and 0 ≤ β(x) ≤ 1 with β(0) = 1. Note that if β(x) = 1 (or 0)
for all x, then shortages are completely backlogged (or lost).
5. The retailer can accumulate revenue and earn interest after
his/her customer pays for the amount of purchasing cost to
the retailer until the end of the trade credit period offered
by the supplier. That is, the retailer can accumulate revenue
and earn interest during the period N to M with rate Ie under
the condition of trade credit. However, beyond the fixed credit
period, the product still in stock is presumed to be financed with
an annual rate Ir .