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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.
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