72 research outputs found
Breast mass segmentation in mammographic images by an effective region growing algorithm
No full textEnvy, regret, and social welfare loss
No full textIncentive compatibility (IC) is a desirable property for any auction mechanism, including those used in online advertising. However, in real world applications practical constraints and complex environments often result in mechanisms that lack incentive compatibility. Recently, several papers investigated the problem of deploying black-box statistical tests to determine if an auction mechanism is incentive compatible by using the notion of IC-Regret that measures the regret of a truthful bidder. Unfortunately, most of those methods are computationally intensive, since they require the execution of many counterfactual experiments. In this work, we show that similar results can be obtained using the notion of IC-Envy. The advantage of IC-Envy is its efficiency: it can be computed using only the auction's outcome. In particular, we focus on position auctions. For position auctions, we show that for a large class of pricing schemes (which includes e.g. VCG and GSP), IC-Envy ≥ IC-Regret (and IC-Envy = IC-Regret under mild supplementary conditions). Our theoretical results are completed showing that, in the position auction environment, IC-Envy can be used to bound the loss in social welfare due to the advertiser untruthful behavior. Finally, we show experimentally that IC-Envy can be used as a feature to predict IC-Regret in settings not covered by the theoretical results. In particular, using IC-Envy yields better results than training models using only price and value features
Elacatinus colini Randall & Lobel, 2009, new species
No full text<i>Elacatinus colini</i>, new species <p>(Figs. 2–7; Tables 1, 2)</p> <p> ? <i>Gobiosoma (Elacatinus) horsti</i> Colin, 1975: 94 (Belize).</p> <p> <i>Elacatinus xanthiprora</i> Smith et al., 2003: 62, fig. 17 (Pelican Cays, Belize).</p> <p> <b>Holotype:</b> USNM 395063, female, 26.8 mm, Belize, Wee Wee Cay, off main dock, 16°45.860’N, 88°8.631’W, patch reef of mixed coral and sponge on sloping sand substratum, 5 m, hand net, P.S. Lobel, 7 December 2008.</p> <p> <b>Paratypes</b>: BPBM 37442, 21.3 mm, Belize, Pelican Cays, coral reef in mangrove area, 14 m, quinaldine, J.E. Randall, 21 March 1997; USNM 347301, 32.0 mm, Belize, Carrie Bow Cay, sand bores south of island, C.C. Baldwin, 28 August 1997; USNM 347401, 27.2 mm, Belize, Pelican Cays, Cat Cay, outside lagoon, <i>Agaricia</i> slope at southwest end of cay, 9–13.5 m, J.C. Tyler, M. Tyler, W.P. Davis, & C.L. Smith, 14 October 1997; USNM 365031, 4: 17.1–25.8 mm, Belize, Pelican Cays, Manatee Cay, west side, outside of coral wall at entrance to lagoon, 12–16.5 m, J.C. Tyler & W.P. Davis, 29 January 2001; USNM 394964, female, 33.5 mm, Belize, Wee Wee Cay, off main dock, same location as holotype, patch reef of mixed coral and sponge on sloping sand substratum, 3 m, hand net, P.S. Lobel, 4 December 2003; ANSP 189240, 4: 18.5–23.2 mm and UF 173046, 28.1 mm, same data as preceding; SIO 08-180, 19.3 mm, Honduras, Islas de la Bahía, Isla de Utila, south-side wall, 16.081°N, 86.921°W, B.C. Victor, 30 June 2008; MNHN 2009-0091, 2: 22.5–24.7, same location as holotype, sloping coral and sponge bottom, 5 m, hand net, P.S. Lobel, 7 December 2008.</p> <p> <b>Diagnosis:</b> Dorsal rays VII + I, 11 (10–12, rarely 10); anal rays I,10 (10–11, two of 15 with 11); pectoral rays 17–19 (usually 18, one of 30 with 19); no scales; body and fins covered with thick adherent mucus; mouth subterminal; tongue truncate; no rostral frenum; color in alcohol pale yellowish gray with a midlateral black stripe from eye nearly to posterior end of caudal fin, about a pupil diameter in width on head, broadening to about an eye diameter in width on body; a blackish line extending posteriorly from above upper edge of eye, continuing below base of dorsal fins, and extending into dorsal part of caudal fin; fins otherwise translucent gray; color in life of body above black lateral stripe bluish to greenish gray with a bright white stripe as wide as pupil passing between black stripe and black line, becoming yellow on head as it nears upper edge of eye; dorsal part of iris brilliant yellow; snout and lips dusky yellow; a median brilliant yellow band on snout extending into anterior interorbital; ventral part of head and body whitish.</p> <p>Species Dorsal rays Anal rays Pectoral rays1</p> <p> 10 11 12 9 10 11 17 18 19 20 <i>E</i>. xanthiprora 4 1 5 4 6 <i>E</i>. colini 9 3 11 1 7 17</p> <p> <i>E. serranilla</i> 4 4 2 6</p> <p> <i>E.