On the complexity of scheduling with negative time lags
Abstract
We investigate the scheduling problems with negative time lags to minimise the makespan. Unlike an ordinary precedence constraint, a negative time lag l between two tasks i and j, means that j cannot start earlier than —l time units before i. In such problems, the precedence graph may have cycles which may lead to unfeasibility. We identify polynomially solvable cases for different precedence graph structures on a single machine and on parallel machines. We also prove that for cyclic and star-like graphs, the problem with negative time lags is NP-complete.