In quantum mechanics, dynamical variables A such as position, translational (linear) momentum, orbital angular momentum, spin, and total angular momentum are each associated with a self-adjoint operator A ^ ^ that acts on the state of the quantum system. The eigenvalues of operator A ^ ^ correspond to the possible values that the dynamical variable can be observed as having. For example, suppose | ψ ψ a ⟩ ⟩ is an eigenket (eigenvector) of the observable A ^ ^ , with eigenvalue a , and exists in a Hilbert space. Then A ^ ^ | ψ ψ a ⟩ ⟩ = a | ψ ψ a ⟩ ⟩ .