In matrix equations, the Pauli matrices are denoted by σ in which μ = 0, 1, 2, 3, where σ is the 2 × 2 identity matrix : σ σ 0 = ( 1 0 0 1 ) and the other matrices have their usual representations. The expression σ σ μ μ ∂ ∂ μ μ ≡ ≡ σ σ 0 ∂ ∂ 0 + σ σ 1 ∂ ∂ 1 + σ σ 2 ∂ ∂ 2 + σ σ 3 ∂ ∂ 3 is a 2 × 2 matrix operator which acts on 2-component spinor fields.