Limiting Sobolev inequalities for vector fields and canceling linear   differential operators
  The estimate [\lVert D^{k-1}u\rVert_{L^{n/(n-1)}} \le \lVert A(D)u \rVert_{L^1} ] is shown to hold if and only if (A(D)) is elliptic and canceling. Here (A(D)) is a homogeneous linear differential operator (A(D)) of order (k) on (\mathbb{R}^n) from a vector space (V) to a vector space (E). The operator (A(D)) is defined to be canceling if [\bigcap_{\xi \in \mathbb{R}^n \setminus {0}} A(\xi)[V]={0}.] This result implies in particular the classical Gagliardo-Nirenberg-Sobolev inequality, the Korn-Sobolev inequality and Hodge-Sobolev estimates for differential forms due to J. Bourgain and H. Brezis. In the proof, the class of cocanceling homogeneous linear differential operator (L(D)) of order (k) on (\mathbb{R}^n) from a vector space (E) to a vector space (F) is introduced. It is proved that (L(D)) is cocanceling if and only if for every (f \in L^1(\mathbb{R}^n; E)) such that (L(D)f=0), one has (f \in \dot{W}^{-1, n/(n-1)}(\mathbb{R}^n; E)). The results extend to fractional and Lorentz spaces and can be strengthened using some tools of J. Bourgain and H. Brezis. 
Jean Van Schaftingen
math.AP