</i> sp. 1 (Nicaragua) 2 2 2 2 <i>E.</i> sp. 2 (Isla de Providencia) 3 8 2 9 8 12 2</p> <p>1Both pectoral fins counted Holotype Paratypes</p> <p> <b>Description:</b> Dorsal rays VII + I,11 (10–12, usually 11, one with 10); anal rays I,10–11 (two of 13 paratypes with 11); dorsal and anal soft rays branched, the last to base; pectoral rays 18 (17–19, usually 18, one with 19 on one side), the uppermost and lowermost unbranched (branched or unbranched in paratypes); pelvic rays I,5; pelvic frenum well developed; branched caudal rays 13 (12–13); upper and lower procurrent caudal rays 9 (8–9); head and body entirely naked, with a thick adherent covering of mucus; gill rakers 2 +7 (1–2 + 7); vertebrae 28.</p> <p>The following morphometrics are given as percentages of the standard length: body depth 18.8 (18.7–23.3); body compressed, the width 12.5 (10.6–15.0); head length 28.3 (28.2–29.2); snout length 6.7 (6.1–6.8); orbit diameter 6.7 (6.6–7.0); interorbital width 4.8 (4.0–5.3); caudal-peduncle depth 12.6 (11.5–13.8); caudal-peduncle length 19.8 (19.6–21.5).</p> <p>Mouth subterminal, U-shaped when viewed ventrally, and oblique laterally, forming an angle of about 18° below horizontal axis of body; maxilla reaching to or slightly posterior to middle of eye, the upper-jaw length 11.5 (11.3–12.7); each side of upper jaw of female holotype with an outer row of 24 slender, conical, slightly incurved and retrorse teeth, the largest about two-thirds pupil diameter, becoming progressively shorter on posterior half of jaw; an inner band of 3 rows of small conical teeth at front of jaw, narrowing to a single row posteriorly; each side of front of lower jaw with an outer row of 7 or 8 slender, incurved, conical teeth about two-thirds size of anterior upper-jaw teeth, followed by about 22 smaller teeth on side of jaw; a band of small conical teeth medial to anterior row of large teeth; an inner row of four larger, strongly retrorse, conical teeth about one-third distance back in jaw; each side of front of upper jaw of males with five progressively larger and more retrorse canine teeth, the last two of about pupil length; corner of lower jaw of males with two retrorse canines comparable to posterior two of upper jaw; no rostral frenum, and no mental flap.</p> <p>Gill opening short, extending ventrally to level of lower edge of pectoral-fin base; gill membranes attached anteriorly to isthmus; gill rakers short, the longest at angle less than one-half length of longest gill filaments.</p> <p>Anterior nostril tubular, at edge of snout above upper lip, in line with ventral edge of pupil; posterior nostril dorsoposterior, with a fleshy rim, nearly in line with upper edge of pupil, the internarial distance threefourths pupil diameter; cephalic sensory pores system of eight pores as diagrammed by Lachner & Karnella (1980: fig. 4a); cephalic sensory papillae as follows: a row of papillae from anterior nostril along edge of upper lip and continuing horizontally half way across cheek, a second row extending ventroposteriorly from posterior nostril to join first row, followed by six rows from ventral edge of eye, the first three progressively less oblique, the fourth perpendicular to eye, the fifth obliquely posterior to eye with a horizontal branch near base; sixth row below posterior interorbital pore; a series of papillae along lower jaw adjacent to lower lip, continuing along edge of operculum; an irregular, double, horizontal row of papillae dorsal to opercle; a vertical row of papillae anteriorly on opercle behind posterior margin of preopercle, with a horizontal row of papillae from its upper end, and another near lower end, the two slightly converging; a series of slightly oblique rows of papillae following midlateral black stripe on body posterior to axil of pectoral fin.</p> <p>Origin of first dorsal fin above rear base of pelvic fins, the predorsal length 34.1 (34.0–35.0); dorsal and anal spines slender and flexible; third dorsal spine longest (but second and fourth spines nearly as long), 17.7 (l7.7–18.7); last membrane of first dorsal fin reaching origin of second dorsal fin; spine of second dorsal fin 11.4 (11.2–13.8); middle dorsal soft rays longest, 17.9 (17.8–18.5); origin of anal fin slightly posterior to base of first dorsal soft ray, the preanal length 58.0 (57.3–59.9); anal spine 9.0 (8.4–9.5); penultimate anal soft ray usually longest 15.8 (15.3–17.5); caudal fin rounded, 23.9 (23.4–25.2); base of pectoral fins directly posterior to and equal to height of gill opening; ninth or tenth pectoral rays usually longest, reaching to between verticals at base of seventh dorsal spine and origin of second dorsal fin, 24.8 (23.3–25.9); origin of pelvic fins slightly posterior to rear edge of pectoral-fin base, the prepelvic length 27.7 (27.2–29.0); pelvic fins joined to form a disk more than twice as long as wide, approaching but not reaching anus, 23.5 (23.3–25.3); genital papilla of male a narrow triangle in ventral view, one-half orbit diameter in length in 32-mm paratype.