 3x3, Aij, and B is 3x3, Bij, each a rank 2 tensor.

Any help is greatly appreciated.

Thanks!

sugarmolecule
 
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I've never heard of a dot product of tensors. Can you give us more details? Tip: If this is from a book, check if it's available at books.google.com. You might even be able to show us the specific page where you found this.
 
nevermind i was thinking of something else.
 
Last edited:
sugarmolecule said:
Hi,

I found this reference online that lists a potential intepretation:

www.math.mtu.edu/~feigl/courses/CFD-script/tensor-review.pdf

It lists the dot product of two rank-2 tensors U, V in 3-space as:

UikVkj

Does that look right?

Thanks,

sugarmolecule
I suspected that. I didn't know that anyone uses term "dot product" about rank 2 tensors, but if they do, it's logical that they mean precisely that. I don't see a reason to call it a dot product though. To me, that's just the definition of matrix multiplication, and if we insist on thinking of U and V as tensors, then the operation would usually be described as a ''contraction" of two indices of the rank 4 tensor that you get when you take what your text calls the "dyadic product" of U and V.
 

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