996 research outputs found
A Theoretical Analysis of Two-Stage Recommendation for Cold-Start Collaborative Filtering
In this paper, we present a theoretical framework for tackling the cold-start
collaborative filtering problem, where unknown targets (items or users) keep
coming to the system, and there is a limited number of resources (users or
items) that can be allocated and related to them. The solution requires a
trade-off between exploitation and exploration as with the limited
recommendation opportunities, we need to, on one hand, allocate the most
relevant resources right away, but, on the other hand, it is also necessary to
allocate resources that are useful for learning the target's properties in
order to recommend more relevant ones in the future. In this paper, we study a
simple two-stage recommendation combining a sequential and a batch solution
together. We first model the problem with the partially observable Markov
decision process (POMDP) and provide an exact solution. Then, through an
in-depth analysis over the POMDP value iteration solution, we identify that an
exact solution can be abstracted as selecting resources that are not only
highly relevant to the target according to the initial-stage information, but
also highly correlated, either positively or negatively, with other potential
resources for the next stage. With this finding, we propose an approximate
solution to ease the intractability of the exact solution. Our initial results
on synthetic data and the Movie Lens 100K dataset confirm the performance gains
of our theoretical development and analysis
Small Orbits
We study both the "large" and "small" U-duality charge orbits of extremal
black holes appearing in D = 5 and D = 4 Maxwell-Einstein supergravity theories
with symmetric scalar manifolds. We exploit a formalism based on cubic Jordan
algebras and their associated Freudenthal triple systems, in order to derive
the minimal charge representatives, their stabilizers and the associated
"moduli spaces". After recalling N = 8 maximal supergravity, we consider N = 2
and N = 4 theories coupled to an arbitrary number of vector multiplets, as well
as N = 2 magic, STU, ST^2 and T^3 models. While the STU model may be considered
as part of the general N = 2 sequence, albeit with an additional triality
symmetry, the ST^2 and T^3 models demand a separate treatment, since their
representative Jordan algebras are Euclidean or only admit non-zero elements of
rank 3, respectively. Finally, we also consider minimally coupled N = 2, matter
coupled N = 3, and "pure" N = 5 theories.Comment: 40 pages, 9 tables. References added. Expanded comments added to
sections III. C. 1. and III. F.
Four-qubit entanglement from string theory
We invoke the black hole/qubit correspondence to derive the classification of
four-qubit entanglement. The U-duality orbits resulting from timelike reduction
of string theory from D=4 to D=3 yield 31 entanglement families, which reduce
to nine up to permutation of the four qubits.Comment: 4 pages, 1 figure, 2 tables, revtex; minor corrections, references
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Biópsia de lesões pulmonares por agulha cortante guiada pela tomografia computadorizada
PURPOSE: To report the experience of a radiology department in the use of computed tomography - guided cutting needle biopsy of pulmonary nodules, by evaluating diagnostic yield and incidence of complications. METHODS: This is a retrospective analysis of 52 consecutive patients who underwent lung lesion biopsy guided by computed tomography, performed between May 1997 and May 2000. Thirty-five patients were male and 17 were female, with ages ranging from 5 to 85 years (median, 62 years). The size of the lesions ranged from 1.8 to 15 cm (median, 5.4 cm). RESULTS: In a total of 52 biopsies of lung lesions, 51 biopsies (98.1%) supplied appropriate material for histopathological diagnosis, with 9 diagnosed (17.3%) as benign and 42 (80.8%) as malignant lesions. Specific diagnosis was obtained in 44 (84.6%) biopsies: 4 benign (9.1%) and 40 (90.9%) malignant lesions. The sensitivity, specificity, and accuracy of the cutting needle biopsies for determining presence of malignancy were 96.8%, 100%, and 97.2%, respectively. Complications occurred in 9 cases (17.3%), including 6 cases (11.5%) of small pneumothorax, 1 (1.9%) of hemoptysis, 1 (1.9%) of pulmonary hematoma, and 1 (1.9%) of thoracic wall hematoma. All had spontaneous resolution. There were no complications requiring subsequent