</p> <p>Color of holotype in alcohol: pale yellowish gray with a midlateral black stripe from middle of eye nearly to posterior end of caudal fin, about a pupil diameter in width on head, broadening to about an eye diameter in width on body; a median white line dorsally on snout, faintly edged in blackish; a blackish line at edge of each eye in interorbital, continuing across postorbital head, below base of dorsal fins, and extending into dorsal part of caudal fin; fins otherwise translucent yellowish gray.</p> <p>Color of holotype in life as illustrated in Fig. 2. Color of other individuals shown in Figs. 3–7. Figs. 6 and 7 extend the range to the island of Utila, Islas de Bahía, Honduras. We also have a paratype collected at Utila by Benjamin B. Victor.</p> <p> <b>Etymology:</b> We are pleased to name this species for Patrick L. Colin, in recognition of his exceptional doctoral thesis on the comparative biology of western Atlantic gobies of the genus <i>Elacatinus</i>, and for his help in our research on the genus.</p> <p> <b>Remarks:</b> We examined the holotype of <i>Elacatinus xanthiprora</i> (ANSP 110898, 28.8 mm) from 23 m at Alligator Reef in the Florida Keys and the two paratypes from Dry Tortugas collected by W.H. Longley (USNM 118105, 22.9–25.6 mm), first identified by Longley in Longley & Hildebrand (1941: 227) as <i>Elacatinus horsti</i> (Metzelaar). The fourth paratype of <i>E. xanthiprora</i> from Jamaica, collected by the first author from 24 m in 1959, is reidentified here as a new species that is described below, along with three specimens from the Serranilla Bank.</p> <p> Two additional specimens of <i>Elacatinus xanthiprora</i> were examined, one collected by Patrick L. Colin at Dry Tortugas in 13.5 m, in 1973 (UF 230715, 22.2 mm) (Fig. 8), and the other from trawl station 50 of the R/ V <i>Albatross IV</i> in 26 m off the west coast of Florida, commencing at 28°40’54”N, 83°45’18”W in 1980 (ANSP 148926, 43.1 mm).</p> <p> An underwater color photograph of <i>E. xanthiprora</i> taken by Paul Humann off Key Biscayne, Miami (Humann, 2002: 265) was provided for this study (Fig. 9). It is the northernmost record of the species for the Atlantic coast of Florida.</p> <p> We present the fin-ray counts of the six available specimens of <i>Elacatinus xanthiprora</i> (all from Florida) in Table 1, along with those of <i>E. colini</i> (types and nontypes). The pectoral-ray counts of 19 and 20 for <i>E. xanthiprora</i> provide complete separation from <i>E. colini</i>.</p> <p> Comparison of the proportional measurements of <i>E. colini</i> (Table 2) with those of <i>E. xanthiprora</i> (Table 3) offers little to separate the species. The only nonoverlapping measurements are the snout length (shorter in <i>E. colini</i>) and the length of base of dorsal fins (longer in <i>E. colini</i>).</p> <p> Most of the specimens of <i>E. colini</i> have been collected in Pelican Cays of the lagoon area of the Belize Barrier Reef. We have not seen the species on the seaward side of the Mesoamerican Barrier Reef, or on the offshore reefs of Glovers and Lighthouse Atolls.</p> <p> Tyler & Böhlke (1972: 619) regarded <i>Elacatinus xanthiprora</i> and relatives as facultative sponge dwellers, defined as species that spend at least a portion of their lives in or on tubular sponges. <i>E. colini</i> clearly falls in this category. It has been observed in association with sixteen different sponges and often seeks refuge in the lumen of its hosts. In addition to the five sponges identified in our Figs. 3–7, we have seen <i>E. colini</i> on or within <i>Aplysina fulva</i>, <i>Aplysina</i> sp. 1, <i>Aplysina</i> sp. 2., <i>Ircinia</i> sp., <i>Niphrates digitalis</i>, <i>Spirastrella coccinea</i>, <i>Verongula</i> sp., and four unidentified sponge species. However, it has also been observed at rest on live coral (Fig. 2), and may hide in cracks in the reef when threatened. The two individuals of Fig. 4 were observed to retire to the sponge for the night.