Matrix Multiplication of \delta_{ij}v_j = v_i

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SUMMARY

The discussion centers on the matrix multiplication expression \(\delta_{ij}v_j = v_i\), where \(\delta_{ij}\) represents the Kronecker delta, functioning as an identity matrix in this context. The confusion arises from the interpretation of \(v_j\) as a row vector instead of a column vector, which is essential for proper multiplication with the 3x3 matrix represented by \(\delta_{ij}\). The conclusion is that \(\delta_{ij}v_j\) effectively sums over all \(j\) for a specific \(i\), yielding the column vector \(v_i\), thereby confirming that \(v_i\) equals the \(i\)-th component of the vector \(v\).

PREREQUISITES
  • Understanding of matrix multiplication
  • Familiarity with the Kronecker delta notation
  • Knowledge of vector types (row vs. column vectors)
  • Basic linear algebra concepts
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  • Study the properties of the Kronecker delta in linear algebra
  • Learn about matrix-vector multiplication techniques
  • Explore the implications of identity matrices in transformations
  • Investigate the differences between row and column vectors in matrix operations
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Students of linear algebra, mathematicians, and anyone involved in understanding matrix operations and vector manipulations.

virgileso
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Homework Statement


Show by matrix multiplication, \delta_{ij}v_j = v_i

The Attempt at a Solution


I'm having trouble understanding how to do this, because I'm under the impression that v_j is a row vector, which can't be multiplied by a 3x3 matrix which \delta_{ij} is; or am I horribly wrong here?
 
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I believe the Kronecker delta is just the identity matrix... if it's a 3x3 matrix, then v_j is 3x1 (3 rows, 1 column)
 
Row vectors (n,1) can be multiplied by matrices (n,n) only if they stay at the right of the matrix, which is the case here.
 
Then shouldn't the unity matrix give another row vector as an answer? I'm trying to understand how v_i = v_j
 
But the unit matrix 0 nondiagonal elements, so that v_{i}=v_{j} only for i=j.
 
Aren't we talking about column vectors here... 3x1 is a column vector... and the result of the multiplication gives the same column vector back...

\delta_{ij}v_j denotes the sum over all j... for a particular i... ie: it is analogous to the multiplying the ith row of the matrix by the column vector v... and the result is v_i
 
Last edited:

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