# Thread: Product Basis?

1. ## Product Basis?

I have this theorem:

Theorem 33.2: Let $\displaystyle \{e_i \}$ and $\displaystyle \{ e^i \}$ be dual bases for $\displaystyle V$ and $\displaystyle V^*$. Then the set of tensor products:

$\displaystyle \{ e_{i_1} \otimes \ldots \otimes e_{i_p} \otimes e^{j_1} \otimes \ldots \otimes e^{j_q}, \ i_1, \ldots , i_p,j_1, \ldots , j_q=1, \ldots , N \}$

forms a basis for $\displaystyle \vartheta_q^p(V)$ (the set of all tensors of order $\displaystyle (p,q)$ on a vector space $\displaystyle V$).
The proof starts by proving that the set is linearly independent. It does this as follows:

"We shall prove that the set of tensor products is a linearly independent generating set for $\displaystyle \vartheta_q^p(V)$. To prove that the set is linearly independent, let

$\displaystyle A^{i_1, \ldots, i_p}_{j_1, \ldots , j_q} e_{i_1} \otimes \ldots \otimes e_{i_p} \otimes e^{j_1} \otimes \ldots \otimes e^{j_q}=0$

where the RHS is the 0 tensor".

Firstly, what is $\displaystyle A^{i_1, \ldots, i_p}_{j_1, \ldots , j_q} e_{i_1}$? Is it a linear map going from $\displaystyle V$ to $\displaystyle V^*$ with the bases outlined in the question?

Secondly, how will this prove that the set is linearly independent?

2. Originally Posted by Showcase_22
I have this theorem:

The proof starts by proving that the set is linearly independent. It does this as follows:

"We shall prove that the set of tensor products is a linearly independent generating set for $\displaystyle \vartheta_q^p(V)$. To prove that the set is linearly independent, let

$\displaystyle A^{i_1, \ldots, i_p}_{j_1, \ldots , j_q} e_{i_1} \otimes \ldots \otimes e_{i_p} \otimes e^{j_1} \otimes \ldots \otimes e^{j_q}=0$

where the RHS is the 0 tensor".

Firstly, what is $\displaystyle A^{i_1, \ldots, i_p}_{j_1, \ldots , j_q} e_{i_1}$? Is it a linear map going from $\displaystyle V$ to $\displaystyle V^*$ with the bases outlined in the question?

Secondly, how will this prove that the set is linearly independent?
$\displaystyle A^{i_1, \ldots, i_p}_{j_1, \ldots , j_q}$ is a scalar, for each set of indices $\displaystyle \{i_1, \ldots, i_p,j_1, \ldots , j_q\}$. You should read this expression as $\displaystyle \sum A^{i_1, \ldots, i_p}_{j_1, \ldots , j_q}(e_{i_1} \otimes \ldots \otimes e_{i_p} \otimes e^{j_1} \otimes \ldots \otimes e^{j_q})$. The summation convention is being used, which means that you are meant to sum (from 1 to N) over each index that appears both as a superscript and as a subscript. So that expression simply denotes a linear combination of the given tensors. To prove that they are linearly independent, you put a linear combination of them equal to 0, and then you are aiming to show that each coefficient $\displaystyle A^{i_1, \ldots, i_p}_{j_1, \ldots , j_q}$ is zero.