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Homework Help: Strange question on matrices

  1. Oct 26, 2014 #1
    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. jcsd
  3. Oct 26, 2014 #2


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    Science Advisor
    Homework Helper

    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.
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