Improved Approximation Algorithms for Stochastic Matching

In this paper we consider the Stochastic Matching problem, which is motivated by applications in kidney exchange and online dating. We are given an undirected graph in which every edge is assigned a probability of existence and a positive profit, and each node is assigned a positive integer called timeout. We know whether an edge exists or not only after probing it. On this random graph we are executing a process, which one-by-one probes the edges and gradually constructs a matching. The process is constrained in two ways: once an edge is taken it cannot be removed from the matching, and the timeout of node $v$ upper-bounds the number of edges incident to $v$ that can be probed. The goal is to maximize the expected profit of the constructed matching. For this problem Bansal et al. (Algorithmica 2012) provided a $3$-approximation algorithm for bipartite graphs, and a $4$-approximation for general graphs. In this work we improve the approximation factors to $2.845$ and $3.709$, respectively. We also consider an online version of the bipartite case, where one side of the partition arrives node by node, and each time a node $b$ arrives we have to decide which edges incident to $b$ we want to probe, and in which order. Here we present a $4.07$-approximation, improving on the $7.92$-approximation of Bansal et al. The main technical ingredient in our result is a novel way of probing edges according to a random but non-uniform permutation. Patching this method with an algorithm that works best for large probability edges (plus some additional ideas) leads to our improved approximation factors

Marketing of Fruits and Vegetables in India: A Study Covering the Ahmedabad, Chennai and Kolkata Markets

There has been concern in recent years regarding the efficiency of marketing of fruits and vegetables, and that this is leading to high and fluctuating consumer prices and only a small share of the consumer rupee reaching the farmers. Marketing of horticultural crops is complex especially because of perishability, seasonality and bulkiness. The study seeks to examine different aspects of their marketing, focusing particularly, on the wholesale markets for fruits and vegetables which have been established to overcome deficiencies and improve the marketing efficiency. Results indicate that in Ahmedabad the direct contact between commission agents and farmers is very low. For vegetables this is 50 percent and for fruits only 31 percent. Further, in the system of transaction, secret bidding and simple transaction dominate and open auction is relatively rare. In KFWVM, Chennai, the wholesalers act as commission agents and receive consignments directly from producing centers through agents or producers. By and large the system of transaction remains traditional and open auction is rarely seen. This is one major reason for poor efficiency. However, in the small AUS market in Chennai, the farmers sell directly to consumers. The share of farmers in the consumer rupee in Ahmedabad was 41.1 to 69.3 percent for vegetables and 25.5 to 53.2 percent for fruits. In Chennai KFWVM, the farmers share was 40.4 to 61.4 percent for vegetables and, 40.7 to 67.6 percent for fruits. In the small AUS market in Chennai, where the farmers sell directly to the consumers, the share of farmers was as high as 85 to 95.4 percent for vegetables. This indicates that if there are few or no middlemen, the farmers’ share could be much higher. In the Kolkata market the share of farmers ranged from 45.9 to 60.94 percent for vegetables and 55.8 to 82.3 percent for fruits. Thus, the shares are frequently very low, but somewhat better in Chennai, lower in Kolkata and even lower in Ahmedabad. The margin as a percentage of farmer-consumer price difference (an efficiency measure) shows that in Ahmedabad, the margins are very high and range from 69 to 94 percent. In Chennai they range from 15 to 69 percent, and in Kolkata they range from 46 to 73 percent. The high percentage of margin to farmer-consumer price difference is indicative of large inefficiencies and relatively poor marketing efficiency. There is great need to improve the marketing of fruits and vegetables. One important measure would be to bring more markets under regulation and supervision of a well-represented market committee. Another measure would be the promotion and perhaps enforcement of open auctions in the markets. Yet another measure could be efforts to bring more buyers and sellers into the markets, bringing them closer to perfect markets. The direct participation of farmers should be increased. Market infrastructure should be improved through storage (go-down) facilities, cold storages, loading and weighing facilities. Improvement in the road network, and cold-chain facilities are also of substantial importance. Greater transparency of the operations through supervision and systems can also help substantially. The market integration and efficiency can also be improved by making up-to-date market information available to all participants through various means, including a good market information systems, internet and good telecommunications facilities at the markets.

Fruit and Vegetable Marketing and its Efficiency in India: A Study of Wholesale Markets in the Ahmedabad

There has been great concern in recent years regarding the efficiency of marketing of fruits and vegetables in India. It is believed that poor efficiency in the marketing channels and poor marketing infrastructure leading to high and fluctuating consumer prices and only a small portion of the consumer rupee reaching the producer farmers. This paper examines these aspects in regulated wholesale markets for fruits and vegetables in Ahmedabad City area. These regulated markets were established to improve the marketing efficiency. The system of sale followed in these markets indicated that open auction as a system of sale is yet to take roots in these markets and the marketing system was dominated by open auction or secret bidding resulting to significant erosion of marketing efficiency. Analysis of marketing costs indicated that on an average they hover around 8 percent of the consumer price for vegetables 11 to 15 percent for fruits. On an average the share of farmers in consumer rupee was hardly 48 percent for vegetables and 37 percent for fruits. The study suggests that regulated wholesale markets can help in improving the marketing efficiency by promoting direct contact with the farmers, increasing the number of buyers and sellers in the market, promoting open auction system of marketing and strengthening or introducing facilities and services such as go-down, cold storage, transparency and access to internal and external market environment.