</p> <p> <b>TABLE 3.</b> Proportional measurements of type specimens of <i>Elacatinus xanthiprora</i> as percentages of standard length. Holotype Nontype Paratype Nontype ANSP 110898 UF 230715 USNM 118105 ANSP 148926 Measurements were not taken from the two smallest paratypes due to their poor condition.</p> <p> <i>Elacatinus colini</i> has been found in the depth range of 2–17 m, but it probably occurs at greater depths. The coral reefs of the Pelican Cays area extend on sloping bottom to only about 18 m. This sponge goby is usually seen in pairs or small groups. It is sympatric with a blue-striped cleaning goby that has long been identified as <i>Elacatinus oceanops</i> Jordan. However, the latter is another undescribed species of the genus (Randall & Colin, MS). A third species of <i>Elacatinus</i>, <i>E. lori</i> Colin (Fig. 10), also a sponge inquiline, was described from Belize, but is known only from the seaward part of the barrier reef. Colin (1975: 98-99) had tentatively identified this species as the white form of <i>E. horsti</i> (Metzelaar). We present an illustration of the true <i>E. horsti</i> as Fig. 11.</p> <p> <i>Elacatinus randalli</i> is similar in color to <i>E. colini</i> and <i>E. xanthiprora</i>, especially in the snout coloration, and it may be found on sponge as in Fig. 12, but it is a cleaner goby.</p> <p> <i>Elacatinus colini</i> has a thick coat of mucus over the head, body, and fins. The mucus congeals in formalin-preserved specimens to a tough, faintly translucent, whitish coating that nearly obliterates the black markings. The first specimen collected for this study was damaged in the attempt to remove the mucus, and no measurements were taken from it for Table 2.</p> <p> Smith & Tyler (1972: 160) reported that the mucus of the sponge-dwelling <i>Elacatinus chancei</i> (Beebe & Hollister) contains a repelling substance after they observed the hamlet <i>Hypoplectrus puella</i> (Cuvier) forcefully reject an individual of this goby, following its initial seizure. Colin (1975: 243–247) discussed experiments demonstrating the noxious quality of the skin of <i>E. horsti</i> and <i>E. chancei</i>.</p> <p> The first author determined the suspected presence of a crinotoxin in the mucus of some species of grammistine, gobiesocid, and callionymid fishes by the bitter taste of the mucus. At his suggestion, the second author and students tasted the mucus of <i>E. colini</i> and found it very bitter and somewhat peppery. The mucus of the sympatric cleaning goby also tasted bitter, but not so strongly. One could assume that the latter would not need as much chemical protection because of its symbiotic association with numerous species of Caribbean reef fishes.</p> <p> Although these two species of <i>Elacatinus</i> on lagoon reefs of Belize are readily separated by the color of the stripe on the upper side of the body in life, white in <i>E. colini</i> and blue in the undescribed cleaner goby, a close examination is needed to distinguish them as preserved specimens. The overhanging snout and ventral mouth of the cleaning goby provides the most obvious difference. Other morphological differences are its shorter pelvic fins, 16.4–17.6% SL, compared to 23.3–25.3% SL of <i>E. colini</i>, and the rounder pelvic disk, reflecting its use as a strong sucking disk when it attaches to the host fishes. There is also a difference in the ventrolateral black stripe. It is broader on the cleaning goby and extends onto the side of the snout.</p> <p> Because Colin (1975: 119) found the parasitic polychaete <i>Haplosyllis spongicola</i> as the principal component of the gut contents of the four species of sponge-dwelling <i>Elacatinus</i> that he was able to dissect, <i>E. chancei</i>, <i>E. horsti</i>, <i>E. louisae</i>, and <i>E. tenox</i>, we expected to find the polychaete in the digestive tract of <i>E. colini</i>. However, the seven adult type specimens that we dissected were largely empty, and none contained anything that we could identify as polychaete. One fish had eaten copepods, which might be anticipated because individuals on the reef occasionally make quick upward forays, as if to feed on zooplankton. Another fish contained only one small larval gnathiid isopod and a tiny fish scale. This supports the fleeting observation by the second author of what appeared to be a brief cleaning of a reef fish by <i>E. colini</i>. Three nontype specimens from Belize were obtained from David G. Smith of the National Museum of Natural History for further dissection. All had empty digestive tracts. Although additional specimens may yet reveal feeding on the parasitic polychaete, our limited data suggest that it is not a major food source.