  For an elliptic, semilinear differential operator of the form $S(u) = A : D^2
u + b(x, u , Du)$, consider the functional $E_\infty(u) = \mathop{\mathrm{ess
\, sup}}_\Omega |S(u)|$. We study minimisers of $E_\infty$ for prescribed
boundary data. Because the functional is not differentiable, this problem does
not give rise to a conventional Euler-Lagrange equation. Under certain
conditions, we can nevertheless give a system of partial differential equations
that all minimisers must satisfy. Moreover, the condition is equivalent to a
weaker version of the variational problem.
