where ħ = ⁠ h / 2 π ⁠, and σ x, σ p are the standard deviations of position and momentum. (Heisenberg only proved relation (A2) for the special case of Gaussian states.) In 1929 Robertson generalized the inequality to all observables and in 1930 Schrödinger extended the form to allow non-zero covariance of the operators; this result is referred to as Robertson-Schrödinger inequality.