1,962 research outputs found

    Dirichlet parabolicity and L1L^1-Liouville property under localized geometric conditions

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    We shed a new light on the L1L^1-Liouville property for positive, superharmonic functions by providing many evidences that its validity relies on geometric conditions localized on large enough portions of the space. We also present examples in any dimension showing that the L1L^1-Liouville property is strictly weaker than the stochastic completeness of the manifold. The main tool in our investigations is represented by the potential theory of a manifold with boundary subject to Dirichlet boundary conditions. The paper incorporates, under a unifying viewpoint, some old and new aspects of the theory, with a special emphasis on global maximum principles and on the role of the Dirichlet Green's kernel

    Risk of cancer after assisted reproduction: a review of the available evidences and guidance to fertility counselors

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    Infertile women requiring ovarian stimulation and assisted reproduction techniques (ART) are faced with difficult issues. The fear that using hormones could increase their risk of cancer is the most significant. One of the main challenges for assessing cancer risk after ART is the difficulty to separate it from the underlying condition of infertility per se. The delay or the inability to achieve a pregnancy is an important risk factor for breast, endometrial and ovarian cancer. We analyzed the current literature on the topic

    Second-level NIST randomness tests for improving test reliability

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    Testing Random Number Generators (RNGs) is as important as designing them. Here we consider the NIST test suite SF 800-22 and we show that, as suggested by NIST itself, to reveal non-perfect generators a more in-depth analysis should be performed using the outcomes of the suite over many generated sequences. Testing these second-level statistics is not trivial and, relying on a proper model that takes into account the errors due to the approximations in the first level tests, we propose a tuning of the parameters in the simplest cases. The validity of our consideration is widely supported by experimental results on several RNG currently employed by major IT players, as well as a chaos-based RNG designed by authors

    A 3 GHz spread spectrum clock generator for SATA applications using chaotic PAM modulation

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    This paper proposes a prototype of a Spread Spectrum Clock Generator which is the first known specifically meant for 3 GHz Serial ATA-II applications. The modulation is obtained from a fractional PLL which employs a Delta-Sigma modulator. A further innovative aspect of our work is that our prototype takes advantage of a chaotic PAM as driving signal, instead a triangular signal as in all spread spectrum generators proposed in literature for SATA-II. In this way we avoid the periodicity of the modulated clock, completely flattening the peaks in the power spectral density. The circuit prototype has been designed n 0.13 μm CMOS technology and achieves a peak reduction greater than 14 dB measured at RBW = 100 kHz. The chip active area is 0.27×0.78 mm2 and the power consumption is as low as 14.7 mW. © 2008 IEEE

    On the Approximation Errors in the Frequency Test Included in the NIST SP800-22 Statistical Test Suite

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    In previous papers we have addressed the problem of testing Random Number Generators (RNGs) through statistical tests, with particular emphasis on the approach we called second-level testing. We have shown that this approach is capable of achieving much higher accuracy in exposing non-random generators, but may suffer from reliability issues due to approximations introduced in the test. Here we consider the NIST Frequency Test and present a mathematical expression of the error introduced by approximating the effective discrete distribution function with its continuous limit distribution. The matching against experimental data is almost perfect. © 2008 IEEE

    Asymptotic expressions of mismatch variance in interdigitated geometries

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    Performance in analog integrated circuits strongly depends on the mismatch between nominally identical devices. In this work we derive closed-form asymptotic expressions describing mismatch variance in multifinger structures, under the assumption of Gaussian autocorrelation for the mismatch-generating stochastic process. The analysis is performed on interdigitated geometries, eventually modified to make them common-centroid. Comparison with the numerical results provided by an independent model validates the theoretical expressions presented here

    Stability and mismatch robustness of a leakage current cancellation technique

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    Leakage discharge currents represent one of the most detrimental factors for the maximum hold time in analog sample-and-hold circuits. Apart from the obvious passive solution of enlarging the sampling capacitor, alternatives based on active circuits have been proposed. We focus here on an existing solution which has proven to be effective in reducing the leakage discharge, hence extending the hold time, by a factor of 20. Being based on a feedback circuit built around the hold capacitor, it is paramount to understand its stability properties. This work tries to close the gap by analyzing the closed-loop stability of the nominal circuit. Classical control systems techniques are employed to thoroughly analyze the dynamic behaviour of the feedback circuit, highlighting the detrimental effect of device mismatches

    Resonate and fire dynamics in Complex Oscillation Based Test of analog filters

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    Recently, proposals have been made for enhancing the Oscillation Based Test (OBT) methodology by using non-plain oscillation regimes, leading to so called Complex Oscillation Based Test (COBT). Here we focus on a recently illustrated strategy for the testing of analog 2nd order filters, showing that the COBT dynamics is quite similar to that expressed by Resonate & Fire (R+F) neuron models. In this interpretation, the testing approach can be related to firing-rate measures. A brief description is given of the mathematical models necessary to achieve a precise characterization of firing times, showing how it can be used for testing purposes. A practical example with simulation data is also provided. © 2011 IEEE

    A Zero-Transient Dual-Frequency Control for Class-E Resonant DC-DC Converters

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    In this paper, a dual-frequency control method for regulating the output power in class-E resonant DC-DC converters has been introduced. As in the standard ON-OFF control or other recently proposed dual-frequency controls, the approach is based on the ability of the converter to alternately operate in a high- and a low-power state. The proposed solution has a twofold advantage: on the one hand, soft-switching capabilities (i.e., Zero-Voltage and Zero-Voltage-Derivative switching) are preserved in both operating states; on the other hand, it is possible to reduce to zero the transient time required to switch from one state to the other one. The most straightforward consequence is the possibility to increase to very large values the frequency at which the two operating states are switched, up to the same order of magnitude as the main switching frequency of the converter. In this way, the additional ripple introduced by the proposed dual-frequency control can be decreased to a negligible value. The approach has been validated by measurements on a prototype operating between 4 MHz and 8 MHz and in which it has been possible to increase the control frequency up to 500 kHz

    Unlocking the gender diversity-group performance link: the moderating role of relative cultural distance

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    PurposeThis study aims to shed light on the relationship between gender diversity and group performance by considering the moderating role of relative cultural distance. Drawing from the categorization-elaboration model (CEM), the authors hypothesize that gender-diverse collaborative learning groups perform better when a low level of relative cultural distance in country-level individualism-collectivism or power distance exists among group members.Design/methodology/approachTo test this hypothesis, the authors conducted a study on 539 undergraduate students organized into 94 groups. The assessment of group performance was based on scores given by external raters.FindingsThe authors found that relative cultural distance significantly moderated the gender diversity-group performance relationship such that gender diversity was positively related to group performance when the collaborative learning group included members who similarly valued individualism-collectivism or power distance (i.e. relative cultural distance was low) and was negatively related to group performance when the collaborative learning group comprised members who differently valued individualism-collectivism or power distance (i.e. relative cultural distance was high).Originality/valueThis study contributes to understanding when gender diversity is positively associated with group performance by expanding the range of previously examined diversity dimensions to include relative cultural distance in country-level individualism-collectivism and power distance
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