Stefan Chanas Pawel Zielinski On the hardness of evaluating criticality of activities in a planar network with duration intervals. 53-59 2003 31 Oper. Res. Lett. 1 http://dx.doi.org/10.1016/S0167-6377(02)00174-8 db/journals/orl/orl31.html#ChanasZ03