The correct proofs for the optimal ordering policy with trade credit under two different payment methods in a supply chain system

Cheng et al. (2010) consider the optimal ordering policy with trade credit under two different payment methods. Under Assumption (5) by Cheng et al., the annual total relevant cost TRC(T) is only defined on a finite interval. However, Cheng et al. treat the domain of TRC(T) to be the set of all positive numbers such that the formulation and optimal solution of TRC(T) cause some errors. So, the main purpose of this paper not only removes those shortcomings by Cheng et al. but also presents the correct proofs for Theorems 1 and 2 of Cheng et al.