How to Decompose a Tensor into Lambda, F, and V Components?

In summary: The purpose of this exercise, as far as I can tell, is for you to separate a general tensor T into parts that transform under different representations of the rotation group.1. The trace is a scalar, and is invariant under rotation: a spin-0 object.2. The antisymmetric part has 3 independent components, and transforms like a vector under rotations: a spin-1 object.3. The symmetric, traceless part has 5 independent components, and transforms like a tensor under rotations: a spin-2 object.You are trying to solve for the symmetric traceless part of a tensor, F.
  • #1
latentcorpse
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0
Show that a tensor T can be written as

[tex]T_{ij}=\lambda \delta_{ij} + F_{ij} +\epsilon_{ijk} v_{k}[/tex]

for the tensor
[itex]\[ \left( \begin{array}{ccc}
1 & 2 & 3 \\
4 & 5 & 6 \\
7 & 8 & 9 \end{array} \right)\] [/itex]

find [itex] \lambda, F_{ij}, v_k[/itex]

i can't get anywhere whatsoever with this question?
 
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  • #2


If i and j are the rows/columns, then what is k? Is there something missing in the problem description?
 
  • #3


i'm not sure about the problem itself, as I'm not sure of the context or meaning of the symbols (what are vk's & F?), but k is a contracted sum, so i think its ok
 
  • #4


hi. it's straight out a past exam q so should be ok.

not sure if this is what you mean but i noted that because of the levi civita,

if k is 3 then you can have either i=1,j=2 or j=1,i=2

i.e. we get [itex]v_3-v_3[/itex] but wouldn't that just make this last term trivial as all the terms would just cancel out similarly for k=1 and k=2?
 
  • #5


I think the tensor F should be symmetric. Basically what the question is asking is to separate the tensor into three components:

1. Its trace.

2. A symmetric, traceless part.

3. An antisymmetric part.
 
  • #6


yes but [itex]\lambda \delta_{ij}[/itex] puts a [itex]\lambda[/itex] on each diagonal element so in order to get 1,5 and 9 on the diagonals, at least one of the other two terms must have non zero diagonal components. clearly the third (antisymmetric) one cannot. so its down to the [itex]F_{ij}[/itex] and it must be symmetric and traceless.

incidentally - this is the only term i don't understand the origin of - why must it be symmetric and traceless?

then how do i solve for F when all i know is that

[itex]F_{11}=1-\lambda.F_{22}=5-\lambda,F_{33}=9-\lambda,F_{11}+F_{22}+F_{33}=0[/itex]
this gives [itex]\lambda=5[/itex]
so then [itex]F_{11}=-4.F_{22}=0,F_{33}=4[/itex]

how do i find the rest of F using v?
 
  • #7


Try first writing

[tex]\epsilon_{ijk}v_k = A_{ij}[/tex]

where A is an antisymmetric tensor. Now relate the components of A, F, and T. Also use the symmetry of F and the antisymmetry of A. You should get enough equations to obtain the rest of the elements.
 
  • #8


latentcorpse said:
incidentally - this is the only term i don't understand the origin of - why must it be symmetric and traceless?

The purpose of this exercise, as far as I can tell, is for you to separate a general tensor T into parts that transform under different representations of the rotation group.

1. The trace is a scalar, and is invariant under rotation: a spin-0 object.

2. The antisymmetric part has 3 independent components, and transforms like a vector under rotations: a spin-1 object.

3. The symmetric, traceless part has 5 independent components, and transforms like a tensor under rotations: a spin-2 object.
 
  • #9


hang on,

first of all how did you know to write [itex]\epsilon_{ijk}v_k=A_{ij}[/itex]

secondly i get equaitons like

[itex]A_{12} + F_{12}=2, A_{21}+F_{21}=4 \Rightarrow F_{12} + F_{21} = 6[/itex]

but none of the other equaitons refer to either the 12 or 21 components so I am struggling to get an exact value here
 
  • #10
Hi latentcorpse! :smile:

Hint: what are Tij + Tji,

and Tij - Tji ? :wink:
 
  • #11


do you want me to apply that to F or A?

for the symmetric F the add one will give twice the original and the subtract one will give 0. vice versa for the antisymmetric tensor A.

how does that help us though?
 
  • #12
latentcorpse said:
do you want me to apply that to F or A?

No … to T :rolleyes:
 
  • #13


[itex]T_{ij}+T_{ji}=2\lambda \delta_{ij} + 2 F_{ij}, T_{ij}-T_{ji}=2 \epsilon_{ijk} v_k[/itex]

can you explain why [itex]\epsilon_{ijk} v_k = A_{ij}[/itex] i can see that it must be anti symmetric because of the levi civita but how does the k fit into it all?

and why is F symmetric and traceless - how do we know that?
 
  • #14
latentcorpse said:
[itex]T_{ij}+T_{ji}=2\lambda \delta_{ij} + 2 F_{ij}, T_{ij}-T_{ji}=2 \epsilon_{ijk} v_k[/itex]

Yes, half the sum (this is for anything, not just tensors) is the symmetric part, and half the difference is the anti-symmetric part. :smile:
can you explain why [itex]\epsilon_{ijk} v_k = A_{ij}[/itex] i can see that it must be anti symmetric because of the levi civita but how does the k fit into it all?

k is a dummy index …

just draw the matrix, and you'll see where the v coordinates fit in :wink:
and why is F symmetric and traceless - how do we know that?

