Resumen
Define the set of zeros of a solution u by Z (t)={xR : u (x, t)=0} for t>0. In the case of =0, we show that the set Z (t) is contained in [-Ct, Ct] for large t>0 with some C>0 and that this order of t is best possible. When >0, we also give estimates of Z (t) for global solutions and prove that Z (t)[-K, K] for all t(0, T) with some K>0 for each blowup solution, where T is the blowup time.