</p> <p> An abundance of polychaete remains were found in two specimens of the undescribed species of spongedwelling <i>Elacatinus</i> from Isla de Providencia. The gut material was sent to Leslie H. Harris of the Natural History Museum of Los Angeles County, who confirmed it as belonging to the genus <i>Haplosyllis</i>.</p> <p> We contacted two aquarium fish collectors in Florida with a request to provide tissue samples of specimens of <i>Elacatinus xanthiprora</i> so that genetric comparison could be made with our tissue samples of <i>E. colini</i>. Both collectors know the species but indicated that they rarely catch it because of its occurrence on deep reefs.</p>Published as part of <i>Randall, John E. & Lobel, Phiillip S., 2009, A literature review of the sponge-dwelling gobiid fishes of the genus Elacatinus from the western Atlantic, with description of two new Caribbean species, pp. 1-19 in Zootaxa 2133</i> on pages 5-11, DOI: <a href="http://zenodo.org/record/274944">10.5281/zenodo.274944</a>
Approximately Efficient Double Auctions with Strong Budget Balance
No full textMechanism design for one-sided markets is an area of extensive research in economics and, since more than a decade, in computer science as well. Two-sided markets, on the other hand, have not received the same attention despite the numerous applications to web advertisement, stock exchange, and frequency spectrum allocation. This work studies double auctions, in which unit-demand buyers and unit-supply sellers act strategically.
An ideal goal in double auction design is to maximize the social welfare of buyers and sellers with individually rational (IR), incentive compatible (IC) and strongly budget-balanced (SBB) mechanisms. The first two properties are standard. SBB requires that the payments charged to the buyers are entirely handed to the sellers. This property is crucial in all the contexts that do not allow the auctioneer retaining a share of buyers' payments or subsidizing the market.
Unfortunately, this goal is known to be unachievable even for the special case of bilateral trade, where there is only one buyer and one seller. Therefore, in subsequent papers, meaningful trade-offs between these requirements have been investigated.
Our main contribution is the first IR, IC and SBB mechanism that provides an O(1)-approximation to the optimal social welfare. This result holds for any number of buyers and sellers with arbitrary, independent distributions. Moreover, our result continues to hold when there is an additional matroid constraint on the sets of buyers who may get allocated an item. To prove our main result, we devise an extension of sequential posted price mechanisms to two-sided markets. In addition to this, we improve the best-known approximation bounds for the bilateral trade problem
Controlled-release oxycodone and pregabalin in the treatment of neuropathic pain: results of a multicenter Italian study
No full textAbstract
AIMS:
The aim of our study was to compare the efficacy, safety, and quality of life of combination therapy with controlled-release (CR) oxycodone plus pregabalin versus monotherapy with either CR oxycodone or pregabalin in patients with neuropathic pain.
MATERIALS AND METHODS:
Patients with moderate to severe neuropathic pain, despite the use of various pharmacologic treatments prior to study entry, were enrolled (n = 409) and treated with CR oxycodone plus pregabalin (n = 169), CR oxycodone (n = 106), and pregabalin (n = 134). Pain intensity was rated on an 11-point numerical rating scale (NRS).
RESULTS:
The combination of CR oxycodone plus pregabalin and CR oxycodone monotherapy were both more effective for alleviating neuropathic pain than pregabalin monotherapy (reduction in NRS value: 80, 76, and 46%, respectively; p </= 0.003). Significantly greater improvements from baseline in quality of life were reported with combination therapy than with monotherapy (p = 0.0009). At the end of treatment, the majority (91.2%) of patients receiving CR oxycodone plus pregabalin found that the treatment had been 'effective' or 'very effective'. Combination therapy also allowed a dose reduction of both agents (22% for CR oxycodone and 51% for pregabalin) compared with the dosages of the respective monotherapies. Combination therapy had a superior safety profile compared with pregabalin monotherapy.
