  For a nonlinear operator $T$ satisfying certain structural assumptions, our
main theorem states that the following claims are equivalent: i) $T$ is
surjective, ii) $T$ is open at zero, and iii) $T$ has a bounded right inverse.
The theorem applies to numerous scale-invariant PDEs in regularity regimes
where the equations are stable under weak$^*$ convergence. Two particular
examples we explore are the Jacobian equation and the equations of
incompressible fluid flow.
  For the Jacobian, it is a long standing open problem to decide whether it is
onto between the critical Sobolev space and the Hardy space. Towards a negative
answer, we show that, if the Jacobian is onto, then it suffices to rule out the
existence of surprisingly well-behaved solutions.
  For the incompressible Euler equations, we show that, for any $p<\infty$, the
set of initial data for which there are dissipative weak solutions in $L^p_t
L^2_x$ is meagre in the space of solenoidal $L^2$ fields. Similar results hold
for other equations of incompressible fluid dynamics.
