In the phase space formulation of quantum mechanics, the Robertson–Schrödinger relation follows from a positivity condition on a real star-square function. Given a Wigner function W ( x , p ) with star product ★ and a function f, the following is generally true: ⟨ ⟨ f ∗ ∗ ⋆ ⋆ f ⟩ ⟩ = ∫ ∫ ( f ∗ ∗ ⋆ ⋆ f ) W ( x , p ) d x d p ≥ ≥ 0 .