3 research outputs found
An information theoretic approach to statistical dependence: copula information
We discuss the connection between information and copula theories by showing
that a copula can be employed to decompose the information content of a
multivariate distribution into marginal and dependence components, with the
latter quantified by the mutual information. We define the information excess
as a measure of deviation from a maximum entropy distribution. The idea of
marginal invariant dependence measures is also discussed and used to show that
empirical linear correlation underestimates the amplitude of the actual
correlation in the case of non-Gaussian marginals. The mutual information is
shown to provide an upper bound for the asymptotic empirical log-likelihood of
a copula. An analytical expression for the information excess of T-copulas is
provided, allowing for simple model identification within this family. We
illustrate the framework in a financial data set.Comment: to appear in Europhysics Letter
Intersubband-induced spin-orbit interaction in quantum wells
Recently, we have found an additional spin-orbit (SO) interaction in quantum
wells with two subbands [Phys. Rev. Lett. 99, 076603 (2007)]. This new SO term
is non-zero even in symmetric geometries, as it arises from the intersubband
coupling between confined states of distinct parities, and its strength is
comparable to that of the ordinary Rashba. Starting from the Kane
model, here we present a detailed derivation of this new SO Hamiltonian and the
corresponding SO coupling. In addition, within the self-consistent Hartree
approximation, we calculate the strength of this new SO coupling for realistic
symmetric modulation-doped wells with two subbands. We consider gated
structures with either a constant areal electron density or a constant chemical
potential. In the parameter range studied, both models give similar results. By
considering the effects of an external applied bias, which breaks the
structural inversion symmetry of the wells, we also calculate the strength of
the resulting induced Rashba couplings within each subband. Interestingly, we
find that for double wells the Rashba couplings for the first and second
subbands interchange signs abruptly across the zero bias, while the
intersubband SO coupling exhibits a resonant behavior near this symmetric
configuration. For completeness we also determine the strength of the
Dresselhaus couplings and find them essentially constant as function of the
applied bias.Comment: 16 pages, 12 figure
Polimorfismos GSTT1 e GSTM1 em indivÃduos tabagistas com carcinoma espinocelular de cabeça e pescoço GSTT1 and GSTM1 polymorphism in cigarette smokers with head and neck squamous cell carcinoma
A variabilidade em genes relacionados aos processos de ativação e detoxificação de carcinógenos pode interferir na suscetibilidade ao câncer. OBJETIVO: Investigar a relação entre os polimorfismos GSTT1 e GSTM1 nulos e o risco para o carcinoma espinocelular de cabeça e pescoço em indivÃduos tabagistas. MATERIAL E MÉTODO: Este estudo caso-controle foi realizado na Faculdade de Medicina de São José do Rio Preto, Brasil. Foram avaliadas as freqüências dos genótipos nulos GSTT1 e GSTM1 por PCR multiplex em 60 pacientes com carcinoma espinocelular de cabeça e pescoço e 60 indivÃduos sem a doença. RESULTADOS: A cavidade oral foi o sÃtio de tumor mais freqüente. O genótipo GSTT1 nulo foi encontrado em 33,3% dos pacientes e em 23,3% dos indivÃduos controles (p=0,311). Os grupos caso e controle apresentaram freqüências do genótipo GSTM1 nulo de 35% e 48,3%, respectivamente (p=0,582). Não foram encontradas associações entre o hábito etilista e genótipos nulos GSTT1 e GSTM1 em ambos os grupos (valores de p>0,05). O gênero masculino e o hábito etilista foram prevalentes em ambos os grupos. CONCLUSÃO: Neste estudo não foi possÃvel estabelecer uma correlação entre os genótipos nulos GSTT1 e GSTM1 e o carcinoma espinocelular de cabeça e pescoço em indivÃduos tabagistas.<br>Gene variability related to carcinogen activation and detoxification may interfere with susceptibility to head and neck cancer. AIM: To investigate the relation between GSTT1 and GSTM1 null polymorphisms and the risk of head and neck squamous cell carcinoma in cigarette smokers. MATERIAL AND METHOD: A case-control study conducted at the Sao Jose do Rio Preto Medical School, Brazil. GSTM1 and GSTT1 null genotype frequencies were evaluated by multiplex PCR in 45 cigarette smokers with head and neck squamous cell carcinomas and 45 cigarette smokers without this disease. RESULTS: The oral cavity was the most prevalent tumor site for squamous cell carcinoma. The GSTT1 null genotype was found in 33.3% of the Experimental Group and 23.3% of the Control Group (p= 0.311). Experimental and Control Groups had GSTM1 null genotype frequencies of 35% and 48.3% (p=0.582). No association between alcohol consumption and GSTT1 and GSTMI null genotypes was found in these groups (p-values>0.05). There were more men, and alcohol consumption was prevalent in both groups. CONCLUSION: In this study we were unable to show a correlation between GSTM1 and GSTT1 genotypes and the development of head and neck squamous cell carcinomas in cigarette smokers