Linear transformations as tensor.

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Discussion Overview

The discussion revolves around the nature of (1,1) tensors and their relationship to linear transformations. Participants explore the definitions and properties of bilinear forms, linear maps, and tensors, particularly in the context of finite-dimensional vector spaces.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why a (1,1) tensor is considered a linear transformation, noting that a (1,1) tensor takes a vector and a one-form to a scalar, while a linear transformation takes a vector to a vector.
  • Another participant suggests that a bilinear form can take a vector to a vector or a covector to a covector, providing examples of how this might work.
  • A different participant asserts that a bilinear form is the tensor product of two one-forms or linear functionals, which would imply it takes two vectors to a scalar.
  • One participant elaborates on the function of a (1,1) tensor, explaining that if T(u,v) is a function where u is a vector and v is a co-vector, then the function f(u) = T(u,-) produces a scalar when a co-vector is plugged in, suggesting a connection to vectors.
  • Another participant discusses the equivalence of bilinear maps and linear maps, stating that bilinear maps from VxW to U correspond to linear maps from V to linear maps from W to U, and provides a series of equivalences involving tensor products and linear spaces.

Areas of Agreement / Disagreement

Participants express differing views on the nature of bilinear forms and their relationship to (1,1) tensors and linear transformations. There is no consensus on the definitions or implications of these concepts.

Contextual Notes

The discussion assumes finite-dimensional spaces and relies on specific definitions of tensors and linear maps, which may not be universally agreed upon. Some mathematical steps and definitions remain unresolved.

The1337gamer
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I was looking at this table here: http://en.wikipedia.org/wiki/Tensor#Examples

And i didn't understand why a (1,1) tensor is a linear transformation, I was wondering if someone could explain why this is.

A (1,1) tensor takes a vector and a one-form to a scalar.
But a linear transformation takes a vector to a vector.

Thanks.
 
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A bilinear form can take a vector to a vector or a covector to a covector:

Auvζv = γu
Auvωu = κv
 
I thought a bilinear form was the tensor product of 2 one-forms/linear functionals, so it would take two vectors to a scalar.
 
The1337gamer said:
I was looking at this table here: http://en.wikipedia.org/wiki/Tensor#Examples

And i didn't understand why a (1,1) tensor is a linear transformation, I was wondering if someone could explain why this is.

A (1,1) tensor takes a vector and a one-form to a scalar.
But a linear transformation takes a vector to a vector.

Thanks.

If your form is T(u,v) where u is a vector and v is a co-vector, consider the function f(u)=T(u,-). f(u) is a function which, when you plug in a co-vector you get a scalar. Those are exactly what vectors are (co-co vectors)
 
Assume all spaces are finite diml.

by plugging into one variable at a time, you see that bilinear maps from VxW to U is the same as linear maps from V to linear maps from W to U.

I.e. Bil(VxW,U) ≈ Lin(V,Lin(W,U)).

Write V* for linear maps from V to k, where k is the scalar field.
For finite diml spaces then Lin(Lin(V,k),k) = V** ≈ V.

essentially by definition of tensors, V*tensorW* ≈ (VtensorW)* ≈ Bil(VxW,k).

Thus V*tensorW ≈ Bil(VxW*,k) ≈ Lin(V,W**) ≈ Lin(V,W).

I.e. if ftensorw is an elementary (1,1) tensor in V*tensW,

then it maps the vector v in V to the vector

f(v).w in W. A general (1,1) tensor is a sum of such elementary ones and maps v to the corresponding sum of vectors in W.
 

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