We don't … we choose F and lambda so that trace(F) = 0, by subtracting trace(Tij + Tji) from Tij + Tji :smile:
 
  • #15


so the sums involving T_ij and T_ji give me the extra equations i was needing to find all the constants but I am still having trouble seeing what's going on:

i agree that k is a dummy index but i can't really see how it works here:
if i pick k=1 that restricts i and j to 1 or 2
so the matrix has a 1 in the (12) component and a -1 in the (21) component - is this along the right lines?

and i still don't follow how/why we choose F and lambda so that trace(F)=0 or why we'd even want trace(F) to be 0 in the first place?
 
  • #16


so i think it means
[itex]\epsilon_{ijk} v_{k} =
\[ \left( \begin{array}{ccc}\epsilon_{11k} v_{k} & \epsilon_{12k} v_{k} & \epsilon_{13} v_{k} \\\epsilon_{21k} v_{k} & .. & .. \\.. & .. & .. \end{array} \right)\] [/itex]

if i remember correctly about the levi civita
[itex]\epsilon_{ijk} = 0 [/itex], if [itex] i=j [/itex] or [itex]j=k [/itex] or [itex]k=i [/itex]
[itex]\epsilon_{ijk} = 1 [/itex], for even permutation of i,j,k
[itex]\epsilon_{ijk} = -1 [/itex], for odd permutation of i,j,k

which gives
[itex]\epsilon_{ijk} v_{k} =
\[ \left( \begin{array}{ccc} 0 & \epsilon_{123} v_{3} & \epsilon_{132} v_{2} \\\epsilon_{213} v_{3} & .. & .. \\.. & .. & .. \end{array} \right)\] [/itex]

which gives
[itex]\epsilon_{ijk} v_{k} =
\[ \left( \begin{array}{ccc} 0 & +v_{3} & -v_{2} \\ -v_{3} & .. & .. \\.. & .. & .. \end{array} \right)\] [/itex]

which is antisymmetric with zero trace
 
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  • #17
latentcorpse said:
if i pick k=1 that restricts i and j to 1 or 2
so the matrix has a 1 in the (12) component and a -1 in the (21) component - is this along the right lines?

No, you're missing the point (and it's not 1, it's 3 :wink:) …

the point of the εijk is to pick up bits of v and plonk them down into a matrix …

if you pick k=3 that restricts i and j to 1 or 2
so the matrix has v3 in the (12) component and -v3 in the (21) component …

see lanedance's :smile: last matrix!
 
  • #18


kl. can i ask a quick unrelated question while your at it:

let [itex]V=span(1,1)^T[/itex]

that is the set of all linear combinations, i.e. [itex]V=\{\lambda \vec{i} + \mu \vec{j} \}[/itex] if we're in [itex]\mathbb{R^2}[/itex]. so would it be correct to plot this in [itex]\mathbb{R^2}[/itex] as a series of infinite lines parallel to the line y=x. or is it just the line y=x?
 
  • #19
latentcorpse said:
that is the set of all linear combinations, i.e. [itex]V=\{\lambda \vec{i} + \mu \vec{j} \}[/itex] if we're in [itex]\mathbb{R^2}[/itex]. so would it be correct to plot this in [itex]\mathbb{R^2}[/itex] as a series of infinite lines parallel to the line y=x. or is it just the line y=x?

uhh? :confused: [itex]\{\lambda \vec{i} + \mu \vec{j} \}[/itex] is R2
 
  • #20


so what does the span of (1,1)^T look like then is it just the line y=x?
 
  • #21
latentcorpse said:
so what does the span of (1,1)^T look like then is it just the line y=x?

Yes, it's [itex]\{\lambda \vec{i} + \lambda \vec{j} \}[/itex] which is y = x.
 

FAQ: How to Decompose a Tensor into Lambda, F, and V Components?

What is the difference between a tensor, matrix, and vector?

A tensor is a mathematical object that represents a multidimensional array of numbers. It can be thought of as a generalization of vectors and matrices. A vector is a one-dimensional array of numbers, while a matrix is a two-dimensional array of numbers.

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Tensors, matrices, and vectors are used in a wide range of scientific fields, including physics, engineering, and computer science. They are used to represent and manipulate data in a concise and efficient way, making them essential tools for data analysis and modeling.

Can you provide an example of a real-world application of tensors, matrices, and vectors?

One example is in image processing, where tensors are used to represent images as multidimensional arrays of pixels. Matrices and vectors are then used to perform operations on these images, such as resizing, filtering, and compression.

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How can someone without a strong mathematical background learn about tensors, matrices, and vectors?

There are many online resources, books, and tutorials available for learning about these concepts. It is helpful to have a basic understanding of algebra and geometry before delving into tensors, matrices, and vectors. It may also be helpful to work through practice problems and examples to gain a better understanding.

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