The covariant formulation is especially useful for constructing the states with definite angular momentum. In this construction, the four-vector ω ω participates on equal footing with other four-momenta, and, therefore, the main part of this problem is reduced to the well known one. For example, as is well known, the wave function of a non-relativistic system, consisting of two spinless particles with the relative momentum k → → and with total angular momentum l , is proportional to the spherical function Y l m ( k → → ^ ^ ) : ψ ψ l m ( k → → ) = f ( k ) Y l m ( k ^ ^ ) , where k ^ ^ = k → → / k and f ( k ) is a function depending on the modulus k = | k → → | . The angular momentum operator reads: J → → = − − i [ k → → × × ∂ ∂ k → → ] . Then the wave function of a relativistic system in the covariant formulation of light-front dynamics obtains the similar form: