A general notion of uniform ellipticity and the regularity of the stress field for elliptic equations in divergence form

For solutions of Div(DF(Du)) = f we show that the quasiconformality of z 7 -> DF(z) is the key property leading to the Sobolev regularity of the stress field DF(Du), in relation with the summability of f. This class of nonlinearities encodes in a general way the notion of uniform ellipticity and encompasses all known instances where the stress field is known to be Sobolev regular. We provide examples showing the optimality of this assumption and present two applications: a nonlinear Cordes condition for equations in divergence form and some partial results on the Cp ' conjecture.