“…As in the Ginzburg-Landau case, topological obstructions may imply the lack of an extension operator W 1−1/k,k (∂Ω, N ) → W 1,k (Ω, N ) (see for instance [10]). As a consequence, minimisers u ε subject to a Dirichlet boundary condition u ε = u bd ∈ W 1−1/k,k (∂Ω, N ) may not satisfy uniform energy bounds with respect to ε. Compactness results in the spirit of the Ginzburg-Landau theory have been shown for minimisers of the Landau-de Gennes functional [49,22,35,23]. However, some points that are understood in the Ginzburg-Landau theory -for instance, a variational characterisation of the singular set of the limit or a description of the problem in terms of Γ-convergence, as in [46,3,4] -are still missing, even for the Landau-de Gennes functional.…”