Dynamic economic lot size models with period-pair-dependent backorder and inventory costs

Inventory and backorder cost functions in the classical Wagner-Whitin econo mic lot size (ELS) models are typically period-pair-independent (pp-indepen dent) in the sense that inventoried units carried (or backorders in existen ce) in a given period are treated tile same regardless of the periods in wh ich they are produced (placed) or the periods, in which they are used (fill ed). we consider versions of tile problem where inventory and backorder cos ts are pp-dependent as well as versions where backorder costs, but not inve ntory costs, are pp-dependent. Recognizing that the problems considered are NP-hard. a e provide cases where the cost structure allows polynomial solv ability via dynamic programming.