  The aim of this short note is to give an alternative proof, which applies to
functions of bounded variation in arbitrary domains, of an inequality by Maz'ya
that improves Friedrichs inequality. A remarkable feature of such a proof is
that it is rather elementary, if the basic background in the theory of
functions of bounded variation is assumed. Never the less, it allows to extend
all the previously known versions of this fundamental inequality to a
completely general version. In fact the inequality presented here is optimal in
several respects.
  As already observed in previous proofs, the crucial step is to provide
conditions under which a function of bounded variation on a bounded open set,
extended to zero outside, has bounded variation on the whole space. We push
such conditions to their limits. In fact, we give a sufficient and necessary
condition if the open set has a boundary with $\sigma$-finite surface measure
and a sufficient condition if the open set is fully arbitrary. Via a
counterexample we show that such a general sufficient condition is sharp.

Analysis of PDEs