  We consider computing optimal k-norm preemptive schedules of jobs that arrive
over time. In particular, we show that computing the optimal k-norm of flow
schedule, is strongly NP-hard for k in (0, 1) and integers k in (1, infinity).
Further we show that computing the optimal k-norm of stretch schedule, is
strongly NP-hard for k in (0, 1) and integers k in (1, infinity).
