| 1. |
- Amaldi, Edoardo, et al.
(författare)
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Solving Nonlinear Covering Problems Arising in WLAN Design
- 2011
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Ingår i: OPERATIONS RESEARCH. - : Informs. - 0030-364X .- 1526-5463. ; 59:1, s. 173-187
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Tidskriftsartikel (refereegranskat)abstract
- Wireless local area networks (WLANs) are widely used for cable replacement and wireless Internet access. Because the medium access control (MAC) scheme of WLANs has a strong influence on network performance, it should be accounted for in WLAN design. This paper presents AP location models that optimize a network performance measure specifically for the MAC scheme of WLANs that represents the efficiency in sharing the wireless medium. For these models, we propose a solution framework based on an effective integer-linear programming Dantzig-Wolfe reformulation. This framework is applicable to any nonlinear covering problem where the objective function is a sum of contributions over the groundset elements (users in WLANs). Extensive computational results show that our solution strategy quickly yields optimal or near-optimal solutions for WLAN design instances of realistic size.
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| 2. |
- Berling, Peter, 1973-, et al.
(författare)
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Optimal Inventory Policies when Purchase Price and Demand are Stochastic
- 2011
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Ingår i: Operations Research. - : Institute for Operations Research and the Management Sciences (INFORMS). - 0030-364X .- 1526-5463. ; 59:1, s. 109-124
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we consider the problem of a firm that faces a stochastic (Poisson) demand and must replenishfrom a market in which prices fluctuate, such as a commodity market. We describe the price evolution as acontinuous stochastic process and we focus on commonly used processes suggested by the financial literature,such as the geometric Brownian motion and the Ornstein-Uhlenbeck process. It is well-known that under variablepurchase price, a price-dependent base-stock policy is optimal. Using the single-unit decomposition approach, weexplicitly characterize the optimal base-stock level using a series of threshold prices. We show that the base-stocklevel is first increasing and then decreasing in the current purchase price. We provide a procedure for calculatingthe thresholds, which yields closed-form solutions when price follows a geometric Brownian motion and implicitsolutions under the Ornstein-Uhlenbeck price model. In addition, our numerical study shows that the optimalpolicy performs much better than inventory policies that ignore future price evolution, because it tends to placelarger orders when prices are expected to increase.
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| 3. |
- Marklund, Johan, et al.
(författare)
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Lower Bounds and Heuristics for Supply Chain Stock Allocation
- 2012
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Ingår i: Operations Research. - : Institute for Operations Research and the Management Sciences (INFORMS). - 0030-364X .- 1526-5463. ; 60:1, s. 92-105
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Tidskriftsartikel (refereegranskat)abstract
- Assume that in periods with stochastic demand remain until the next replenishment arrives at a central warehouse. How should the available inventory be allocated among N retailers? This paper presents a new policy and a new lower bound for the expected cost of this problem. The lower bound becomes tight as N -> infinity. The infinite horizon problem then decomposes into N independent m-period problems with optimal retailer ship-up-to levels that decrease over the in periods, and the warehouse is optimally replenished by an order-up-to level that renders zero (local) warehouse safety stock at the end of each replenishment cycle. Based on the lower bound solution, we suggest a heuristic for finite N. In a numerical study it outperforms the heuristic by Jackson [Jackson, P. L. 1988. Stock allocation in a two-echelon distribution system or what to do until your ship comes in. Management Sci. 34(7) 880-895], and the new lower bound improves on Clark and Scarf's [Clark, A. J., H. Scarf. 1960. Optimal policies for a multi-echelon inventory problem. Management Sci. 6(4) 475-490] bound when N is not too small. Moreover, the warehouse zero-safety-stock heuristic is comparable to Clark and Scarf's warehouse policy for lead times that are not too long. The suggested approach is quite general and may be applied to other logistical problems. In the present application it retains some of the risk-pooling benefits of holding central warehouse stock.
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| 4. |
- Guo, Baolin, et al.
(författare)
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Molecular Achitecture of electroactive and biodegradable copolymers composed of polyactide and carboxyl-capped aniline trimer
- 2010
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Ingår i: Biomacromolecules. - : American Chemical Society (ACS). - 1525-7797 .- 1526-4602. ; 11:4, s. 855-863
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Tidskriftsartikel (refereegranskat)abstract
- wo-, four-, and six-armed branched copolymers with electroactive and biodegradable properties were synthesized by coupling reactions between poly(l-lactides) (PLLAs) with different architecture and carboxyl-capped aniline trimer (CCAT). The aniline oligomer CCAT was prepared from amino-capped aniline trimer and succinic anhydride. FT-IR, NMR, and SEC analyses confirmed the structure of the branched copolymers. UV−vis spectra and cyclic voltammetry of CCAT and copolymer solution showed good electroactive properties, similar to those of polyaniline. The water contact angle of the PLLAs was the highest, followed by the undoped copolymer and the doped copolymers. The values of doped four-armed copolymers were 54−63°. Thermal properties of the polymers were studied by DSC and TGA. The copolymers had better thermal stability than the pure PLLAs, and the Tg between 48−58 °C and Tm between 146−177 °C of the copolymers were lower than those of the pure PLLA counterparts. This kind of electroactive and biodegradable copolymer has a great potential for applications in cardiovascular or neuronal tissue engineering.
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