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I'm just doing the Problems of Chapter 1 of Sakurai: Modern Quantum Mechanics. On page 60, problem 2 he writes:

"Suppose a 2x2 matrix X, (not necessary Hermitian, nor unitary) is written as

[itex] X = a_0 + \mathbf{\sigma \cdot a} [/itex],

where [itex] a_0 [/itex] and [itex] a_{1,2,3}[/itex] are numbers."

which confuses me a bit, because you can't add a number and a matrix, and what is [itex] \sigma [/itex] anyway? Would be nice if someone knows what is meant by this (and tells me ).

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# Sakurai Problems - strange notation

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