Frobenius rigidity in A1-homotopy theory

We study the homotopy fixed points under the Frobenius endomorphism on the stable A1-homotopy category of schemes in characteristic p > 0 and prove a rigidity result for cellular objects in these categories after inverting p. As a consequence we determine the analogous fixed points on the K-theory of algebraically closed fields in positive characteristic. We also prove a rigidity result for the homotopy fixed points of the partial Frobenius pullback on motivic cohomology groups in weights at most 1.