Groundwater Irrigation in India: Gains, Costs and Risks

Groundwater has rapidly emerged to occupy a dominant place in India.'s agriculture and food security in the recent years. It has become the main source of growth in irrigated area over the past 3 decades, and it now accounts for over 60 percent of the irrigated area in the country. It is estimated that now over 70 percent of India.'s food grain production comes from irrigated agriculture, in which groundwater plays a major role. Since the development of groundwater irrigation has not largely been government or policy driven . has happened gradually through highly decentralized private activity, this revolution has often gone largely unrecognized. However, despite this huge significance, groundwater irrigation is heading for a crisis in India and needs urgent understanding and attention. The number of irrigation blocks considered overexploited is increasing at an alarming rate of 5.5 percent per year. The number of blocks in which, officially, the creation of wells must completely stop is scaling new heights every year. Yet, the sinking of wells continues rapidly at enormous private, public and environmental cost. The way India will manage its groundwater resource in the future will clearly have very serious implications for the future growth and development of the agriculture sector in India, as well as the alleviation of poverty in India.

The Adoption and Economics of Bt Cotton in India: Preliminary Results from a Study

The paper presents preliminary results from a study of the economics and adoption of Bt cotton in India. Biotech crops, which made their appearance in the world about a decade ago, have gained substantial popularity and acceptance in many parts of the world including US, China, Australia, Mexico, Argentina and South Africa. However, their introduction in India has been relatively late and controversial and they still have considerable ground to cover in the country. Cotton is a major commercial crop in India but has substantial problems particularly from extensive pest damage and poor yields. Bt cotton offers a promising solution to these serious problems. Data from the survey, which covered the important cotton states of Gujarat, Maharashtra, Andhra Pradesh and Tamil Nadu, and 694 farmers, indicates that Bt cotton offers good resistance to bollworms as well as several other pests. The incidence of these pests is reported to be considerably lower in Bt cotton as compared to Non-Bt cotton. The yields of Bt cotton are found to be higher and the yield increase/ difference statistically significant in all the states under both irrigated and rain-fed conditions. As a result, given the good market acceptance of the product, the value of output per hectare is higher in all the states and conditions. The question of higher cost of cultivation exists, and is confirmed, mainly because of high seed cost and not commensurate reduction in pesticide cost. However, the profit is found to be higher in all the states to the estimated extent of about 80-90 percent on an average when the effects of associated inputs are included. The returns are highest in Maharashtra followed by Gujarat and then Andhra Pradesh. Subjective assessment indicates that farmers see advantage in Bt cotton in pest incidence, pesticide cost, cotton quality, yield and profit. Almost all farmers indicate that they plan to plant Bt cotton in the future. To increase the benefits from the technology, the farmers strongly urge reduction in the seed cost, greater field extension and demonstration work on the correct practices, and more Bt cotton varieties to suit the diverse agro-ecological settings.

Evidence that indirect inhibition of saccade initiation improves saccade accuracy

Saccadic eye-movements to a visual target are less accurate if there are distracters close to its location (local distracters). The addition of more distracters, remote from the target location (remote distracters), invokes an involuntary increase in the response latency of the saccade and attenuates the effect of local distracters on accuracy. This may be due to the target and distracters directly competing (direct route) or to the remote distracters acting to impair the ability to disengage from fixation (indirect route). To distinguish between these we examined the development of saccade competition by recording saccade latency and accuracy responses made to a target and local distracter compared with those made with an addition of a remote distracter. The direct route would predict that the remote distracter impacts on the developing competition between target and local distracter, while the indirect route would predict no change as the accuracy benefit here derives from accessing the same competitive process but at a later stage. We found that the presence of the remote distracter did not change the pattern of accuracy improvement. This suggests that the remote distracter was acting along an indirect route that inhibits disengagement from fixation, slows saccade initiation, and enables more accurate saccades to be made

Three-dimensional unstructured grid generation via incremental insertion and local optimization

Algorithms for the generation of 3D unstructured surface and volume grids are discussed. These algorithms are based on incremental insertion and local optimization. The present algorithms are very general and permit local grid optimization based on various measures of grid quality. This is very important; unlike the 2D Delaunay triangulation, the 3D Delaunay triangulation appears not to have a lexicographic characterization of angularity. (The Delaunay triangulation is known to minimize that maximum containment sphere, but unfortunately this is not true lexicographically). Consequently, Delaunay triangulations in three-space can result in poorly shaped tetrahedral elements. Using the present algorithms, 3D meshes can be constructed which optimize a certain angle measure, albeit locally. We also discuss the combinatorial aspects of the algorithm as well as implementational details