On the Bandpass problem

The complexity of the Bandpass problem is re-investigated. Specifically, we show that the problem with any fixed bandpass number B≥2 is NP-hard. Next, a row stacking algorithm is proposed for the problem with three columns, which produces a solution that is at most 1 less than the optimum. For the special case B=2, the row stacking algorithm guarantees an optimal solution. On approximation, for the general problem, we present an O(B ²)-algorithm, which reduces to a 2-approximation algorithm for the special case B=2.