  In this paper we obtain an exact periodic solution for the system of
equations of state corresponding to the spiral phase of crystalline ordering.
We used method of integral rational basis of invariants to construct an
inhomogeneous Landau potential. As a result the inhomogeneous Landau potential
takes account of anisotropic invariants consisting of OP components as well as
anisotropic invariants that comprise space derivatives of OP components. It is
demonstrated that it is taking into account the latter that leads to the exact
periodic solution for the system of equations of state that describes the
spiral phase. A phase diagram of states is built for active Lifshitz
representations at the second-order phase transition point. Analogy between
inhomogeneous states of A-phase and second-order superconductors in the
magnetic field is discussed. It is suggested to describe the vortex state in
the magnetic field as a deformation of the magnetic sublattice with
dislocations.
