-(iii) The convexity: Hadley and Whitin [11] show that the total annual cost function of the basic EOQ (economic order quantity) without backorders model is convex.
-Rachamadugu [16] demonstrates that the discounted total annual cost function is also convex. ---Chiu et al. [17] presents a better proof of convexity of a long-run average cost function for an inventory system with stochastic machine breakdown and rework process.
-Based on the above truths, in fact, if the total annual cost function of the inventory model is convex, it is easier to find the optimal solution by using the convexity property.
-Therefore, it is worth to study the convexity of the total annual cost function of the inventory model to locate the optimal solution.
-Consequently, the exploration of the convexity of the total annual cost function is still one of the main research topics of the inventory models.