A question about the derivation of Fermion Quantization in QFT

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SUMMARY

The discussion centers on the derivation of Fermion Quantization in Quantum Field Theory (QFT), specifically addressing the contraction of the index ##c## as a representation of matrix multiplication. The participants clarify that the element of the product of two matrices follows the formula $$(AB)_{mn} = \sum_{k}A_{mk}B_{kn}$$, adhering to Einstein summation convention for indices appearing twice. Additionally, they note that lifting and lowering indices is straightforward due to the trivial nature of the metric involved.

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Homework Statement
I am studying the part of Fermion Quantization in QFT. However, I am puzzled about the derivation in one class in youtube.
https://www.youtube.com/watch?v=FNxespJgDDE&list=PLbMVogVj5nJQ3slQodXQ5cSEtcp4HbNFc&t=826s
At 13:46, why does (sigma^3)_a^c (sigma^1)_{cb}=(sigma^3 sigma^1)_{ab}?
Relevant Equations
sigma^0 is the unit. sigma^k is the pauli matrices (k=1,2,3). 'a','b' and 'c' denotes the indexes of matrices elements.
fermionqm1.png
fermionq0.png
fermionq1.png
fermionq2.jpg
 

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Because contraction of the ##c## index is just an index form of noting matrix multiplication. What he wrote is just matrix multiplication law for those matrices. For example, the element of product of two matrices is:
$$(AB)_{mn} = \sum_{k}A_{mk}B_{kn}$$
In the notation on the board, the sum is implied over indices that appear twice(that's Einstein summation convention), and also lifting and lowering indices is probably trivial since metric is trivial.
 

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