Assembling these pieces, the full positive energy solution is conventionally written as ψ ψ ( + ) = u ( ϕ ϕ ) ( p → → ) e − − i p ⋅ ⋅ x = E + m 2 m [ ϕ ϕ σ σ → → ⋅ ⋅ p → → E + m ϕ ϕ ] e − − i p ⋅ ⋅ x The above introduces a normalization factor E + m 2 m , derived in the next section.