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International Journal of Modern Engineering Research (IJMER) www.ijmer.com Vol. 2, Issue. 5, Sep.-Oct. 2012 pp-3650-3652 ISSN: 2249-6645 Inventory Model: Deteriorating Items with Price and Time Dependent Demand Rate Jasvinder Kaur1, Rajendra Sharma2 *(Department Of Mathematics, Graphic Era University Dehradun,India) ** (Department Of Mathematics Graphic Era University Dehradun, india) ABSTRACT: This study presents a deterministic e t inventory model for deteriorating products under the D(t , p ) condition of instantaneous replenishment. The rate of d( p ) deterioration is assumed to be a constant fraction of on is constant governing the decreasing rate of demand. hand inventory and demand is a function of selling price p is the selling price per unit and d( p) is the function and decreases exponentially with time. It is shown that the of p . developed model can be related to Cohen Model and Standard model without deterioration. A numerical 3 Lead time is zero, shortages are not allowed. example demonstrates the effectiveness of the developed 4 The replenishment rate is infinite and T is the cycle model. time. 5 There is no replacement or repair of the decayed units during the period under consideration. Keywords: Demand; deterioration; inventory; optimal; shortage. 6 The unit purchase cost is C , h is the holding cost per unit per unit time, K is the ordering cost per order. I. INTRODUCTION 7 I (t ) is the inventory at any time t . It is of considerable interest and important to analysis the inventory models for deteriorating items. In many inventory III. MATHEMATICAL MODELLING AND systems, the effect of deterioration is an important factor ANALYSIS and cannot be ignored. Deterioration may be defined as decay or damage or spoilage, so that the item can not be The differential equation governing the system is given by used as in its original state. Thus for example, blood, dI (t ) certain food items, photographic films, fruits, chemicals, I (t ) D( p , t ) (1) dt radio active substances are some examples of items in which deterioration plays a major role. Solution of (1) is given by after adjusting constant of Ghare and Schrader (Ghare and Schrader, 1963) integration. developed a model for exponentially decaying inventory e t e t considering constant demand. Emmons (1968) also I (t ) I (0)e t (2) d( p)( ) developed a model for exponential decaying products, where the product decayed at one rate into a new product, Inventory without decay at time t is given by the which decayed at a second rate. differential equation. Cohen (1977) developed a model for joint pricing d e t and ordering policy for exponentially decaying inventory I W (t ) (3) with known demand and constant decay rate. Mukherjee dt d( p) (1987) extended it by considering time varying decay rate. The solution of which gives Kumar and Sharma (2009) has extended Mukherjee’s (1999) model by considering shortages. ( e t 1) I W (t ) I (0) (4) In this study, we consider demand as a function of .d( p) time and price both and decreases exponentially. Deterioration is assumed to be constant fraction of on hand The stock loss Z(t ) due to decay in [0, T ] is given by inventory. More ever the developed model reduces Cohen’s z(t ) I W (t ) I(t ) model (1977) and standard model without deterioration. e t 1 II. PROPOSED ASSUMTIONS & NOTATIONS I ( o) I (t ) (5) .d( p) The model is developed under following assumptions and notations. Using (2), equation (5) reduced to 1 Deterioration rate is a constant fraction of on hand e( )t 1 e t 1 inventory. Z(t ) I (t )( et 1) (6) 2 Demand rate D(t , p) is known and decreases d( p)( ) .d( p) exponentially, i.e. at time Total demand D during (0, T ) is given by t, t 0 T e t ( e T 1) D dt (7) 0 d( p) .d( p) www.ijmer.com 3650 | Page International Journal of Modern Engineering Research (IJMER) www.ijmer.com Vol. 2, Issue. 5, Sep.