In contrast the term e − − i k x 2 2 z can be eliminated from the equation, since when bracketed it gives 1. ⟨ e − − i k x 2 2 z | e − − i k x 2 2 z ⟩ = e − − i k x 2 2 z ( e − − i k x 2 2 z ) ∗ ∗ = e − − i k x 2 2 z e + i k x 2 2 z = e 0 = 1