What is the purpose of contraction in tensor algebra?

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    Contraction Tensor
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Discussion Overview

The discussion revolves around the concept of contraction in tensor algebra, specifically how it applies to tensor fields and vector fields. Participants explore the mechanics of contraction, its definitions, and its implications in various contexts, including specific examples like the dot product of vectors.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant asks for an explanation of how applying contractions to a tensor field and vector fields results in a specific scalar output.
  • Another participant suggests that the definition of contraction is essentially what is being discussed.
  • A request for more detailed explanation of contraction is made, with a reference to an external definition.
  • One participant describes the process of contracting indices in a tensor product to yield a scalar, linking it to the definition of a 1-form applied to a vector field.
  • A participant notes that the dot product of two vectors serves as a common example of contraction, reflecting a general understanding of the concept.

Areas of Agreement / Disagreement

Participants appear to have varying levels of understanding and clarity regarding the definition and application of contraction, with no consensus reached on the explanations provided.

Contextual Notes

Some participants reference definitions and examples that may depend on specific interpretations of tensor algebra, and the discussion includes varying degrees of detail and clarity regarding the mechanics of contraction.

joe2317
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Let Y_{1},..,Y_{k} be vector fields and let A be a tensor field of type ^{k}_{1}. Could you explain how applying k contractions to A\otimesY_{1}\otimes...Y_{k} yields A(Y_{1}...Y_{k})?

Actually, could you first explain why contraction of w\otimesY is equal to w(Y)?
Here, w is a 1-form and Y is a vector field.
Thank you.
 
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Isn't that pretty much the definition of "contraction"?
 
w \otimes Y has incides w_{i} \otimes Y^{j}. Contract the indices to make w \otimes Y into a scalar gives w_{i} \otimes Y^{i}. This is the definition of w(Y).

Similarly for everything else.
 
a special case is the dot product of two vectors, this is how everyone really things about contraction anyway
 

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