CONCLUSIONS:
The combination of CR oxycodone plus pregabalin may represent a valuable addition to the existing pharmacotherapy for neuropathic pain and warrants further investigation
Fully Dynamic Online Selection through Online Contention Resolution Schemes
No full textWe study fully dynamic online selection problems in an adversarial/stochastic setting that includes Bayesian online selection, prophet inequalities, posted price mechanisms, and stochastic probing problems subject to combinatorial constraints. In the classical “incremental” version of the problem, selected elements remain active until the end of the input sequence. On the other hand, in the fully dynamic version of the problem, elements stay active for a limited time interval, and then leave. This models, for example, the online matching of tasks to workers with task/worker-dependent working times, and sequential posted pricing of perishable goods. A successful approach to online selection problems in the adversarial setting is given by the notion of Online Contention Resolution Scheme (OCRS), that uses a priori information to formulate a linear relaxation of the underlying optimization problem, whose optimal fractional solution is rounded online for any adversarial order of the input sequence. Our main contribution is providing a general method for constructing an OCRS for fully dynamic online selection problems. Then, we show how to employ such OCRS to construct no-regret algorithms in a partial information model with semi-bandit feedback and adversarial inputs
Stochastic bandits for multi-platform budget optimization in online advertising
No full textWe study the problem of an online advertising system that wants to optimally spend an advertiser's given budget for a campaign across multiple platforms, without knowing the value for showing an ad to the users on those platforms. We model this challenging practical application as a Stochastic Bandits with Knapsacks problem over T rounds of bidding with the set of arms given by the set of distinct bidding m-tuples, where m is the number of platforms. We modify the algorithm proposed in Badanidiyuru et al., [11] to extend it to the case of multiple platforms to obtain an algorithm for both the discrete and continuous bid-spaces. Namely, for discrete bid spaces we give an algorithm with regret , where OPT is the performance of the optimal algorithm that knows the distributions. For continuous bid spaces the regret of our algorithm is . When restricted to this special-case, this bound improves over Sankararaman and Slivkins [34] in the regime OPT < < T, as is the case in the particular application at hand. Second, we show an lower bound for the discrete case and an ?(m1/3B2/3) lower bound for the continuous setting, almost matching the upper bounds. Finally, we use a real-world data set from a large internet online advertising company with multiple ad platforms and show that our algorithms outperform common benchmarks and satisfy the required properties warranted in the real-world application
Investigation of the active Celano-L'Aquila fault system, Abruzzi (central Apennines, Italy) with combined ground-penetrating radar and palaeoseismic trenching
No full textFixed price approximability of the optimal gain from trade
No full textBilateral trade is a fundamental economic scenario comprising a strategically acting buyer and seller (holding an item), each holding valuations for the item, drawn from publicly known distributions. It was recently shown that the only mechanisms that are simultaneously dominant strategy incentive compatible, strongly budget balanced, and ex-post individually rational, are fixed price mechanisms, i.e., mechanisms that are parametrised by a price p, and trade occurs if and only if the valuation of the buyer is at least p and the valuation of the seller is at most p. The gain from trade (GFT) is the increase in welfare that results from applying a mechanism. We study the GFT achievable by fixed price mechanisms. We explore this question for both the bilateral trade setting and a double auction setting where there are multiple i.i.d. unit demand buyers and sellers. We first identify a fixed price mechanism that achieves a GFT of at least 2/ r times the optimum, where r is the probability that the seller’s valuation does not exceed that of the buyer’s valuation. This extends a previous result by McAfee. Subsequently, we improve this approximation factor in an asymptotic sense, by showing that a more sophisticated rule for setting the fixed price results in a GFT within a factor O(log (1/ r)) of the optimum. This is asymptotically the best approximation factor possible. For the double auction setting, we present a fixed price mechanism that achieves for all ϵ> 0 a gain from trade of at least (1 - ϵ) times the optimum with probability 1-2/e#Tϵ2/2, where # T is the expected number of trades of the mechanism. This can be interpreted as a “large market” result: Full efficiency is achieved in the limit, as the market gets thicker
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