# Strange question on matrices

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1. Oct 26, 2014

### Josh Conneely

1. The problem statement, all variables and given/known data
Suppose a 2x2 matrix X (not necessarily hermitian, nor unitary) is written as

X = a0 + sigma . a (the sigma . a is a dot product between sigma and a)

where a0 and a1, a2 and a3 are numbers.

How on earth does X represent a matrix? it's a number added to another number (dot product).

2. Relevant equations

3. The attempt at a solution

2. Oct 26, 2014

### Dick

I suspect this is a question about Pauli matrices. That notation is shorthand for X=a0*I+sigma1*a1+sigma2*a2+sigma3*a3 where I is the 2x2 identity matrices and sigma1, sigma2 and sigma3 are 2x2 matrices collectively referred to as sigma. The result X is a 2x2 matrix.