We study the one‐dimensional symmetry of solutions to the nonlinear Stokes equation
{left−Δu+∇W(u)=∇pin ℝd,left∇⋅u=0in ℝd,
which are periodic in the d − 1 last variables (living on the torus 𝕋d−1) and globally minimize the corresponding energy in Ω = ℝ × 𝕋d−1, i.e.,
E()u=∫Ω12∇u2+W()uitalicdx,1em∇⋅u=0.
Namely, we find a class of nonlinear potentials W ≥ 0 such that any global minimizer u of E connecting two zeros of W as x1 → ± ∞ is one‐dimensional; i.e., u depends only on the x1‐variable.
In particular, this class includes in dimension d = 2 the nonlinearities W=12w2 with w being a harmonic function or a solution to the wave equation, while in dimension d ≥ 3, this class contains a perturbation of the Ginzburg‐Landau potential as well as potentials W having d + 1 wells with prescribed transition cost between the wells. For that, we develop a theory of calibrations relying on the notion of entropy (coming from scalar conservation laws). We also study the problem of the existence of global minimizers of E for general potentials W providing in particular compactness results for uniformly finite energy maps u in Ω connecting two wells of W as x1 → ± ∞. © 2019 Wiley Periodicals, Inc.