Unary NP-hardness of preemptive scheduling to minimize total completion time with release times and deadlines

We revisit the single-machine preemptive scheduling problem to minimize total completion time with release times and deadlines. Du and Leung (1993) showed that the problem is binary NP-hard. But the exact complexity (unary NP-hardness or pseudo-polynomial-time solvability) of the problem is a long standing open problem. We show in this paper that the problem is unary NP-hard.