Hello!(adsbygoogle = window.adsbygoogle || []).push({});

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 ).

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Sakurai Problems - strange notation

**Physics Forums | Science Articles, Homework Help, Discussion**