where α α → → = ( α α 1 , α α 2 , α α 3 ) are the off-diagonal Dirac matrices α α i = β β γ γ i , with β β = γ γ 0 and the remaining constants are c the speed of light, ℏ ℏ being the Planck constant, and m the mass of a fermion (for example, an electron). It acts on a four-component wave function ψ ψ ( x ) ∈ ∈ L 2 ( R 3 , C 4 ) , the Sobolev space of smooth, square-integrable functions. It can be extended to a self-adjoint operator on that domain. The square, in this case, is not the Laplacian, but instead D 2 = Δ Δ + m 2 (after setting ℏ ℏ = c = 1. )