The rotation operator R ⁡ ⁡ ( z , θ θ ) , with the first argument z indicating the rotation axis and the second θ θ the rotation angle, can operate through the translation operator T ⁡ ⁡ ( a ) for infinitesimal rotations as explained below. This is why, it is first shown how the translation operator is acting on a particle at position x (the particle is then in the state | x ⟩ ⟩ according to Quantum Mechanics).