This article presents a group-theoretical investigation of an incommensurate magnetic structure in the case where the crystal structure is centrosymmetric while the structure vector k is not conserved by inversion. A first method starts with the conventional group-theoretical treatment taking into account the transformations which conserve k and then combines the solutions to obtain a centrosymmetric and real magnetization. A second method includes inversion in the group-theoretical treatment, for instance by introducing the complex conjugation operator and corepresentations. The general theory is applied to CeAl2 with the scope of improving the knowledge of its magnetic structure.