-Oct. 2012 pp-3650-3652 ISSN: 2249-6645 e T 1 e( )T 1 For optimal price decision, consider the profit rate function Also Z(T ) (8) for a fixed period length. .d( p) d( p)( ) eT ( )T f (T , p) p. C(T , p) 1e d( p) Lot Size QT (9) d( p)( ) Differentiating with respect to p , we get Also I (0) QT , then Using (9), (2) reduces to C {1 e( )T } h T 2 e( )T ( eT 1) f eT d'p (T ) e t T e .e ( t T ) p {d( p)} 2 d( p) pd '( p) 2 T( ) T( ) 2 I (t ) [d( p)] (10) d( p)( ) f Cost per cycle becomes Also 0 implies T p C (T , p) K C.QT h I (t )dt d( p) T C 1 e( )T h T 2 e( )T ( eT 1) 0 e (T ) pT d '( p) T ( ) T ( ) 2 For a fixed price level p cost per unit time C(T , p) is C(T, p) = C*(T, p)/T IV. EXAMPLE AND TABLES K C[1 e( )T ] h 1 eT e( )T (1 eT ) SPECIAL CASE T Td( p)( ) Td( p)( ) Case 1 If = 0 and Demand d * ( p)(say ) (11) Then this model reduces to Cohen model (2). By holding p fixed, the necessary conditions for Case 2 If = 0, = 0 and demand is constant. Then this minimizing C(T , p) with respect to T is model reduces to the standard formula for non decaying inventory. C(T , p) 0 T NUMERICAL EXAMPLE implies Consider an inventory system such that C (C h )e ( )T {T ( ) 1} 2 h T h K = Rs. 50 per order, h = Rs. 0.50 per unit per week, K 1 d( p)( ) d( p)( ) 2 d( p)( ) d( p)( ) d( p ) = 25-0.5 p . 0 T2 h2T 2 + The optimum or time period. 2d( p)( ) Table 1 Tabulation of Tp for different values of the (C h)e( )T [T ( ) 1] h T 2 C h parameter. K P* C Tp d( p)( ) 2d( p) d( p)( ) 0.02 0.0 40 30 21.32 (12) 0.06 0.02 40 30 17.14 From this equation by substituting known values 0.06 0.02 30 30 24.25 C , , h , , d( p) and K we can find the optimum value of 0.10 0.02 30 40 16.43 0.10 0.06 30 40 21.82 T. An approximate solution to (12) can be obtained by using a We have found the values of Tp for a fixed set of values turcated Taylor series expansion for exponential function as and are very small. Using Taylor series expansion K and h varying values of C , p , and . Table 1 equation (12) reduces to indicates that with the increasing , Tp decreases and with 1/2 2K d( p) the increasing value of , Tp increases and the same will Tp (13) increase with the decrease in p . h ( )(C h ) The effect of variation in perishability and price changes on V. CONCLUSION In this paper, we developed a deterministic inventory model the optimal order decision can be obtained from equation for deteriorating items when demand is a function of time (9) and (13) we can get and price both. QTp 1 Tp The result of the model is important for 1 ( ) Tp d( p) 2 formulating the decisions when the inventory decay with constant rate and demand is a function of time and price The sensitivity of the order rate to change in the both. perishability is determined by Two special cases illustrates the effectiveness of Tp the developed model. The future study will incorporate any QT p /Tp 2 d( p) 2 d( p) . 0 factual relation that may exists between time and price both www.ijmer.com 3651 | Page International Journal of Modern Engineering Research (IJMER) www.ijmer.com Vol. 2, Issue. 5, Sep.-Oct. 2012 pp-3650-3652 ISSN: 2249-6645 in the demand rate function and variable rate of deterioration. AKNOWLEGEMENT We are thankful to Dr.A.P.Singh of SGRR (PG) College Dehradun for his constant support and review of our research paper. We also thank Prof. Yudhveer Singh Moudgil of Uttranchal Institute of Technology for reviewing our research paper. REFERENCES [1] Cohen M.A. (1976). Analysis of single critical number ordering policies for perishable inventories. Operations Research 24, 726-741. [2] Cohen M.A. (1977). Joint pricing and ordering policy for exponentially decaying inventory with known demand. Nav. Res. Log. Quar. 24(2) 257-268. [3] Emmons H. (1968). A replenishment model for radio active nullide generators. Management Science 14, 263- 274. [4] Ghare P. M. and Schrader G. F. (1963). A model for an exponentially decay inventory. The Journal of Industrial Engineering 14, 238-243. [5] Gupta P. N. and Jauhari R. (1993). Optimum ordering interval for constant decay rate of inventory, discrete-in- time. Ganita Sandesh 7(1) 12-15. [6] Mukherjee S. P. (1987). Optimum ordering interval for time varying decay rate of inventory, Opsearch 24(1) 19- 24. [7] Naresh Kumar and Sharma A. K. (1999). On deterministic production inventory model for deteriorating items with an exponential declining demand. Presented at 11 th RGP held at SGN Khalsa College, Sri Ganga Nagar (Dated 21, 22 Nov. 1999). [8] Naresh Kumar and Sharma A. K. (2009). Optimum ordering interval with known demand for items with variable rate of deterioration and shortages (Accepted for publication in A.S.P.). www.ijmer.com 3652 | Page

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