where the diagonal matrix ημν has diagonal elements 1,−1,−1,−1, and I4 is the unit 4× 4 matrix. The Einstein summation convention is used for repeated indices in equations, with the sense that ab= a μbμ = a0b0 − a ·b and a μ = aμ =(a0,a). A particular representation of the γ-matrices was given by Dirac [1],4,5 γ0 = I2 0 0 −I2 ⎠,γi = 0 σ i
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