Let μ μ p p be atomic (i.e. the set A ⊂ ⊂ X in A is an atom); specifying the measure of any discrete variable x ∈ A equal to 1. The amplitudes are composed of state vector |Ψ⟩ indexed by A ; its components are denoted by ψ (x) for uniformity with the previous case. If the ℓ -norm of |Ψ⟩ is equal to 1, then | ψ (x) | is a probability mass function.