Vladimir G. Deineko Gerhard J. Woeginger Hardness of approximation of the discrete time-cost tradeoff problem. 207-210 2001 29 Oper. Res. Lett. 5 http://dx.doi.org/10.1016/S0167-6377(01)00102-X db/journals/orl/orl29.html#DeinekoW01