Qi Feng 0002 Suresh P. Sethi Houmin Yan Hanqin Zhang Are Base-Stock Policies Optimal in Inventory Problems with Multiple Delivery Modes? 801-807 2006 54 Oper. Res. 4 https://doi.org/10.1287/opre.1050.0271 db/journals/ior/ior54.html#FengSYZ06