Das, B., N. K. Mahapatra, and M. Maiti. “Initial‐valued first‐order Fuzzy Differential Equation in Bi‐level Inventory Model With Fuzzy Demand”. Mathematical Modelling and Analysis, vol. 13, no. 4, Dec. 2008, pp. 493-12, doi:10.3846/1392-6292.2008.13.493-512.