A two-sided estimate for the minimal norm of the extension operator for Sobolev spaces

Let Omega be a domain in {bf R}^n satisfying a cone condition or partialOmegain {rm Lip},1. The authors estimate the minimal norm of extension operators W^l_p(Omega)to W^l_p({bf R}^n), lin{bf N}, 1leq pleq infty, especially in dependence on l.