intervention. CONCLUSION: The high sensitivity and specificity of the method and the low rate of complications have established cutting needle biopsy as an efficient and safe tool for the diagnosis of lung lesions. In our hospital, cutting needle biopsy is considered a reliable procedure for the evaluation of indeterminate pulmonary nodules.OBJETIVO: Apresentar a experiência inicial de um serviço de radiologia, no uso de biópsia de lesões pulmonares por agulha cortante guiada por tomografia computadorizada, avaliando o valor diagnóstico e incidência de complicações. MÉTODOS: Realizamos análise retrospectiva de 52 pacientes submetidos à biópsia de lesão pulmonar guiada pela tomografia computadorizada. Analisamos o perÃodo de maio de 1997 até maio de 2000. Trinta e cinco pacientes eram de sexo masculino e 17 feminino, com idade variando de cinco a 85 anos (mediana de 62 anos). O tamanho das lesões pulmonares variou de 1,8 a 15 cm (mediana de 5,4 cm). RESULTADOS: Entre os 52 casos, 51 (98,1%) forneceram material adequado para diagnóstico histopatológico, com nove diagnósticos (17,3%) de lesão benigna e 42 (80,8%) de lesão maligna. Quarenta e quatro (84,6%) obtiveram diagnósticos especÃficos a partir de análise histopatológica, sendo quatro benignos e 40 malignos. A sensibilidade, a especificidade e a acurácia do método para lesões malignas foi de 96,8%, 100% e 97,2%, respectivamente. Nove casos (17,3%) de complicações ocorreram, sendo seis casos (11,5%) de pneumotórax pequeno, um (1,9%) de hemoptise, um (1,9%) de hematoma intrapulmonar e um outro caso (1,9%) de hematoma na parede torácica. Não houve complicação que necessitasse de intervenção posterior. CONCLUSÃO: As altas sensibilidade e especificidade, e o baixo Ãndice de complicações tornam o método de biópsia por agulhas cortantes uma ferramenta eficiente e segura para diagnóstico de lesões pulmonares. Em nosso hospital é considerado um procedimento confiável para a avaliação de nódulos pulmonares indeterminados
Biópsia de massas mediastinais guiadas por tomografia computadorizada: agulhas finas versus cortantes
PURPOSE: To report the experience of a radiology department in the use of computed tomography guided biopsies of mediastinal lesions with fine and cutting needles, describing the differences between them. The results of adequacy of the sample and histologic diagnoses are presented according to the type of needle used. METHODS: We present a retrospective study of mediastinal biopsies guided by computed tomography performed from January 1993 to December 1999. Eighty-six patients underwent mediastinal biopsy in this period, 37 with cutting needles, 38 with fine needles, and 11 with both types (total of 97 biopsies). RESULTS: In most cases, it was possible to obtain an adequate sample (82.5%) and specific diagnosis (67.0%). Cutting-needle biopsy produced a higher percentage of adequate samples (89.6% versus 75.5%, P = 0.068) and of specific diagnosis (81.3% versus 53.1%, P = 0.003) than fine-needle biopsy. There were no complications that required intervention in either group. CONCLUSION: Because they are practical, safe, and can provide accurate diagnoses, image-guided biopsies should be considered the procedure of choice in the initial exploration of patients with mediastinal masses. In our experience, cutting needles gave higher quality samples and diagnostic rates. We recommend the use of cutting needles as the preferred procedure.OBJETIVO: Apresentar a experiência de um serviço de radiologia na prática de punções biópsias de massas mediastinais guiadas por tomografia computadorizada com agulhas finas ou cortantes, descrevendo as diferenças entre elas. Os resultados referentes a material suficiente e diagnóstico histológico são apresentados de acordo com o tipo de agulha utilizado. MÉTODOS: Apresentamos um estudo retrospectivo de biópsias mediastinais guiadas por tomografia computadorizada realizadas em nosso hospital no perÃodo de janeiro de 1993 a dezembro de 1999. Oitenta e seis pacientes foram submetidos a biópsia mediastinal neste perÃodo, sendo 37 realizadas com agulhas cortantes, 38 com agulhas finas e 11 com ambas, (total de 97 biópsias). RESULTADOS: Na maioria dos casos foi possÃvel se obter material adequado (82.5% ) e diagnóstico especÃfico (67 %). As agulhas cortantes apresentaram maior porcentagem de material suficiente (89.6% versus 75,5%, P=0,068) e de diagnóstico especÃfico (81,3% versus 53,1%, p=0,003 ) do que as agulhas finas. Não houveram complicações que requisessem intervenção em nenhum dos grupos. CONCLUSÃO: Pela praticidade, segurança e grande probabilidade de diagnóstico acurado sem procedimentos mais invasivos, as biópsias guiadas por imagem devem ser consideradas como a primeira etapa na investigação de massas mediastinais. Pela nossa experiência as agulhas cortantes fornecem material de maior qualidade e maior taxa de diagnóstico. Nós recomendamos o uso das agulhas cortantes como procedimento preferencial
