Difference between Kronecker delta and identity matrix

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Discussion Overview

The discussion revolves around the differences between the Kronecker delta and the identity matrix, exploring their definitions, properties, and roles in mathematical contexts. Participants examine whether the distinctions are merely terminological or if they reflect deeper conceptual differences.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants note that the Kronecker delta returns a single number (1 or 0) based on two indices, while the identity matrix is an nxn matrix.
  • Others argue that the elements of an identity matrix can be expressed using the Kronecker delta, suggesting a relationship between the two.
  • One participant emphasizes that the Kronecker delta is a function that takes a pair of indices and returns a value, while the identity matrix is a matrix with elements.
  • Another participant asserts that the Kronecker delta is not a matrix and highlights that it serves a different purpose in mathematical operations compared to the identity matrix.
  • Some participants express that the differences are not merely terminological, pointing out that the identity matrix is a linear map, whereas the Kronecker delta is not.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the differences between the Kronecker delta and the identity matrix are significant or merely terminological. Multiple competing views remain regarding their definitions and roles.

Contextual Notes

Participants reference the notation used for both the Kronecker delta and the identity matrix, which may contribute to the confusion regarding their differences. The discussion highlights the need for clarity in definitions and the implications of viewing them as distinct entities.

loom91
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Hi,

As in the title, what's the difference between the Kronecker delta and the identity matrix? They seem to have the exact same definition, so why are they differentiated? Thanks.

Molu
 
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Well, the kronecker delta returns, given two indices, either 1 or 0, so just a number. An identity matrix of size nxn is of course a matrix, not just a number! However, the elements of an identity matrix, which can be expressed with two indices as well, can be written with the kronecker delta since [tex]\left( {I_n } \right)_{i,j} = \delta _{i,j}[/tex].
 
But the Kronecker delta considered as a whole is no more a single number than a matrix of numbers is a single number! The elements of an identity matrix are equivalent to the elements of the Kronecker delta, so why make the difference?
 
The Kronecker delta does not have elements. It is not a matrix. It is a function it takes as input the pair (i,j) and returns 1 if they are the same and zero otherwise.

The identity matrix is a matrix, the Kronecker delta is not. There is no simpler way to say it than that.
 
So the only difference is in terminology? They even have the same notation!
 
No, one is a linear map the other is not a linear map, that is not just a terminological difference: they are completely different things. They are just different things. You can use one of them to describe the coordinate functions on the other but that does not make them equal in any sense.

The kronecker delta is what you use when you want to work componentwise with matrices. It is not a matrix. You can make a matrix from it in the obvious way and that will be the identity. That does not make it the identity matrix. It is not the identity matrix.
 
Last edited:
No, not only terminilogy. What I was trying to point out - and matt as well I believe - is that there is a fundamental difference. The identity matrix is a matrix (and consists of elements, as matt said), the kronecker delta is not.
 

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