Inf-sup condition for Stokes with outflow condition

The inf-sup condition is one of the essential tools in the analysis of the Stokes equations and especially in numerical analysis. In its usual form, the condition states that for every pressure $p\in L^2(\mathrm{\Omega })\setminus \mathbb{R}$, (i.e. with mean value zero) a velocity $u\in H_0^1(\mathrm{\Omega }{)}^d$ can be found, so that $(divu,p)=\mathrm{\parallel }p{\mathrm{\parallel }}^2$ and $\mathrm{\parallel }\mathrm{\nabla }u\mathrm{\parallel }\le c\mathrm{\parallel }p\mathrm{\parallel }$ applies, where $c>0$ does not depend on $u$ and $p$. However, if we consider domains that have a Neumann-type outflow condition on part of the boundary ${\mathrm{\Gamma }}_N\subset \mathrm{\partial }\mathrm{\Omega }$, the inf-sup condition cannot be used in this form, since the pressure here comes from $L^2(\mathrm{\Omega })$ and does not necessarily have zero mean value. In this note, we derive the inf-sup condition for the case of outflow boundaries.