Landau Subgroup Criterion

The Landau Subgroup Criterion (LSC) is one of the necessary conditions for a given PT to be a continuous, namely: The group H₁ of the low-symmetry phase should be a subgroup of the high-symmetry phase - $H_1 \subset G_0$. This means that all possible low-symmetry phases may be classified using the lattice of subgroups of G₀. This criterion can be formulated in the term of colour groups as follows: Each PT between $\rho_0$-phase and $\rho_1$-phase, where H₁ is a subgroup of G₀ of finite index n, corresponds to one and only one n-colour group G₀/H₁/H(A,A')_n, isomorphic to G₀.