Choosing Well Your Opponents: How to Guide the Synthesis of Programmatic Strategies
This paper introduces Local Learner (2L), an algorithm for providing a set of
reference strategies to guide the search for programmatic strategies in
two-player zero-sum games. Previous learning algorithms, such as Iterated Best
Response (IBR), Fictitious Play (FP), and Double-Oracle (DO), can be
computationally expensive or miss important information for guiding search
algorithms. 2L actively selects a set of reference strategies to improve the
search signal. We empirically demonstrate the advantages of our approach while
guiding a local search algorithm for synthesizing strategies in three games,
including MicroRTS, a challenging real-time strategy game. Results show that 2L
learns reference strategies that provide a stronger search signal than IBR, FP,
and DO. We also simulate a tournament of MicroRTS, where a synthesizer using 2L
outperformed the winners of the two latest MicroRTS competitions, which were
programmatic strategies written by human programmers.Comment: International Joint Conference on Artificial Intelligence (IJCAI)
202
Black holes admitting a Freudenthal dual
The quantised charges x of four dimensional stringy black holes may be
assigned to elements of an integral Freudenthal triple system whose
automorphism group is the corresponding U-duality and whose U-invariant quartic
norm Delta(x) determines the lowest order entropy. Here we introduce a
Freudenthal duality x -> \tilde{x}, for which \tilde{\tilde{x}}=-x. Although
distinct from U-duality it nevertheless leaves Delta(x) invariant. However, the
requirement that \tilde{x} be integer restricts us to the subset of black holes
for which Delta(x) is necessarily a perfect square. The issue of higher-order
corrections remains open as some, but not all, of the discrete U-duality
invariants are Freudenthal invariant. Similarly, the quantised charges A of
five dimensional black holes and strings may be assigned to elements of an
integral Jordan algebra, whose cubic norm N(A) determines the lowest order
entropy. We introduce an analogous Jordan dual A*, with N(A) necessarily a
perfect cube, for which A**=A and which leaves N(A) invariant. The two
dualities are related by a 4D/5D lift.Comment: 32 pages revtex, 10 tables; minor corrections, references adde
Idade e crescimento do boca-negra, Helicolenus dactylopterus dactylopterus (Delaroche, 1809) dos Açores
A idade e o crescimento de boca-negra, Helicolenus dactylopterus dactylopterus (Delaroche, 1809), foram estudadas pela observação dos otólitos (sagittae) esquerdos inteiros (n = 401) obtidos de exemplares (14-47 cm de comprimento total) capturados em águas Açoreanas. Enumeraram-se os anéis opacos observados na face anti-sulcal dos otólitos. Os intervalos de idades foram 3-14 anos para machos e 3-12 anos para fêmeas. Estimaram-se e compararam-se os parâmetros da equação de crescimento de von Bertalanffy, entre sexos e entre métodos (leitura directa de otólitos, retrocálculo e análise de distribuições de frequências de comprimentos). Não se verificaram diferenças importantes. Os resultados são diferentes da literatura publicada para a região. Discutem-se as causas e implicações dos resultados obtidos.ABSTRACT: Bluemouth, Helicolenus dactylopterus dactylopterus (Delaroche, 1809), age and growth were studied by whole-view examination of left sagittae (n = 401) obtained from specimens (14-47 cm in total length) caught off the Azores. Opaque rings observed on the anti-sulcal surface of sagittae were enumerated as age estimates. Ages ranged from 3 to 14 years in males and 3 to 12 years in females. The von Bertalanffy growth equation was fitted to average length at age data, and compared between sexes and methods (direct examination of otoliths, backcalculation and length-frequency analysis). No important differences in growth between sexes were found. Results are different from published literature for the region. The causes and implications of the results are discussed
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